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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Available online at www.sciencedirect.com Earth-Science Reviews 86 (2008) 62 – 88 www.elsevier.com/locate/earscirev Enargite oxidation: A review Pierfranco Lattanzi a,⁎, Stefania Da Pelo a , Elodia Musu a , Davide Atzei b , Bernhard Elsener b , Marzia Fantauzzi b , Antonella Rossi b a b Dipartimento di Scienze della Terra, Università di Cagliari via Trentino 51, I-09127 Cagliari, Italy Dipartimento di Chimica Inorganica e Analitica, Università di Cagliari Cittadella Universitaria, I-09042 Monserrato (CA), Italy Received 31 October 2006; accepted 27 July 2007 Available online 25 August 2007 Abstract Enargite, Cu3AsS4, is common in some deposit types, e.g. porphyry systems and high sulphidation epithermal deposits. It is of environmental concern as a potential source of arsenic. In this communication, we review the current knowledge of enargite oxidation, based on the existing literature and our own original data. Explicit descriptions of enargite oxidation in natural environments are scarce. The most common oxidized alteration mineral of enargite is probably scorodite, FeAsO4.2H2O, with iron provided most likely by pyrite, a phase almost ubiquitously associated with enargite. Other secondary minerals after enargite include arsenates such as chenevixite, Cu2Fe2(AsO4)2(OH)4.H2O, and ceruleite, Cu2Al7(AsO4)4.11.5H2O, and sulphates such as brochantite, Cu4(SO4)(OH)6, and posnjakite, Cu4(SO4)(OH)6 H2O. Detailed studies of enargite field alteration at Furtei, Sardinia, suggest that most alteration occurs through dissolution, as testified by the appearance of etch pits at the surface of enargite crystals. However, apparent replacement by scorodite and cuprian melanterite was observed. Bulk oxidation of enargite in air is a very slow process. However, X-ray photoelectron spectroscopy (XPS) reveals subtle surface changes. From synchrotron-based XPS it was suggested that surface As atoms react very fast, presumably by forming bonds with oxygen. Conventional XPS shows the formation, on aged samples, of a nanometer-size alteration layer with an appreciably distinct composition with respect to the bulk. Mechanical activation considerably increases enargite reactivity. In laboratory experiments at acidic to neutral pH, enargite oxidation/dissolution is slow, although it is accelerated by the presence of ferric iron and/or bacteria such as Acidithiobacillus ferrooxidans and Sulfolobus BC. In the presence of sulphuric acid and ferric iron, the reaction involves dissolution of Cu and formation of native sulphur, subsequently partly oxidized to sulphate. At alkaline pH, the reactivity of enargite is apparently slightly greater. XPS spectra of surfaces conditioned at pH 11 have been interpreted as evidence of formation of a number of surface species, including cupric oxide and arsenic oxide. Treatment with hypochlorite solutions at pH 12.5 quickly produces a coating of cupric oxide. Electrochemical oxidation of enargite typically involves low current densities, confirming that the oxidation process is slow. Important surface changes occur only at high applied potentials, e.g. + 0.74 V vs. SHE. It is confirmed that, at acidic pH, the dominant process is Cu dissolution, accompanied (at +0.56 V vs. SHE, pH = 1) by formation of native sulphur. At alkaline pH, a number of surface products have been suggested, including copper and arsenic oxides, and copper arsenates. XPS studies of the reacted surfaces demonstrate the evolution of Cu from the monovalent to the divalent state, the formation of As–O bonds, and the oxidation of sulphur to polysulphide, sulphite and eventually sulphate. In most natural and quasi-natural (mining) situations, it is expected that enargite reactivity will be slow. Moreover, it is likely that the release of arsenic will be further slowed down by at least temporary trapping in secondary phases. Therefore, an adequate management of exposed surfaces and wastes should minimize the environmental impact of enargite-bearing deposits. · ⁎ Corresponding author. Fax: +39 070 282236. E-mail address: lattanzp@unica.it (P. Lattanzi). 0012-8252/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2007.07.006 Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 63 In spite of an increasing body of data, there are several gaps in our knowledge of enargite oxidation. The exact nature of most mechanisms and products remains poorly constrained, and there is a lack of quantitative data on the dependence on parameters such as pH and dissolved oxygen. © 2007 Elsevier B.V. All rights reserved. Keywords: enargite; secondary minerals; laboratory experiments; surface oxidation 1. Introduction Enargite is a copper–arsenic sulphide with formula Cu3AsS4. It is common and, locally, abundant in some types of ore deposits, notably epithermal “high sulphidation” deposits (e.g., Arribas, 1995), and some porphyry copper systems. The web database www.mindat.org1 lists as many as 461 localities where enargite occurs. Occasionally enargite is a valuable copper ore, but more frequently (along with its polytype luzonite) it is considered a potential hazard to the environment, requiring special precautions in smelting. Therefore, most smelters consider enargite a penalty. While the potential threat from copper cannot be dismissed, the most feared environmental impact of enargite is certainly the release of arsenic. Arsenic is currently regarded as one of the most dangerous inorganic pollutants, causing environmental and health emergencies in several areas of the world (e.g., Mandal and Suzuki, 2002, and references therein). Although reliable, accurate estimates are difficult, it is likely that the most important primary (hypogene) sources of arsenic are arsenian pyrite and arsenopyrite, followed by sulphosalts of the tetrahedrite– tennantite series; enargite and the allied phase luzonite could rank fourth, possibly in close competition with other sulphides and sulphosalts (e.g., realgar, As4S4, orpiment, As2S3, proustite, Ag3AsS3, and cobaltite, CoAsS) that have a comparable diffusion, but are rarely abundant, except in specific deposits.2 Enargite, like most sulphides, is intrinsically unstable in the exogenous environment. Therefore, its oxidation behaviour is critical for the assessment of its potential environmental impact. Our group is involved in the study of enargite oxidation under different conditions, with the ultimate goal of understanding and predicting the potential environmental impact of enargite-bearing ores in mining and mineral processing operations. In this communication, we review the current knowledge on 1 Last visited June 2007. This estimate is based on the number of reported occurrences in www.mindat.org. 2 enargite oxidation, based on a survey of the existing literature, including our own data. Emphasis is placed on information relevant to natural processes that may occur in mineralised rocks and mining sites. 2. The properties of enargite Enargite is a blackish gray mineral with a metallic luster, Mohs hardness = 3, and density = 4.5 g/cm3 . Enargite crystallises in the orthorhombic system, pyramidal class, space group Pnm21. It occurs in granular masses, but well-formed crystals are not rare; its habit may be tabular (001), or prismatic, elongated along c. Enargite shows an excellent cleavage along (110); other cleavage planes are (100) and (010). The crystal structure is well known (Pauling and Weinbaum, 1934). It is derived from that of wurtzite (ZnS), with Zn positions occupied by Cu and As; both elements are in fourfold coordination with S. Hence, enargite is not a sulphosalt in the sense of Takeuchi and Sadanaga (1969) because AsS3 pyramids are not present in the structure. Subsequent refinements (Adiwidjaja and Lohn, 1970; Henao et al., 1994; Karanovic et al., 2002; Pfitzner and Bernert, 2004) confirmed Pauling and Weinbaum's structure, and provided more accurate information on atomic positions. The unit cell parameters (Pfitzner and Bernert, 2004) are a = 7.399 Å, b = 6.428 Å, and c = 6.145 Å. Enargite is a semiconductor of the type A3IBVC4II (Pauporté and Lincot, 1995). The flat band potential is about −0.16 V vs. the standard hydrogen electrode, SHE3 (Pauporté and Schuhmann, 1996). Copper is nominally in the monovalent state, and arsenic in the pentavalent state. However, X-ray absorption near-edge spectra (Li et al., 1994) at the S-K and S-L edges indicate that Cu+ d10 electrons are involved in metal–sulphur bonding. Recently, Reddy et al. (2006), based on electron 3 In the original paper, as well as in others cited here, the value was referred to the standard calomel electrode (SCE); throughout this paper, in agreement with the most accepted recent use, all values have been recalculated to the SHE, assuming + 0.244 V as the potential of SCE vs. SHE. Author's personal copy 64 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 paramagnetic resonance (EPR) and UV-vis (ultravioletvisible) spectroscopic data, suggested the presence of Cu in the divalent state (Cu2 + ion); such a presence is probably referable to partial oxidation of the specific sample (see more in the following sections). Thermodynamic data for enargite are reported by Seal et al. (1996). Thermodynamic diagrams portraying selected phase relations involving enargite in the systems Cu–As–S, Cu–As–S–O, Cu–As–S–H2O and Cu–As– S–Cl–H2O have been presented by Knight (1977), Welham (2001), Kantar (2002) and Castro and Baltierra (2005), and Asbjornsson et al. (2004a,b), respectively. Enargite phase relations in the system Cu–As–S are fairly well known above 350 °C (e.g., Maske and Skinner, 1971; Clarke and Helz, 2000; Muller and Blachnik, 2002), whereas at lower temperature they are poorly defined. Enargite is the high-temperature modification of Cu3AsS4; inversion to luzonite (which has a sphaleritetype structure) should occur below about 300 °C; according to Bernardini et al. (1973), however, enargite and luzonite apparently coexist (metastably?) between 215 ± 15° and 315 ± 15 °C. Studies by Posfai and Sundberg (1998) indicate that both polytypes may occur in highly disordered structures, whereby luzonite-type and enargite-type structures are intergrown at the atomic scale. The results of Clarke and Helz (2000) may suggest that the stable phase at room temperature is a cubic polymorph, with structure similar to arsensulvanite Cu3 (As, V)S4. In most natural occurrences, enargite is associated with pyrite, and other copper and/or arsenic and/or base metal sulphides (chalcopyrite, chalcocite, covellite, digenite, tennantite, sphalerite, galena). Enargite may contain minor amounts of other elements (Sb, Ag, Fe). The presence of Sb (up to 6 wt.%; Springer, 1969) is quite common, and environmentally relevant; enargite is frequently associated with Sb-bearing minerals, such as members of the solid solution series tetrahedrite– tennantite and luzonite–famatinite. Phase relations in the Cu–As–Sb–S system were studied by Bernardini et al. (1973), Luce et al. (1977), and Sugaki et al. (1982); Posfai and Buseck (1998) investigated the relationship between composition and luzonite–enargite polytypism. Whereas luzonite shows a complete solid solution with the corresponding Sb compound, the maximum content of Sb in enargite is 14% mol. Cu3SbS4 at 600 °C, and is lower at lower temperatures. 2.1. The enargite surface The surface structure of enargite at the atomic scale is not known in detail. Owing to the perfect (110) cleavage, this is the face that has the greatest probability of interacting with the environment. However, there have been no detailed studies of the enargite surface with respect to orientation. Enargite has a wurtzite-type structure. Sulphides with such structures have surfaces that are not simple truncated bulk solids, but undergo some kind of relaxation (Vaughan et al., 1997; Rosso and Vaughan, 2006a – see in particular their Fig. 17). X-ray photoelectron spectroscopy (XPS) on fresh surfaces, exposed by fracturing in high vacuum or under an inert atmosphere, gives data entirely consistent with copper in a monovalent state and sulphur in a monosulphide state (Nakai et al., 1978; Da Pelo, 1998; Rossi et al., 2001; Velazquez et al., 2000, 2002; Pratt, 2002, 2004). Velazquez et al. (2000, 2002) and Rossi et al. (2001) further noticed that the composition of this surface (estimated from the photoelectron peaks) is not far from stoichiometric enargite (Velasquez et al., 2002, report Cu3As0.7S4.2). Rossi et al. (2001) provided evidence that electron microprobe results and XPS results are in excellent agreement, suggesting that surface and bulk composition of freshly fractured enargite are the same: no enrichment of any element was observed. This match was not observed for polished and Ar-sputtered (4 keV Ar+ ions) surfaces. Velasquez et al. (2002) documented significant differences between fractured, polished, and Ar-sputtered surfaces of natural enargite. Polishing of the surface resulted in both a shift to lower energies of the As3d and S2p energies, and a composition with excess sulphur (compare the similar results obtained by Velazquez et al., 2000). The results were ascribed to “changes of arsenic and sulphur coordination”, and to formation of polysulphide species. Finally, the Ar-sputtered surface showed a dramatically different composition, with strong depletion in sulphur and arsenic with respect to copper (atomic ratios Cu/As ∼14, Cu/S ∼2). This was interpreted as a preferential loss of As and S during the ion bombardment under the reported sputtering conditions. Sputtering was also performed using a lower energy beam, and the surface modifications were less severe. One can conclude that sputtering conditions, especially energy and beam current, can play an important role in determining enargite surface composition, as reported in the literature for many other materials. By comparing As3d XPS signals under a conventional source with those obtained by synchrotron light excitation, Pratt (2004) found that, in the latter case, a lower-binding energy component at 42.1–42.8 eV is distinctly enhanced. The author concluded that surface arsenic atoms in enargite are different from the bulk crystal structure, and occur as individual protrusions from the surface. The low-BE Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 shoulder in the As3d signal was tentatively attributed to “low coordinate surface As species” also by Asbjornsson et al. (2004a,b). 3. Nature of data sources The literature pertaining to oxidation of enargite can be grouped into the following categories: 1. Field studies of secondary supergene alteration minerals in (quasi) natural environments: typically, mineralised bodies and mine sites. This type of literature comprises a fairly large number of mineralogy and economic geology papers. In general, the focus of these papers is far different from systematic studies of enargite oxidation, and information on this subject is usually of descriptive, non systematic nature. Most papers list secondary minerals with no information as to whether they derive from enargite or from other ore components. Little attention is paid to the mechanism (s) of the alteration process(es). Therefore, this type of study provides a wealth of empirical evidence; however, this can be difficult to interpret in terms of specific understanding of enargite oxidation. 2. Laboratory studies of enargite oxidation and/or dissolution. These studies document the oxidation/ dissolution of enargite under a variety of conditions and reactants, including aqueous solutions, ball milling, and electrochemical oxidation. An important number of these studies address bioleaching processes, i.e. enargite oxidation-dissolution in presence of microorganisms. There is specific attention to reaction processes/products; however, most studies are focused on practical implications for mineral processing, rather than details of the specific mechanisms, and experimental conditions are usually not representative of natural oxidation of enargite in the supergene environment. Moreover, several studies have been conducted on enargite-rich mixtures, often with little detail on the associated phases. An important point that is often overlooked is the possible presence of the enargite polymorph, luzonite. As previously stated, luzonite and enargite may occur intimately intergrown down to the submicroscopic scale. Their X-ray diffraction (XRD) patterns are quite different, hence the presence of luzonite as a major phase can be easily detected (down to approximately 5% weight with routine XRD tests); also the optical properties of enargite and luzonite in reflected light microscopy are appreciably different (see e.g. Ramdohr, 1980). However, without such specific tests, the presence of luzonite may remain undetected. 65 3. Much effort has been devoted in recent years to the study of mineral surfaces. These are the interfaces between minerals and the environment. Therefore, surface properties represent the ultimate control on mineral reactivity. In this context, an increasing body of literature is being devoted to the characterisation of surface properties and surface reactivity of enargite. Again, much effort was addressed at conditions typical of mineral processing (e.g., alkaline environments such as are used in some flotation processes). There are, however, exploratory surface studies of enargite exposed to air, or to a variety of reactants simulating natural environments. These studies are of special interest for acid mine drainage. Acid mine drainage is recognised as one of the most serious environmental problems related to mining (e.g., Blowes et al., 2003); dissolution of enargite is relevant to generation of acid mine drainage and release of “heavy metals” (see more in the next section). 4. Evidence from natural assemblages Weathering of minerals may result in either dissolution or formation of secondary minerals, or, more commonly, a combination of the two (incongruent dissolution). Waters draining mineralised rocks and ore bodies where enargite is a major mineral are typically rich in both copper and arsenic (see e.g. Plumlee et al., 1995; Da Pelo, 1998; Plumlee et al., 1999; Cidu et al., 1999; Cidu, 2000); copper to arsenic ratios are typically much higher than required by enargite stoichiometry (Fig. 1). This water chemistry can be controlled by direct dissolution of enargite, or of other primary or secondary minerals. At Summitville, Colorado, high copper pulses in drainage waters have been ascribed to dissolution of secondary copper minerals (mainly sulphates – Plumlee and Edelmann, 2005). At Furtei, Sardinia, Italy, Musu et al. (2007a,b) have explained the high copper/arsenic ratios in waters draining waste piles and mineralised rocks with the relative solubility of secondary phases: copper is only temporarily trapped in highly soluble minerals such as cuprian melanterite, whereas arsenic release is slowed down by adsorption onto iron oxyhydroxides-sulphates and/or formation of comparatively less soluble secondary minerals (scorodite). Oyarzun et al. (2006) believe that adsorption of arsenic onto goethite partly limits the As contents of waters in the Elqui river basin, Chile, where high concentrations of copper and arsenic (plus iron and sulphate) arise from both natural and mine-related contamination, including operations at the enargite-rich El Indio deposit. Most drainage of enargite-bearing deposits and rocks is acidic; this is probably due to the almost ubiquitous Author's personal copy 66 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 presence of pyrite along with enargite. However, the oxidative dissolution of enargite could be per se an acidgenerating process (Plumlee, 1999), e.g.: Cu3 AsS4 þ 8:75O2 þ 2:5H2 O→3Cu2þ þ AsO3− 4 þ þ 4SO2− þ 5H 4 ð1Þ or else, in the presence of ferric iron, Cu3 AsS4 þ 35Fe3þ þ 20H2 O→3Cu2þ þ 35Fe2þ 2− þ þ HAsO2− 4 þ 4SO4 þ 39H ð2Þ As previously noted, there are no specific studies of secondary supergene minerals developed after enargite. There are, on the other hand, many descriptions of mineral assemblages resulting from weathering of mineralised bodies and/or mineral wastes where enargite is present, and may locally be a major mineral. However, in most cases there is a regrettable scarcity of detailed textural information. A commonly described feature (e.g., Ramdohr, 1980) is the replacement of enargite by one or more copper sulphides (ranging in composition from chalcocite, Cu2S, to covellite, CuS). However, such assemblages may develop in either supergene (below the water table) or hypogene environments, and the distinction can be problematic. Note that, as pointed out by Clarke and Helz (2000), any three-phase assemblage in the system Cu– As–S (e.g., digenite–covellite–enargite) is invariant at constant temperature and pressure. Therefore, oxidation of any such assemblage in supergene systems, developing at practically constant pressure and (quasi) constant temperature, will not modify the mineral assemblage until one of the phases is used up (consider for instance the reaction 2.5Cu1.8S + O2 + 2H2S → 4.5CuS + 2H2O, Clarke and Helz, 2000). Eventually, however, sulphides will be oxidised to a stable assemblage of sulphates–arsenates. Among the oxidised minerals reported as an alteration of enargite, scorodite, FeAsO4 2H2O, is probably the most widespread. Explicit reports of scorodite resulting from alteration of enargite include those of Gray and Coolbaugh (1994), Perello (1994), Sewell and Wheatley (1994), and Chavez (2000). The exact conditions of formation are never specified. In general, scorodite is stable in oxidising, moderately acid environments (cf. Vink, 1996). Given the very common coexistence of enargite and pyrite, the formation of scorodite from enargite can be depicted as a simple mechanism, whereby copper is leached away and arsenic is combined with Fe from pyrite oxidation: · Cu3 AsS4 þ FeS2 þ 12:5O2 þ 5H2 O→FeAsO4 2H2 O þ 3Cu2þ þ 6SO2− 4 þ 6Hþ d Under dry conditions, solid copper sulphates may form. Notice that the reaction would produce more acidity than produced by the same amount of pyrite alone. Chavez (2000) lists chenevixite4 as an alteration product of mixtures of enargite and pyrite (see also Cook, 1978), and chenevixite, ceruleite, and lavendulan as the alteration products of mixtures of enargite and chalcopyrite with minor pyrite. Because of the presence of pyrite and/or chalcopyrite, the evolution of these assemblages likely occurs under acid conditions. In this context, the formation of chenevixite can be schematically described by a reaction similar to that previously written for scorodite: 2Cu3 AsS4 þ 2FeS2 þ 25O2 þ 11H2 O→Cu2 Fe2 ðAsO4 Þ2 ðOHÞ4 H2 O þ þ 4Cu2þ þ 12SO2− 4 þ 16H d Fig. 1. Cu/As molar ratios in waters draining high sulphidation deposits. Data for Furtei refer to open pit lake waters and seepages from waste piles (Da Pelo et al., 2005, and unpublished data by R. Cidu, S. Da Pelo and E. Musu). Other data sources: Summitville, Colorado: Plumlee et al. (1999); Kirki, Greece: Skarpelis and Triantafyllidis (2004); Recsk-Lahoca, Hungary: Rukezo (2003). ð3Þ ð4Þ Reactions (3) and (4) only differ in the presence of slightly more water in the second (molar ratio of water to enargite of 5.5 instead of 5). Table 1 lists a number of other minerals supposedly formed from alteration of enargite. Most of these reports consist of just brief statements, with no details of the specific textures, nor of the formation conditions. The report for the Burrus mine, Nevada, was included in the table, although it is not an explicit statement of enargite 4 See Table 6 for the formula of this and other minerals mentioned below. Author's personal copy 67 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 1 Reports of minerals supposedly formed from alteration of enargite (see Table 6 for formulae) Mineral Locality Arthurite Unspecified Bayldonite Brochantite Posnjakite Cornubite Description Source “secondary mineral in the oxidation zone of some copper deposits, formed by the alteration of arsenopyrite or enargite” Yellow-green crusts and globular aggregates (up to 1–2 mm) in Meleg Hill, Nadap, cavities of hydroquartzite (sic), as an alteration product of Velence Mountains, enargite, associated with plumbojarosite, cornubite, and other Hungary arsenates Summitville, “enargite is partly oxidized and is coated with fine-grained blue Colorado to green-blue Cu-sulfate minerals, including brochantite and posnjakite” Flohr et al. (1995) also report alteration of enargite to “a black sooty phase”. They list additional secondary minerals, including scorodite, chalchantite, and unidentified Cu-sulphates and Fe-arsenates. They also describe hinsdalite intergrown with and encrusting chalcocite in samples where enargite is also present. Lime Bluff Quarry, Blue-green crystalline cornubite, alteration of enargite, with green Lycoming County, Penn. conichalcite in white calcite Anthony et al. (2000) http://www. webmineral.com/data/ Cornubite.shtml Flohr et al. (1995) http://www. webmineral.com/data/ Cornubite.shtml “copper arsenates” Unspecified Enargite rimmed by (supergene?) chalcocite, extensively replaced by Ramdohr, 1980, copper arsenates Fig. 407 Juabite Centennial Eureka mine, Intimately mixed with enargite; believed to be formed by replacement of Roberts et al. (1997) Tintic district, Utah enargite by Te-bearing fluids. Other strictly associated minerals are the arsenates beudantite and arsenobismite. Samples collected from reworked dumps, “characterised by corroded enargite and diverse secondary hydrated arsenate assemblages” (Roberts et al., 1994). At the same locality, enargite is also found associated with other copper tellurates, and connellite (Roberts et al., 1995) Ceruleite Lammerite El Guanaco, Chile “many… secondary minerals… resulted from the oxidation of … enargite” (C. Lemanski, Lavendulan personal “Supergene processes have produced a variety of secondary arsenic Lemanskiite mineral after enargite” communication, 2004; Olivenite Other secondary Cu–As minerals at El Guanaco include arhbarite, Petersen et al., 1999) Scorodite conichalcite, and brochantite (www.mindat.org) Chalcophyllite Burrus Mine, Nevada Cited as being “in near-contact” with enargite A. Christy, personal Cornwallite Other secondary Cu–As minerals present at the same locality include communication, 2004 cyanotrichite, Ba-pharmacosiderite, olivenite, parnauite, scorodite, strashimirite. (www.mindat.org) Richelsdorfite alteration. With the exception of brochantite, cornubite, olivenite, posnjakite, and, possibly, of the “copper arsenates” cited by Ramdohr (1980), all other phases do not belong to the simple system Cu–As–S–O–H, and require the presence of additional components. Other literature information is limited to the lists of secondary minerals from deposits where enargite occurs as a primary mineral. These are summarised in Table 2. Examples for high sulphidation epithermal deposits include the previously mentioned Summitville, Burrus, El Guanaco, and the gold deposit at Furtei (Sardinia, Italy). Here, the primary (hypogene) assemblage contains abundant enargite, and the most widespread secondary arsenic mineral is scorodite. Ruggieri et al. (1997) and Da Pelo (1998) list a large number of oxidised secondary minerals (Table 2), but no specific association of any of these minerals and enargite is reported. Additional information was recently obtained by Musu (2007; see also Musu et al., 2007b), in a detailed study of the alteration processes of enargite in open pit exposures and waste dumps at Furtei. Cu–As oxidised minerals are not abundant, and are mostly represented by scorodite and cuprian iron sulphates (mainly melanterite). Arsenic is also bound to iron oxyhydroxides-sulphates, presumably as an adsorbed species. Eriochalcite and another unidentified copper chloride were occasionally observed. Most enargite alteration apparently occurs through dissolution, as testified by the appearance of etch pits at the surface of enargite crystals (Figs. 2 and 3). In some instances, textures suggestive of a reaction similar to (3) have been observed (Fig. 3). In porphyry-related deposits, enargite abundance ranges from practically nil to abundant. In these systems, enargite typically occurs in high level, late-stage veins, Author's personal copy 68 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 2 Selected alteration minerals reported in enargite-bearing deposits (see also text, and Table 6 for formulae) Locality Minerals Reference Furtei, Sardinia atacamite, azurite, boothite, chalcanthite, chrysocolla, cuprian iron sulphates (melanterite, rozenite, siderotile), cyanotrichite, eriochalcite, lavendulan, magnesioaubertite, mansfieldite, olivenite, unidentified copper chloride antlerite, arsenolite, azurite, brochantite, caledonite, chalcanthite, cornwallite, chrysocolla, cuprite, libethenite, linarite, malachite, pseudomalachite, tenorite, turquoise azurite, bellingerite, chenevixite, cuprite, libethenite, lindgrenite, malachite, metatorbernite, olivenite, paratacamite, pharmacosiderite, salesite, sampleite, scorodite, tenorite, turquoise, plus five Cu-bearing sulphates. antlerite, aurichalcite, bayldonite, beaverite, chalchantite, chenevixite, conichalcite, crednerite, cuprite, cuprogoslarite (=cuprian goslarite), linarite, olivenite, pisanite (=cuprian melanterite), scorodite, tenorite too numerous to be listed individually. There are at least 25 (including varieties) Cu-bearing arsenates, 13 Cu-bearing sulphates, 4 Cu-bearing carbonates, three copper silicates, and the oxides claudetite, cuprite, and tenorite (Ruggieri et al., 1997; Da Pelo, 1998; Musu et al., 2007b) Butte, Montana Chuquicamata, Chile East Tintic district, Utah Tsumeb, Namibia www.mindat.org Cook (1978) Morris and Lovering (1979) www.mindat.org reflecting the transition between the porphyry and the epithermal environment (see e.g. Muntean and Einaudi, 2001). Enargite was particularly abundant in the Butte, Montana, district. Here, an early, large, low-grade porphyry copper (±molybdenum) mineralisation was overprinted by late-stage, very rich polymetallic veins (e.g., the famous Anaconda vein). In the central part of the system, these veins were copper-rich, and the typical assemblage was chalcocite–djurleite–digenite–enargite. Extensive supergene oxidation produced a large variety of secondary minerals (Table 2). Davis et al. (1992), describing soils from this area, report the presence of enargite and tennantite, making up almost 85% of the total arsenic content in these soils. The authors state that “No alteration products were observed for the As-bearing phases, however, K-jarosite (KFe3(SO4)2(OH)6) was a ubiquitous precipitate around enargite and tennantite grains”. This occurrence implies a complex process of dissolution/transport/reprecipitation. Enargite is also well developed in late main-stage veins at Chuquicamata, Chile (Ossandon et al., 2001). Here too, supergene phenomena are extensive, and led to the development of chalcocite blankets, subsequently oxidised to an assemblage where the main phases are antlerite, brochantite, atacamite, chrysocolla, and “copper pitch” – a mixture of chrysocolla and limonite. Cook (1978) lists a large number of other Cu-oxidised minerals (Table 2). The oxidised blankets are underlain and Fig. 2. Scanning electron microscope (SEM) secondary electron (SE) image of the surface of an enargite crystal from Furtei (Sardinia, Italy), showing etch pits from dissolution. Fig. 3. SEM secondary electron image of scorodite (sc) aggregates on top of pyrite (py) and enargite (en) crystals. Notice etch pits on the enargite surface. Author's personal copy 69 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 3 Characteristics of samples, and experimental details, of laboratory studies on enargite Author(s) Mineralogy Chemistry (wt.% if not otherwise specified) Experimental setup Asbjornsson et al. (2004a) Natural enargite (provenance not given) Enargite formula from electron microprobe analysis Cu2.95As1.01Sb0.05S3.98 Balaz et al. (2000) Enargite concentrate from El Indio, Chile. Contains quartz (XRD) Cu/As atomic ratio 3.8 (suggests the presence of additional Cu-rich phases) Gold concentrate (El Indio, Chile), containing 42.8% pyrite, 40.7% enargite (particle size less than 75 μm) Castro and Hand sorted enargite from El Indio, Baltierra (2005) Chile. Reflected light microscopy and X-ray diffraction indicate the presence of 7% quartz and 0.9% pyrite. Cordova et al. Massive enargite from El (1997) Indio, Chile. Checked by X-ray diffraction (XRD) Cu 21.1, Fe 22.6, S 37.8, As 7.7 (42 g Au/tonne, 440 g Ag/tonne) Electrochemical study in 0.1 M HCl. Electrodes prepared as disks, either directly drilled from the mineral, or by mixing mineral powder with carbon paste in 1:1 mass ratio. Ball mill grinding in water; leaching for 120 min at 60°, 80° and 90 °C in alkaline medium (100 g l− 1 Na2S + 50 g l− 1 NaOH); solid to liquid ratio 1:400 Continuous biooxidation at 33 °C and pH 1.8 (kept constant by automatic addition of 5 N NaOH) with Acidithiobacillus ferrooxidans R2 Curreli et al. (1997) Gold-bearing sulphide concentrate (pyrite, chalcopyrite, enargite, tetrahedrite–tennantite, sphalerite, galena, hematite, quartz, and limestone) Au–Cu concentrate obtained by flotation of the sulphide ores from Serrenti-Furtei Cu 2.7; As 1.06; S 15.09; Sb 614 ppm Natural enargite from Furtei (Sardinia – Italy); contains luzonite, pyrite, quartz, kaolinite, and gypsum; natural enargite from Peru (BM1931,462); contains pyrargirite Synthetic enargite – contains 0.7% vol. CuS; natural samples from Butte (Montana) and Poopo (Bolivia) – contain minor quartz, pyrite, and covellite Not given Formulas of enargite from microprobe analysis Furtei: Cu3.01As0.95Sb0.02S4 BM1931,462:Cu3.04As1.01Sb0.06S4 Canales et al. (2002) Curreli et al. (2005) Da Pelo (1998) Dutrizac and Macdonald (1972, 1974) Ehrlich (1964) Cu 40.9, As 18.9 (presumably by weight: this corresponds to an atomic ratio Cu/As of about 2.5) Zeta potential measurements on ground sample in 0.001 M NaNO3 solutions; pH adjusted by either NaOH or HNO3 Not reported Electrochemical study at pH 0.5–13, obtained either by appropriate buffers – acetate buffer 0.5 M, pH 4.6; 0.1 M KH2PO4 + 0.1 M NaOH, pH 6.8; 0.05 M Na2B4O7, pH 9.2 – or by addition of either HClO4 or NaOH. Electrode prepared by grinding (SiC) and polishing (Al2O3) in argon atmosphere. Comparison between procedures for gold extraction by cyanidation and bioleaching plus cyanidation. Cu 31.15, As12.55, Fe 9.16 Leaching of concentrate in NaClO solutions at different concentrations, solid to liquid ratios, pH (10.5–12.5), and temperatures Polished sample kept for 28 days in a climatic chamber: 80 °C, 80% RH. XPS analyses on BM sample freshly cleaved and exposed to 3 M H2SO4 for 30' and 150'. Synthetic enargite formula (electron microprobe) Cu3AsS3.94; CuS contains 0.5% As. Not given for natural samples. Polished pellets leached in 0.1 M H2 SO4, 0.1 M Fe+ 3. Temperature 60°– 95 °C; at 90 °C experiments at different FeSO4 saturations. Cu 38.2, As 7.2 (atomic ratio Cu/As 6.25) Treatment with solution at pH = 3.5, containing 3 g/l (NH4)2SO4, 0.1 g/l KCl, 0.5 g/l K2HPO4, 0.5 g/l MgSO4.7H2O, 0.01 g/l Ca(NO3)2, with and without bacteria (Acidithiobacillus-Ferrobacillus) (continued on next page) Author's personal copy 70 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 3 (continued) Author(s) Mineralogy Chemistry (wt.% if not otherwise specified) Elsener et al. (2007) Enargite from San Genaro (Peru) Semiquantitative SEM/EDS analysis consistent with pure enargite Natural enargite (location not given). Contains arsenopyrite and chalcocite (XRD). Size fractions 147–104 μm: Cu 15.82; As 10.5; Fe 15.5. 104–53 μm: Cu 28.15; As 13.12; Fe 12.2. 53–38 μm: Cu 30.07; As 17.2; Fe 15.0. Escobar et al. (1997) Escobar et al. (2000) Fantauzzi (2001) Fantauzzi et al. (2004) Fantauzzi et al. (2006) Fantauzzi et al. (in press) Fornasiero et al. (2001) Fullston et al. (1999a) Fullston et al. (1999b) Fullston et al. (1999c) Gajam and Raghavan (1983) Experimental setup Electrochemical and XPS study after exposure to FeCl3 and Fe2(SO4)3 solutions ([Fe] = 0.025 M, pH = 1.7). Natural enargite (location not given) – Cu 46.2, As 16.3, Fe 0.55 (from these Batch chemical leaching and bioleaching (Acidithiobacillus ferroxidans) at 30 °C in contains minor chalcopyrite and quartz. data one can estimate that the 0.4(NH4)2SO4, 0.4MgSO4•7H2O, Particle size range of material used sample might contain about 0.056KH2PO4 g/l acidified with sulphuric 90% enargite, and 5% chalcopyrite) for the experiment −147 + 104 μm. acid to pH 1.6. Without iron or with 3.0 g/l Fe3 + added as ferric sulphate. Solid/solution ratio 5 g/250 ml. See Escobar et al. (1997) See Escobar et al. (1997) Batch chemical leaching and bioleaching (Sulfolobus BC) at 70 °C in 0.4(NH4)2SO4, 0.4MgSO4•7H2O, 0.056KH2PO4 g/l acidified with sulphuric acid to pH 1.6. Without iron or with 1.0 g/l Fe3+ added as ferric sulphate. Solid/solution ratio 2 g/250 ml. Natural enargite from Peru Peru: Cu 45.9; As 17.3; S 36.7. XPS analyses on 1) “as received” minerals; and from Furtei. Furtei: Cu 43.1; As 19.4; S 37.3 2) freshly cleaved minerals surfaces; 3) cleaved enargite samples from Furtei immersed in aqueous solutions at pH 1, 4 and 7; 4) cleaved samples from Peru immersed in aqueous solutions at pH 1, 4, 7 and 13. Enargite from Furtei Not given XPS measurements on natural sample and synthetic enargite “as received” and on powdered synthetic material Natural enargite from Furtei Same material as in Fantauzzi et al. XPS and XAES characterization of and Peru, and synthetic enargite (2004), Elsener et al. (2007), samples «as received» and/or powdered and Rossi et al. (2001) and/or freshly cleaved Natural enargite from Same material as Elsener et al. XPS and XAES characterization San Genaro (Peru) (2007) of samples subjected to the electrochemical studies by Elsener et al. (2007) See next item See next item Selective oxidation of mineral mixtures (enargite–chalcocite, enargite–covellite, enargite–chalcopyrite, tennantite– chalcocite, tennantite–covellite and tennantite–chalcopyrite, all in 1:1 weight ratio). Samples grain size:16 μm (d50). DEDTP N 95% pure was added to the mineral slurry for flotation at a concentration of 2 × 10− 5 mol dm− 3 Exposure to 0.01 M KNO3 solution at Wet chemical analysis of enargite: Synthetic enargite; natural Cu 57.2; As 11.7; S 28.3; Sb 0.29; enargite (sampling site pH = 11 in a nitrogen, oxygen, and Fe 3.0; Pb 0.02; Zn 0.2; Ag 0.03 unknown) – contains small oxygen + H2O2 environment. amounts of bornite and Zeta potentials were derived from chalcocite. electrophoretic measurements at pH 11 to 5 and backwards. See Fullston et al. (1999a) See Fullston et al. (1999a)) XPS analysis of the same material with the same treatment of the previous item See Fullston et al. (1999a) See Fullston et al. (1999a) See Fullston et al. (1999a) Leaching experiment (batch reactor) in ammonia solutions (0.1522 to 0.6088 M), oxygen pressures 5 to 50 psi, temperature 30° to 82 °C Author's personal copy 71 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 3 (continued) Author(s) Mineralogy Chemistry (wt.% if not otherwise specified) Experimental setup Natural enargite: Cu 47.4, As 18.94, Potential applied to an enargite S 32.3, plus minor amounts of Fe, Pb, electrode in potassium amyl xanthate (PAX) solutions at pH 10 and 7 after Zn, Ni and Si. conditioning for 10 min. See previous item Voltammetric studies, contact angle Guo and Yen Synthetic enargite and natural measurements, collector and collectorless (2005) enargite from Maria Elana microflotation tests in the same Mine (Chile); mixed with experimental setup of Guo and Yen (2002). synthetic and natural chalcopyrite. Cu 40.8, As 18.2 Leaching of 2.1 g of sample in 1200 ml Herreros et al. Enargite with minor quartz and of “in situ generated” chlorine solutions, (2002) gersdorffite from El Indio mine (Chile) at different. chlorine concentrations, temperatures,and particle sizes. Kantar (2002) Natural enargite from Butte Not given Flotation characteristics investigated in a (Montana) microflotation cell – solution potential controlled with H2O2 and Na2S. Not given Leaching of samples in 0.05N sulphuric Koch and Pure selected enargite, ore acid, in 0.05N sulphuric acid Grasselly mixture of pyrite and enargite +0.2236 g/l Fe, and in 0.05N (1952) and pure selected pyrite from sulphuric acid +0.2847 g/l Cu. Recsk mineralization (Hungary) Cu 26.25, As 10.34, S 19.48, Oxidative roasting in an electric Mihajlovic et al. Natural enargite from Bor resistance furnace at T = 400–800 °C Fe 1.62, Al2O3 3.18, SiO2 38.12; (2007) copper mine (Serbia and Leaching kinetic experiments with NaClO Montenegro), deposit H. traces Ba, Mn, Sb, Ge, Pb, Sn, Ti, 0.3 M solution with 5 g/L NaOH Ca, V, Zn Contains quartz (XRD). (pH = 12), at T = 25–60 °C. Solution volume 800 mL with 0.5 g solid (ground at − 100 μm). Cu 46, S 31, As 17, Sb 1.8 Biooxidation kinetics of Chilean Muñoz et al. I) Fragments of an enargite concentrate II) by Acidithiobacillus. (2006) single crystal with quartz ferrooxidans (33 °C) and Sulpholobus inclusions from Huancavelica, metallicus (68 °C). Initial pH and Peru. Eh 1.8 and 500 mV respectively. Not reported Cyclic voltammetry and polarization II) Sulphide concentrate, curves on resin embedded massive El Indio Chile, containing 16% enargite electrodes. Previous enargite, 38% pyrite, 11% grey biotreatment with mesophiles copper (Cu12As4S13), 11% (A ferroxidans, A. thiooxidans and chalcopyrite, traces of chalcocite, Leptospirillum ferroxidans, 35 °C, covellite and bornite and 23% gangue. initial pH = 2.0), and thermophiles Enargite crystals of high purity, (Sulpholobus sp. at 68 °C, initial pH = 2.0) El Indio Chile. Musu (2007) Natural enargite from Furtei, Sardinia Semiquantitative SEM/EDS Dissolution experiments on cleavage (same bulk sample as in Da Pelo, 1998) analyses consistent with the (presumably, {110}) faces in flow-through composition given by Checked by XRD and SEM/EDS – reactor. Input solutions open to exchange Da Pelo (1998) minimal pyrite present with atmosphere. Initial pH 1 and 4 (HCl). Nakai et al. (1978) Natural enargite Not given XPS spectra (Cu) of powdered material Pauporté and Enargite from El Indio, Chile Not reported Electrochemical study in 0.25 M KNO3 Schuhmann (no details given) solution, buffered with 0.05 M Na2B4O7. (1996) 10H2O (pH = 9.3) Padilla et al. Enargite sample from El Indio Mine Three size fractions of the sample: Treatment in H2SO4/NaCl solutions with (2005) (Chile). Sample is reported to contain 75–63 μm: As 15 bubbling oxygen at different stirring enargite (84.1 wt.%) pyrite (9.6 wt.%), 63–53 μm: As 15.6 speeds, oxygen flow rates, sulphuric 53–45 μm: As 15.3 gangue minerals (5.8 wt.%), acid and chloride concentrations, and chalcopyrite (0.45 wt.%), tetrahedrite temperatures (80 to 100 °C). (0.08 wt.%), molibdenite (0.07 wt.%) Pratt (2002) Natural enargite from Enargite formula XPS analyses on enargite surface exposed Cerro de Pasco, Peru. (from microprobe analysis) by fracturing in UHV and on enargite Cu3As0.9Sb0.1S4 surfaces exposed to air for 2 min. Comparison with SRXPS measurements. Guo and Yen (2002) Synthetic enargite and natural enargite from Maria Elana Mine (Chile). (continued on next page) Author's personal copy 72 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 3 (continued) Author(s) Mineralogy Chemistry (wt.% if not otherwise specified) Experimental setup Pratt (2004) See previous item See previous item Rossi et al. (2001) Synthetic enargite, natural enargite from Perù (42977 and BM1931,462) and natural enargite from Furtei. Velasquez et al. (2002) Natural enargite from El Salvador, Chile. 42977: Cu 47.7; As18.1; Sb 0.8; S 33; BM1931,462: Cu 48.1; As 18.8; Sb 2; S 32. EnF: Cu 48.86; As 18.27; Sb 0.54; S 32.79. Enargite formula (XPS): Cu3As0.7S4.2 Velazquez et al. (2000a) Enargite from El Indio Mine, Chile with minor contamination of quartz. Atomic concentration (%) of fractured enargite measured by XPS: Cu 23.39; As 7.75; S 33.05 corresponding to enargite formula of Cu2.92As0.97S4.12 Velasquez et al. (2000b) Enargite from Chañaral (Chile) + minor quartz Enargite formula by SEM/EDX Cu3AsS3.5 Viñals et al. (2003) Natural enargite from Huancavélica, Peru Cu 46; As17; Sb 1.8; S 31 Welham (2001) Enargite – additional recognized minerals: quartz, chalcopyrite, tennantite Cu 41.5, As 15.5 (Cu/As atomic ratio 3.16) Comparison between SRXPS and XPS measurements on enargite surface exposed by fracturing in UHV. XPS analyses on synthetic enargite as powder on tape and on natural enargite as powder on tape, as crystal as received and as sputtered with 3 keV Ar+ ions for 30 s and on freshly cleaved surface. XPS analyses on 1) a surface after fracture in Ar atmosphere; 2) a polished (SiC + alumina wet polishing) surface; 3) the fractured surface after 28 min of etching with 4 keV Ar+ ions. Cyclic voltammetry on a polished electrode of natural enargite with an electrolyte solution of disodium tetraborate decahydrate at pH 9.2 at room temperature. XPS analysis on enargite electrode surface at different applied potentials. Electrochemical experiment (by cyclic voltammetry and electrochemical impedance spectroscopy) on wet-abraded and polished surface: area exposed 0.2 cm2; electrolyte: 0.05 M borax solution (I = 0.2; pH = 9.2) Leaching of enargite in NaOCl solution for ranges of 20–60 °C, 0.07– 0.47 M ClO− and 0.003–0.03 M OH−. XPS measurements on original surface and on leached enargite after 1 s at pH 12.5, 0.34 M ClO−, 20 °C. Ball mill grinding in air, argon or oxygen; leaching in 0.5 M HCl for 24 h. overlain by leaching zones, where the acidity generated by pyrite oxidation removed most copper. This copper was redeposited to form, some kilometres away, a very large (300 million tonnes) secondary deposit (Exotica), where the main assemblage includes atacamite, chrysocholla, “copper pitch”, and “copper wad” – a mixture of copper and manganese oxyhydroxides. This sequence of events: supergene replacement of primary copper minerals by Cu-rich sulphides, “in situ” oxidation of these to oxidised copper minerals, and/or acid leaching with removal and transport of much copper, is documented for several porphyry deposits, e.g. in Northern Chile (El Salvador, Mote et al., 2001; Escondida, Padilla Garza et al., 2001). Enargite is also an important copper mineral in some vein-replacement hydrothermal deposits in carbonate rocks. Unlike the above mentioned deposit types, in these cases, the country rocks should buffer the weathering environment at circumneutral to mildly alkaline pH. Examples of this deposit type include the Tintic district in Utah (cf. Table 1), and the peculiar Tsumeb, Namibia, deposit. From the descriptions by Morris and Lovering (1979) of the East Tintic ore bodies, it appears that the most common Cu-oxidised minerals are the carbonates azurite and malachite. Other minerals reported by the same authors are listed in Table 2. At Tsumeb, the combination of an unusual geochemical association (major Pb–Zn–Cu with economic As–Ag– Ge–Cd, and subordinate Ga–Sn–W–Mo–Co–Ni–Sb– Hg–V) and an extensive supergene oxidation of the primary ore produced an astounding diversity of mineral species, including very rare or unique findings (see e.g. Wilson, 1977; Gebhard, 1999). Keller (1977) believes that the first alteration minerals formed directly from Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 primary sulphides are the arsenates arsentsumebite, beudantite, carminite, and scorodite (plus the lead sulphate anglesite). Pinch and Wilson (1977) report that enargite crystals “sometimes have an alteration coating of other [unspecified] minerals”. Enargite is locally replaced by tennantite (Cu12As4S13), and the descriptions of secondary Cu–As minerals typically refer to association with tennantite rather than enargite (e.g. Pinch and Wilson, 1977; Gebhard, 1999, p. 286; http://www. mindat.org/picshow.php?id=14579). As detailed below, elemental sulphur is commonly formed during laboratory oxidation of enargite at low pH. Native sulphur is not rare in enargite-bearing deposits, including many of those mentioned above (see e.g. Table 3 in Arribas, 1995), but seldom is it a major phase. Moreover, in some cases, it is thought to be a latestage hydrothermal mineral and not of supergene origin (e.g., Sillitoe and Lorson, 1994). To our knowledge, there has been no claim that native sulphur is anywhere formed in nature from supergene oxidation of enargite. 5. Laboratory experiments Laboratory studies of enargite oxidation were scarce until the late 1990s; in recent years, the literature on the subject has been rapidly growing, mostly with reference to ore processing (cf. Senior et al., 2006, and references therein). A limitation of several studies, especially the early ones, is that the purity of the enargite studied has not been demonstrated by any reliable micro-scale methods. As shown by Dutrizac and Macdonald (1972), minor impurities in natural enargites may result in significantly different apparent dissolution rates. Table 3 reviews the available information on enargites used in the studies described below, whereas Table 4 reports the solid products observed during these studies. Essentially, we can consider three different environments: oxidation by air; oxidation in acidic aqueous solutions; and oxidation in neutral to alkaline aqueous solutions. 5.1. Oxidation in air There are very few systematic studies of enargite oxidation in air; additional evidence arises from scattered observations in the course of other experiments. Like most sulphides, enargite is decomposed by roasting, i.e. reaction with air at high temperatures (e.g., Mihajlovic et al., 2007). However, at room temperature bulk oxidation of enargite in air is apparently a rather inconspicuous process. Indeed, museum specimens keep a brilliant metallic luster on crystal surfaces even 73 after many years. Padilla et al. (2005) suggested that a quick dissolution step observed during their solution experiments (described in more detail below) of natural enargite should be ascribed to the presence of a soluble weathering product, possibly arsenolite (As2O3). Da Pelo (1998) kept a polished slab of natural enargite from Furtei (Sardinia) for 28 days in a climatic chamber where temperature was 85 °C and relative humidity 80%. In these rather severe conditions, the only remarkable change was a tarnishing of the mineral surface. However, along cracks and in cavities, minimal amounts of a green phase were observed. From the X-ray diffraction (Gandolfi camera) pattern, this phase turned out to be a copper sulphate equivalent to the mineral antlerite. As mentioned above, antlerite occurs in several enargitecontaining deposits. More subtle changes are revealed by X-ray photoelectron and Auger spectroscopies. With these techniques, analyzing the surface of a natural museum specimen “as received”, i.e. exposed to the exogenous environment for an unspecified time (presumably years), Rossi et al. (2001) and Fantauzzi (2001) observed changes at the enargite surface. Specifically, by application of Rossi and Elsener's (1992) algorithm for quantitative XPS analysis, they demonstrated that the surface of such crystals is covered with an oxidised layer about 0.5 nm thick, with a remarkably different composition (enriched in arsenic) from the bulk crystal underneath. This oxidised layer is likely to affect the interaction of enargite with the environment. In the material studied by Rossi et al. (2001), the binding energies of Cu2p and As3d suggest that, in the oxidation layer, Cu may be present in both + 1 and +2 states, with establishment of Cu(II)–O bonds (as shown by the appearance of the “shake up” bands, that are not present in the signal of Cu(II) bound to sulphur – Chawla et al., 1992). In this layer, As is also bound (at least partly) to oxygen, and not to sulphur. Evidence of partial oxidation of sulphur to sulphate was provided by the appearance of a peak at 168.7 eV in the S2p signal. This S2p signal at 168.7 eV, assigned to sulphates, was not observed by Fantauzzi et al. (2004) in an “as received” natural enargite from Furtei, Sardinia, collected few months before analysis, neither was it seen by Viñals et al. (2003) in untreated samples from Huancavelica, Peru (previous history unknown). However, the latter authors found evidence of As–O bonds (by the presence of a doublet in the As3d signal at 44.2–44.9 eV) and of polysulphide species (doublet at 163.2–164.4 eV in the S2p signal). Hence, they concluded that “natural enargite has an external layer… which contains a copper(I), arsenicdeficient sulfoarsenide with monosulfide and polysulfide components… Part of As… is oxidized to As2O3”. Author's personal copy 74 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 4 Summary of solid phases detected or inferred in laboratory studies of enargite oxidation (does not include hypothetical phases not known to occur as bulk stable phases) Phase Identification Nature of the experiment Reference As2O3 Positive (XRD) Welham (2001) Inferred (XPS) Inferred (XPS) Ball milling (dry in oxygen atmosphere; wet in air) of an enargite-rich mixture (minor phases: quartz, chalcopyrite, tennantite) Untreated surface of natural enargite Conditioning at pH = 11 of a mixture of enargite, bornite, and chalcocite Inferred (EIS, XPS) Natural enargite, electrochemically oxidised at +742 mV vs. SHE, pH = 9.2 Inferred (XPS, electrophoresis) Positive (XRD) Electrophoresis (pH 11 to 5 and backwards), and conditioning at pH = 11 of a mixture of enargite, bornite, and chalcocite Oxidation in climatic chamber (28 days, 85 °C, 80% humidity) of natural enargite with minor quartz and pyrite Electrochemical oxidation of natural enargite at applied potential between 0.54 and 0.74 V vs. SHE, pH = 9.2 Cu(OH)2 Cu3(SO4) (OH)4 Cu3(AsO4)2 CuO CuS CuSO4.5H2O S° Viñals et al. (2003) Fullston et al. (1999a,b,c) (Velazquez et al., 2000a,b) Fullston et al. (1999a,b,c) Da Pelo (1998) Inferred (electrochemical behaviour) Inferred (electrochemical behaviour) Inferred (EIS, XPS) Electrochemical oxidation of natural enargite in aqueous solutions, 4.6 b pH b 11 Cordova et al. (1997) Electrochemical oxidation at +742 mV vs. SHE of natural enargite, pH = 9.2 Positive (SEM, XRD) Treatment with hypochlorite solutions at pH 12.5 (NaOH) of natural enargite Positive (XRD) Positive (XRD) Positive (XRD) Positive (Soxhlet extraction) Positive (SEM) Positive (Raman) Alkaline leaching of a previously ball-milled enargite-rich mixture Acid leaching of a previously ball-milled enargite-rich mixture Ball milling (dry in oxygen atmosphere; wet in air) of an enargite-rich mixture Leaching with sulphuric acid/ferric sulphate of synthetic and natural enargite Enargite oxidation at pH with Cl2/Cl− solutions Electrochemical oxidation at pH = 1 (HCl) of natural enargite Inferred (XPS) Surface of natural and synthetic enargite ground in air (Velazquez et al., 2000a,b) Viñals et al. (2003) Balaz et al. (2000) Welham (2001) Welham (2001) Dutrizac and Macdonald (1972) Herreros et al. (2002) Asbjornsson et al. (2004a,b) Rossi et al. (2001) Pratt (2002) noted that exposure of the enargite surface to air for two minutes results in the complete disappearance of a low-binding energy (∼42 eV) component in the As3d signal. He suggested that this is due to the establishment of As–O bonds with the surface As atoms. Fantauzzi et al. (2006), on consideration of both photoelectron and Auger peaks, concluded that the α′ parameter (that takes into account the energies of both the photoelectron and the Auger electron) of sulphur is the most sensitive indication of chemical state change at the enargite surface upon exposure to air. Mechanical activation (grinding) strongly speeds up oxidation in air. Thus, Balaz et al. (2000) demonstrated that grinding in a stirring ball mill increased enargite reactivity significantly with respect to alkaline (sodium sulphide) leaching (see also Balaz and Achimovicova, 2006). Specifically, alkaline leaching of the activated material induces a substantial removal of As. Indeed, Xray diffraction shows that covellite is the dominant Cuphase in the residual material. Welham (2001) attempted Cordova et al. (1997) oxidation of an enargite-rich concentrate by ball milling. 10 hours of dry milling in argon or air produced little effect, whereas dry milling in an oxygen atmosphere, or wet milling in air, resulted in significant oxidation of the starting mixture. In agreement with thermodynamic calculations, As2O3, CuSO4.5H2O, and SO2 were observed as oxidation products. They should result from reactions 4Cu3 AsS4 þ 7O2 →12CuS þ 4SO2 þ 2As2 O3 ð5Þ CuS þ 2O2 →CuSO4 ð6Þ The presence of CuS, indicating incomplete progress of reaction (6), was also detected (cf. Balaz et al., 2000). Finally, enargite crystals powdered in an agate mortar in air have been analysed by XPS/XAES by Fantauzzi (2005). Both arsenic As3d and sulfur S2p spectra show a higher binding energy component, assigned to oxidised arsenic and to sulphate, respectively. Despite the fact that they exhibit the same Auger parameter α′ as freshly Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 cleaved enargite, binding energies and X-ray excited Auger lines of arsenic and sulfur are shifted by 0.7 eV due to grinding effects. 5.2. Reaction with pure water To our knowledge, there are very few studies of interaction between enargite and pure water. Da Pelo (1998) and Fantauzzi (2001) found no change in any photoelectron peak (As3d, Cu2p, S2p) at the enargite surface after exposure to pure water for 210 minutes to several hours. Fantauzzi (2005) and Elsener et al. (2007) confirmed that the peak positions and chemical state of As3d, Cu2p and S2p did not change after 24 h immersion in aerated distilled water (pH 6). They report a surface composition depleted in copper and enriched in sulfur. Musu (2007) kept a fragment of natural enargite immersed in pure (milli-Q) water for 24 hours, and could not detect (by ICP-AES) any copper or arsenic in the liquid phase. On the other hand, working with a larger exposed surface (about half cm2), Elsener et al. (2007) in the same conditions found 0.6 μg/l of copper, but no detectable (by AAS) arsenic. From these data, one can calculate an overall average bulk dissolution rate in the order of 10− 10 mol m− 2s− 1. 5.3. Reaction with acidic solutions Like most sulphides, enargite should be intrinsically unstable in acidic environments. For instance, Davis et al. (1992) state that “Simulation [by the MINTEQA2 code] of enargite dissolution under oxidizing conditions of pH 2.0, Eh + 200 mV, and 0.01 M Cl− …. indicates that this mineral phase is nearly infinitely soluble (4.7 × l05 mg/L As dissolved at equilibrium)”. However, as detailed below, reaction rates are extremely slow, and significant oxidation/dissolution typically occurs only in the presence of a strong oxidant. Early studies of enargite reactivity involved treatment with acid solutions containing ferric iron. They were conceived for hydrometallurgical applications, but they have implications for acid mine drainage environments where acidity and ferric iron typically arise from oxidation of pyrite and/or other iron-bearing sulphides. The first studies (as quoted by Dutrizac and Macdonald, 1972, 1974) were conducted by Sullivan (1933), and Brown and Sullivan (1934). By treating natural enargite with sulphuric acid and acidified ferric sulphate solutions, they established two important points that were confirmed by all subsequent studies: enargite is comparatively resistant to oxidation (only 3% copper was extracted after 146 days of treatment with 5% ferric 75 sulphate solution at 25 °C – compare e.g. the remarkably faster dissolution rates of other copper sulphides – Dutrizac and Macdonald, 1974); the presence of ferric iron is of major importance (negligible amounts of copper were dissolved by sulphuric acid alone). Subsequent studies by Koch and Grasselly (1952) and Ehrlich (1964) further investigated the behaviour of enargite ores exposed to acidic ferric sulphate solutions. Working with pyrite-rich enargite ores, Koch and Grasselly (1952) observed that the slow enargite reaction is slightly accelerated by the presence of pyrite. They postulated an overall reaction 4Cu3 AsS4 þ 35O2 þ 10H2 O→12CuSO4 þ 4H3 AsO4 þ 4H2 SO4 ð7Þ but did not provide any specific evidence for the presence of any of these products, nor they specified how does the presence of pyrite affect this reaction. An important contribution (reported in detail below) by Ehrlich (1964) was the consideration of the influence of bacterial action. The first complete systematic study was carried out by Dutrizac and Macdonald (1972; summarised in Dutrizac and Macdonald, 1974). They treated both synthetic and natural enargite at various temperatures (60°–95 °C) with sulphuric acid/ferric sulphate solutions of variable concentrations. They could ascertain the presence of elemental sulphur as the only solid reaction product, and therefore suggested a reaction of the type Cu3 AsS4 þ 11Fe3þ þ 4H2 O→3Cu2þ þ AsO3− 4 þ 4S- þ 8Hþ þ 11Fe2þ ð8Þ where 5–50% of produced elemental sulphur is subsequently oxidised to sulphate. In agreement with Eq. (8), the copper/arsenic ratio in solution is close to that required by enargite stoichiometry. In the specific conditions, dissolution rates5 are in the order of 10− 6–10− 7 mol m− 2s− 1; rates are strongly dependent (apparent reaction order = 0.55) on Fe3 + concentration up to 0.2 M Fe, and to a lesser extent on H+ (order = 0.20). Based on the temperature dependence of dissolution rates, they could calculate an activation energy of 13.3 Kcal/mol (55.6 KJ/mol); the authors believe that this value suggest a surface-controlled reaction. In the next two decades, there were few further studies of enargite oxidation, and these were addressed to the development of specific process techniques (e.g., Gajam and Raghavan, 1983). Since the mid 1990s, a new wave of enargite oxidation studies started, mostly based on 5 Calculated from graphic interpolation of data in their Fig. 1. Author's personal copy 76 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 attempts at characterising surface reactions by a variety of techniques. Although the majority of these studies (described in a following section) were focused on alkaline environments, of interest to flotation and metallurgical processes, some studies did consider the behaviour of enargite in acidic solutions, especially in recent years. Thus, the electrochemical study by Cordova et al. (1997), although carried out in more detail at alkaline pH, includes data obtained at acidic pH (see later description). Da Pelo (1998) exposed to 3 M H2SO4 solutions for 30 and 150 minutes a freshly cleaved enargite surface (fractured in nitrogen atmosphere). She could not detect any change in the XPS signals of Cu and As; only in the sample treated for 150' a change was observed in the S2p peak, with appearance of a second component at 168 eV, referable to sulphate sulphur. Asbjornsson et al. (2004a) performed an electrochemical study of enargite in 0.1 M HCl solution. The open circuit potential EOCP is reported as ∼0.44 V vs. SHE, consistent with the results of other studies (Table 5), and comparable with the values of other copper sulphides (Arce and Gonzalez, 2002). Under applied potential, the increasing currents at N 0.36 V are referred to copper dissolution through reactions such as Cu3 AsS4 þ 19H2 O→3Cu2þ þ H2 AsO−4 þ 4HSO−4 þ 31Hþ þ 35e− 6 ð9Þ However, the active–passive transition associated with the current peak at ∼0.56 V (similar to that found in other studies, e.g. Cordova et al., 1997) is referred to the formation of elemental sulphur: Cu3 AsS4 þ 4H2 O→3Cu2þ þ H2 AsO−4 þ 4S þ 6Hþ þ 11e− ð10Þ The formation of elemental sulphur was confirmed by in situ Raman spectroscopy. XPS analyses show that at applied potential N 0.4 V vs. SHE there is a progressive formation of Cu(II), sulphate and As–O species at the surface. The binding energies for As are mostly indicative of As(III), however with increasing applied potential a weak contribution from As(V) is possible. Although Eqs. (9) and (10) suggest dissolution of copper and arsenic in stoichiometric proportions, measured (ICP-AES) arsenic/copper ratios in solution were lower than required by stoichiometry. 6 The reaction is reported as written in the original paper, but it is not balanced. Herreros et al. (2002) and Padilla et al. (2005) studied the oxidation of enargite in acidic media under strongly oxidising conditions (respectively, in Cl2/HCl solution, and with bubbling oxygen in H2SO4/NaCl solutions). In the first case, after a first fast reaction step, the formation of a surface sulphur layer strongly slows down the reaction. In the second study, elemental sulphur does form, presumably through the reaction 2Cu3 AsS4 þ 6Hþ þ 5:5O2 →6Cu2þ þ 2AsO3− 4 þ 8S þ 3H2 O 7 ð11Þ but apparently does not prevent further reaction. The authors developed a kinetic model whereby the dissolution process is surface-controlled, with an activation energy of 65 KJ/mol (comparable to the value found by Dutrizac and Macdonald, 1972). Elsener et al. (2007) and Fantauzzi et al. (2006, in press) carried out new studies on the reactivity of enargite in acidic solutions with 0.025 M Fe3 +. After 24 hours of exposure in these oxidising solutions, they could not observe any change in the XPS signals of Cu and As, nor in the corresponding modified Auger parameters α′. The only detectable change was an increase of the 163.4 eV component of the S2p signal, which could be attributed to a strongly copper-deficient sulphide layer. The layer thickness, evaluated by quantitative XPS analyses of the reacted surfaces according to the three-layer model (Rossi and Elsener, 1992) was found to be about 1 nm (approximately mono-layer thickness). Atomic absorption spectroscopy analysis of the reacted solutions showed the presence of Cu, whereas the amount of dissolved As was below the detection limit. Based on the copper concentration in solution, the volume of dissolved enargite was calculated to have a thickness of 60–130 nm. The material beneath the polysulphide film (5–10 nm) showed a depletion in Cu, no changes for As, and a slight enrichment in sulphur. A model based on the dissolution of homogeneous binary metal alloys has been proposed by Elsener et al. (2007) to describe the steady state enargite dissolution. The oxidative dissolution of enargite under open circuit conditions can be described by a first step in which copper and arsenic dissolve into solution, forming at the surface a thin metal-deficient layer. Enargite beneath this layer is slightly depleted in copper and enriched in sulphur. Once this layered interface has been formed, the dissolution of enargite continues stoichiometrically: this layered interface 7 At pH b 2, the dominant arsenate species in solution is H3AsO4. Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 77 Table 5 Summary of open circuit potential (OCP) measurements on enargite. SCE and SHE are, respectively, the standard calomel electrode and the standard hydrogen electrode Ref. OCP Pauporté and Schuhmann (1996) With O2 at 100 mbar Experimental setup (see also Table 6) Mechanically cleaned electrodes rinsed with tri-distilled water. Upon addition of Ethylxanthate 10− 4 M OCP vs OCP vs OCP vs OCP vs Buffer solution pH 9.2 0.05 M Na2B4O7⁎10H20 + 0.025 M K2SO4 as SCE (V) SHE (V) SCE (V) SHE (V) supporting electrolyte. OCP measured after 15 min on three different electrodes. +0.031 0.275 − 0.036 0.208 Electrode 1 +0.036 0.280 − 0.028 0.216 Electrode 2 +0.034 0.278 − 0.042 0.202 Electrode 3 Velazquez et al. OCP vs SCE OCP vs SHE Wet abraded and polished surfaces, rinsed with deoxygenated deionized waters; − 0.1 +0.14 (2000) buffer solution pH 9.2 0.05 M Na2B4O7⁎10H20; measurement times not reported. Cordova et al. Aerated solutions Argon atmosphere Wet abraded surface rinsed with distilled, deoxygenated water. Constant OCP values (1997) OCP vs OCP vs OCP vs OCP vs attained in about 2 hrs (no significant variations over 24 hrs) SCE (V) SHE (V) SCE (V) SHE (V) 0.2 0.44 − 0.04 0.20 pH 0.5 (obtained with HClO4 addition) 0.16 0.40 − 0.04 0.20 pH 1.25 (ditto) 0.03 0.27 − 0.04 0.20 pH 3.25 (ditto) 0.0 0.24 − 0.04 0.20 pH 4.6 (buffer 0.5MCH3COOH–0.5MCH3COONa) 0.0 0.24 − 0.04 0.20 pH 6.8 (buffer 0.1 M KH2PO4–0.1 M NaOH) 0.0 0.24 − 0.04 0.20 pH 9.2 (0.05 M Na2B4O7) − 0.04 0.20 − 0.04 0.20 pH 11 (by addition of NaOH) − 0.2 0.04 − 0.2 0.04 pH 14(ditto) Asbjornsson et al. OCP vs SCE (V) OCP vs SHE (V) Rotating electrode. (2004a,b) 0.2 0.4(44) HCl 0.1 M solution Measurement times not reported OCP vs SHE (V) Aerated solutions; freshly cleaved surfaces. (Fantauzzi, 2005; OCP vs SCE (V) OCP constant after 15 min (measured up to 24 h) Elsener et al., 0.480 0.724 FeCl3 [Fe3+] = 0.025 M pH 1.87 2007) 0.480 0.724 Fe2(SO4)3 [Fe3+] = 0.025 M pH 2.04 0.200 0.724 H2O pH = 6 0 0.244 H2SO4 pH = 4 remains unchanged (steady state) on top of the dissolving surface of enargite, and it is controlling the kinetics of the dissolution reaction. EOCP measured under these conditions is about 0.71–0.73 V vs. SHE. This is close to the standard potential for the couple Fe+ 2/Fe+ 3 (0.68 V), i.e. in these conditions this redox couple strongly polarizes the enargite surface towards positive potentials. A surface reaction scheme consistent with these results should be similar to reaction (8), with polysulphide instead of elemental sulphur. Finally, Musu (2007) carried out dissolution experiments on cleavage faces of single crystals at acidic pH (1 and 4, HCl) in a flow-through cell open to exchange with the atmosphere. Although the study should be considered of exploratory nature, a number of points appear established: dissolution rates are consistently lower than determined in the presence of Fe3 + (e.g., at 75 °C, pH = 1, the rate based on Cu2 + concentration is at least two orders of magnitude lower than the results of Dutrizac and Macdonald, 1972, at 70 °C); the release of Cu is faster than that of As (measured Cu/As molar ratios in effluent solutions are systematically higher than 3). 5.4. Reactions with alkaline solutions As noted above, since the mid 1990s the majority of enargite dissolution studies have been carried out in alkaline environments. Most of these studies include the use of techniques that give information on surface processes, either indirectly (e.g., electrochemistry), or directly (e.g., X-ray photoelectron spectroscopy). Pauporté and Schuhmann (1996) carried out the first systematic electrochemical study of enargite, working with natural crystals at pH = 9.3. EOCP was about +0.28 V vs. SHE in presence of bubbling oxygen, and about +0.21 V in presence of xanthate.8 A detailed electric impedance spectroscopic (EIS) study was performed in 8 For the behaviour of enargite in the presence of xanthate, see also Guo and Yen (2002, 2005), Kantar (2002), and references therein. Author's personal copy 78 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 Table 6 Formulae of minerals cited in Tables 1 and 2 antlerite arhbarite arsenobismite arsenolite arsentsumebite arthurite atacamite aurichalcite azurite Ba-pharmacosiderite bayldonite beaverite bellingerite beudantite boothite brochantite caledonite carminite ceruleite chalchantite chalcophyllite chenevixite chrysocolla claudetite conichalcite connellite cornubite cornwallite crednerite cuprite cyanotrichite Cu3(SO4)(OH)4 Cu 2Mg(OH) 3(AsO4) Bi2(AsO4)(OH)3 As2O3 Pb2Cu(AsO4)(SO4)(OH) CuFe2(AsO4,PO4,SO4)2(O,OH)2·4H2O Cu2Cl(OH)3 (Zn,Cu)5(CO3)2(OH)6 Cu3(CO3)2(OH)2 BaFe4(AsO4)3(OH)5·5H2O PbCu3(AsO4)2(OH)2.H2O PbCu(Fe,Al)2(SO4)2(OH)6 Cu3(IO3)6·2H2O PbFe3(AsO4)(SO4)(OH)6 CuSO4·7H2O Cu4(SO4)(OH) 6 Pb5Cu2(CO3)(SO4)3(OH)6 PbFe2(AsO4)2(OH)2 Cu2Al7(AsO4)4.11.5H2O CuSO4·5H2O Cu18Al2(AsO4)3(SO4)3(OH)27·33H2O Cu2Fe2(AsO4)2(OH)4.H2O (Cu,Al)2H2Si2O5(OH)4·nH2O As2O3 CaCu(AsO4)(OH) Cu19Cl4(SO4)(OH)32·3H2O Cu5(AsO4)2(OH)4 Cu5(AsO4)2(OH)4 CuMnO2 Cu2O Cu4Al2(SO4)(OH)12·2H2O the range of applied potential +0.14 to +0.44 V vs. SHE. The relationship between impedance and polarisation time was interpreted as due to the formation and dissolution of a surface hydroxide layer at V N 0 vs. SHE. Finally, cyclic voltammograms between +0.44 and –0.46 V vs. SHE were interpreted to suggest a two-step reaction at the electrode surface, each involving the transfer of one electron, and firstly yielding a surface product, and secondly a soluble species. Except for the above reported mention of “a surface hydroxide layer”, there are no specific suggestions of the chemical nature of the reaction products. A more extended electrochemical study of natural enargite was performed by Cordova et al. (1997). The mineral was exposed to aqueous solutions at pH 0.5– 13.8. Measurements were carried out both in air and under argon atmosphere. EOCP measurements, cyclic voltammetry, and electric impedance spectroscopy were performed. In an argon atmosphere, EOCP is independent of pH up to 11, and is + 0.20 V vs. SHE. In air, EOCP shows an inverse relation with pH (increases as pH decreases) in the pH range 0.5–4, attaining a maximum eriochalcite goslarite hinsdalite juabite lammerite lavendulan lemanskiite libethenite linarite lindgrenite magnesioaubertite malachite mansfieldite melanterite metatorbernite olivenite paratacamite parnauite pharmacosiderite plumbojarosite posnjakite pseudomalachite richelsdorfite rozenite salesite sampleite scorodite siderotile strashimirite tenorite turquoise CuCl2·2H2O ZnSO4·7H2O (Pb,Sr)Al3(PO4)(SO4)(OH)6 Cu5(TeO4)2(AsO4)2.3H2O Cu3[(As,P)O4]2 NaCaCu5(AsO4)4Cl.5H2O NaCaCu5(AsO4)4Cl Cu2(PO4)(OH) PbCu(SO4)(OH)2 Cu3(MoO4)2(OH)2 (Mg,Cu)Al(SO4)2Cl·14H2O Cu2(CO3)(OH)2 AlAsO4·2H2O FeSO4·7H2O Cu(UO2)2(PO4)2·8H2O Cu2AsO4OH (Cu,Zn)2(OH)3Cl Cu9(AsO4)2(SO4)(OH)10·7H2O KFe4(AsO4)3(OH)4·6–7H2O PbFe6(SO4)4(OH)12 Cu4(SO4)(OH)6·H20 Cu5(PO4)2(OH)4 Ca2Cu5Sb(AsO4)4Cl(OH)6·6H2O FeSO4·4H2O Cu(IO3)(OH) NaCaCu5(PO4)4Cl·5H2O FeAsO4·2H2O FeSO4·5H2O Cu8(AsO4)4(OH)4·5H2O CuO CuAl6(PO4)4(OH)8·4H2O value of ca. + 0.4 V at pH = 0.5. This behaviour was interpreted to suggest the reaction Cu3 AsS4 þ 3O2 þ 6Hþ →AsS4 þ 3Cu2þ þ 3H2 O2 9 ð12Þ Between pH 4.6 and 9.2, EOCP is independent of pH, and the hypothesised reaction is Cu3 AsS4 þ 3O2 þ 3H2 O→AsS4 þ 3CuO þ 3H2 O2 ð13Þ Finally, at pH N 11, EOCP shows again an inverse relationship with pH, ascribed to the reaction Cu3 AsS4 þ 3O2 þ 6OH− →AsS4 þ 3HCuO−2 þ 3HO−2 ð14Þ 9 Reactions (12), (13) and (14) imply the formation of highly oxidizing species such as H2O2. Alternative possibilities could be: Cu 3 AsS 4 + 1.5O 2 + 6 H + → AsS 4 + 3Cu 2 + + 3H 2 O; Cu 3 AsS 4 + 1.5O2 → AsS4 + 3CuO; Cu3AsS4 + 1.5O2 + 3OH− → AsS4 + 3HCuO−2. Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 In cyclic voltammetry experiments, positive potential was applied from EOCP up to + 0.76 V vs. SHE. A distinct anodic current peak occurred at + 0.56 V, nearly constant for pH ≤ 4.6; between 4.6 and 11, the potential of this peak decreases by 0.03 V per pH unit. This behaviour was interpreted as two reactions: For pH≤4:6: Cu3 AsS4 →Cu3−x AsS4 þ xCu2þ þ 2xe− ð15Þ For 4:6bpHb11: Cu3 AsS4 þ H2 O→Cu3−x AsS4 þ xCuOHþ þ xHþ ð16Þ enargite and other copper sulphide minerals conditioned at pH = 11 (Fullston et al., 1999a,b,c; Fornasiero et al., 2001). They used both natural and synthetic enargite. The samples were treated in a KNO3 solution at pH = 11 in different environments: nitrogen, oxygen, and oxygen + H2O2. Zeta potentials were derived from electrophoretic measurements upon pH change from 11 to 5, and backwards from 5 to 11. From zeta potential values, it was suggested that a copper oxide/hydroxide layer is formed at the enargite surface. The corresponding reaction was written as Cu3 AsS4 þ x=2O2 þ xH2 O→Cu3−x AsS4 …ðCuðOHÞ2 Þx ð20Þ with a possible secondary reaction CuOHþ þ OH− →CuO þ H2 O ð17Þ It is suggested that the copper-depleted layer could retain the original enargite structure, perhaps with “partially oxidized” Cu and S atoms. Extreme copper removal would lead to the formation of a metastable surface species AsS4. It is also suggested that at applied potentials N 0.6 V “the oxidation of enargite produces arsenate and sulfate ions, so the formation of a surface film containing cupric arsenate… is possible”. The stationary polarization curve at pH = 9.2 was interpreted to indicate that, between EOCP and an applied potential of 0.44 V, the dominant process is copper release and the formation of a copper depleted/CuO film, followed, between 0.44 and 0.64 V, by electrochemical dissolution of this film. At more positive potentials, there is a decrease in current, ascribed to the surface formation of a passivating cupric arsenate film. Formation of this film is inhibited at pH 3 and 11, where dissolution processes prevail. Similar conclusions were derived from analysis of electrochemical impedance spectra through nonlinear fit routines of the system transfer function. It was deduced that “impedance spectra is (sic) dominated by the electro-oxidation of the nonstoichiometric surface film” associated with “chemical dissolution of the Cu–O containing passive surface”, according to reactions AsS4 þ 20H2 O→H2 AsO−4 þ 4SO2− 4 þ þ 38H þ 29e From the dissolution behaviour during electrophoretic measurements, it was concluded that at pH 11 and in an oxygen-saturated environment, enargite oxidises less readily than chalcocite and tennantite, and more readily than chalcopyrite and bornite. Natural enargite is reported to oxidise more readily than synthetic enargite, but this behaviour is influenced by the presence of additional phases. An additional electrophoretic study of enargite was conducted by Castro and Baltierra (2005). The rather uncommon peak-and-valley profile of the zeta potential vs. pH plot was interpreted in terms of pH-dependent ionization/dissociation/precipitation reactions at the enargite surface (e.g., adsorption of H2AsO4− , AsS43 − , H2AsO2S2 −, and H2AsO3S− ions at pH 4.5; predominance of HAsO42 −, HAsO3S2 −, and HAsO2S22 − ions at pH 9; and precipitation of a layer of copper hydroxide at pH N 6). In the study by Fullston et al. (1999b), XPS spectra were fitted with model functions to resolve the experimental photoelectron peaks into single components. These were then assigned to specific chemical states that were used to interpret the changes in XPS spectra at different oxidation steps. On these grounds, an initial reaction of the type was suggested: Cu3 AsS4 þ ðx=2 þ 3=4yÞO2 →Cu3−x As1−y S4 þ xCuO þ y=2As2 O3 ð21Þ − ð18Þ CuO þ 2Hþ →Cu2þ þ H2 O 79 ð19Þ In the potential range 0.64–0.74 V, the concentration of copper and arsenate ions is supposedly high enough to cause precipitation of Cu3(AsO4)2. Fornasiero and coworkers devoted a series of papers to the electrochemical (zeta potential) and XPS study of Subsequent changes would involve formation of polysulphide and then sulphite, and oxidation of As2O3 to As2O5. An intermediate step with the formation of As4S4 or As2S3 was also hypothesised. Velazquez et al. (2000a) studied a sample of natural enargite by cyclic voltammetry and XPS. The XPS spectra were first collected on a mineral surface resulting from fracturing in Ar atmosphere, then on the polished surface. On this surface, immersed in a solution at Author's personal copy 80 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 pH = 9.2, cyclic voltammetry was performed. XPS was performed on the surface at selected values of the applied potential. From all XPS spectra, the quantitative surface composition was calculated by a commercial software, assuming homogeneity of the surface. The pristine fractured surface showed a composition (Cu2.83As0.94S4) not far from stoichiometric enargite. Polishing of the surface caused an apparent slight variation in composition (decrease of both Cu and As with respect to S), as well as a moderate shift to lower energies of the As3d and S2p photoelectron peaks. In agreement with previous studies, application of potential in the positive sweep direction caused an apparent removal of Cu and, subordinately, of As from the electrode, so that at +444 mV vs. SHE the apparent composition was circa Cu1.1As0.7S4. However, there were no major changes in the Cu2p photoelectron peak, whereas the energies of the As3d and S2p peaks slightly increased, and a second component in the S2p peak appeared. At +744 mV vs. SHE, a dramatic change in the electrode surface occurred. The calculated composition jumped to Cu5.5As1.6S4 (i.e., there was a substantial decrease of sulphur with respect to Cu and As); all photoelectron peaks were shifted to higher energies. Specifically, the Cu2p signal showed features typical of Cu(II), and in the S2p region a new peak at 168.9 eV appeared, ascribed to sulphate sulphur. It was then suggested that “CuO, As 2 O x ( x = 3 or 5), the corresponding hydroxides and CuSO4 have been formed at the electrode surface”. These species disappeared upon reverse potential cycling, indicating that they are stable only on application of high oxidation potentials. In a companion paper, Velasquez et al. (2000b) studied the evolution of a sample of natural enargite under the scanning electron microscope (SEM) upon application of selected potential values at pH = 9.2. SEM/EDS analyses10 indicate an initial composition of Cu3AsS3.5. Between 194 and 444 mV vs. SHE, no appreciable change was observed at the sample surface. At + 744 mV, there is a distinct appearance of “bright spots”, where Cu, As and S concentrations decrease (reported as 30%, 9.3% and 34% respectively, presumably by weight), and O appears (26.5%). These findings were interpreted as suggesting the presence of “oxide, hydroxide, and sulphate”. Upon potential reversal (i.e., in negative sweeping direction), the O-bearing “bright spots” persist at + 444, − 356, and even − 556 mV vs. SHE. This is ascribed to either residual Al2O3 from the polishing process, or to “contamination”. Electric impedance spectroscopy (EIS) data are interpreted as 10 No details given about instrumental parameters, standards, data reduction procedure. supporting SEM data. Specifically, the main transformation in the characteristic parameters occurred at +744 mV vs. SHE and was attributed to formation on the surface of the electrode, “of a layer of oxidized material” composed of “CuO, As2O3, As2O5, CuSO4 and their hydroxides”. This layer would be inhomogeneous, reflecting “initial inhomogeneities of the mineral at the electrode surface”. Guo and Yen (2005) compared the electrochemical oxidation of chalcopyrite and enargite at pH 10 in the presence of potassium amyl xanthate. They found that chalcopyrite is more readily oxidised than enargite (i.e., the reverse of what was reported at pH = 11 by Fullston et al., 1999c), presumably because it is a better conductor. Finally, a study by Viñals et al. (2003) was directed at examining changes on enargite surfaces upon treatment with hypochlorite solutions at pH 12.5 (NaOH). Enargite reacted very fast, and was rapidly covered by a thick (several micrometers) rim of copper oxide (SEM/EDS analysis). Apparently, such a layer does not inhibit the reaction progress, presumably because it is porous. XRD showed that the copper oxide is crystalline CuO, equivalent to the mineral tenorite. The XPS spectra of the reacted enargite surface showed clear evidence of divalent copper (bound to oxygen), of As(III)–O bonds, and of sulphate. The authors estimated an activation energy of 58 kJ/mol for the leaching reaction in the temperature interval 25 °C–60 °C. The porous nature of the reaction product was recently confirmed in our laboratory. A freshly cleaved enargite crystal surface was exposed for 4 hours to a NaClO/NaOH (0.27 M/0.03 M) solution (pH 12.28). Several points of the surface became covered by porous aggregates (Fig. 4). Semiquantitative SEM/EDS analyses are consistent with a CuO composition of this material. The strong reactivity of enargite with hypochlorite solutions was confirmed by Curreli et al. (2005), in an exploratory atomic force microscopy study of the enargite surface by Musu (2007 – see a summary in Musu and Cama, 2006), and by Mihajlovic et al. (2007). These latter authors estimated an activation energy of 30 ± 1 kJ/mol for the leaching reaction (i.e., quite lower than the previous estimated by Viñals et al. (2003). Mihajlovic et al. (2007) also report that, in the same experimental conditions, realgar, As4S4, has about the same reactivity of enargite, whereas chalcocite, chalcopyrite and covellite react at a much lesser extent. 5.5. Bioleaching Bioleaching, that is, microbially mediated oxidation and solubilisation of ore minerals, is a cost-effective, Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 low environmental impact technique of metal recovery, especially suited for the treatment of low-grade and/or refractory ore (e.g., Ehrlich and Brierley, 1990; Rossi, 1990). There are a number of bioleaching studies of enargite (see e.g. Curreli et al., 1997; Escobar et al., 1997; Acevedo et al., 1998; Escobar et al., 2000; Canales et al., 2002; Muñoz et al., 2006; and the recent review by Watling, 2006). However, most are of applied technical nature, i.e. they are more addressed at establishing the performances in terms of recovery and efficiency, than to definition of the specific reaction mechanisms. In his pioneering study, Ehrlich (1964) observed that a culture of bacterium presently known as Acidithiobacillus gen. nov. (Kelly and Wood, 2000) considerably enhanced the kinetics of enargite dissolution, and that these bacteria could survive a comparatively arsenic-rich environment. Most later studies were carried out with cultures of Acidithiobacillus ferrooxidans, on mineral concentrates where enargite was accompanied by appreciable amounts of other phases (in some cases unspecified). In these conditions, oxidation/ dissolution of enargite is enhanced with respect to abiotic conditions; for instance, Fantauzzi (2005) found that the dissolution of enargite in the presence of A. ferrooxidans is 3 to 5 times faster than in abiotic conditions, and is stoichiometric. However, Watling (2006) estimates that chalcopyrite and enargite are the most refractory copper minerals with respect to bioleaching, requiring months to years to achieve significant results (although Inoue et al., 2001, report that the presence of 81 silver catalyses enargite bioleaching by mesophilic bacteria). For instance, after an 8-day treatment of a concentrate containing about equal proportions of pyrite and enargite, Canales et al. (2002) report that the action of A. ferrooxidans was mainly directed against pyrite. Working with (nearly) pure enargite at 30 °C, Escobar et al. (1997) also found a very slow dissolution rate (about 11% Cu released after 800 hours of treatment in a ferric sulphate medium at pH = 1.6). Slightly better results were obtained (with the same sample and in the same medium) at higher temperatures (70 °C) with thermophilic bacteria (Sulfolobus BC): Escobar et al. (2000) were able to dissolve more than 50% Cu from enargite in 550 hours. The better performance of thermophilic bacteria (Sulfolobus metallicus) with respect to mesophilic ones (A. ferrooxidans) in acidic (pH = 1.8) solutions was documented also by Muñoz et al. (2006). They believe that the efficiency of bacteria for enargite dissolution is related to a double effect: on one hand, iron-oxidising bacteria promote recycling of the oxidising agent Fe3+; on the other hand, the sulphur layer formed in these conditions (cf. reaction (8)) is partially removed by bacterial metabolism. The same authors report that enargite biooxidation was associated with the precipitation of iron phosphate and iron and potassium basic sulphate on the mineral surface. Possibly because of this precipitate, enargite submitted to bacterial attack was less reactive than untreated enargite in electrochemical experiments. The mechanism of bacterial interaction with the enargite surface was investigated also by Fantauzzi (2005). By means of XPS, the nitrogen N1s signal was observed on the mineral treated with A. ferrooxidans, thus confirming the hypothesis of adhesion of bacterial cells to the mineral surface by a matrix of extracellular polymeric material. Quantitative XPS analysis performed before and after immersion in the microbial solutions showed the enargite surface to be depleted in copper and enriched in sulfur after leaching. A polysulphide dissolution mechanism for the bioleaching of enargite has been proposed. 5.6. Comparison with other copper–arsenic sulphides Fig. 4. SEM secondary electron image of a porous aggregate of copper oxide developed on an enargite crystal exposed for 4 hours to a NaClO/NaOH (0.27 M/0.03 M) solution (pH 12.28). In this section, we briefly review the oxidation behaviour of the most studied sulphide (pyrite), and of other common copper and/or arsenic sulphides: chalcopyrite (CuFeS2), arsenopyrite, FeAsS, and tennantite (nominally Cu12As4S13, but almost ubiquitously showing partial substitution of Ag, Fe and Zn for Cu, and of Sb for As: see e.g. Sack and Ebel, 2006). The ultimate goal is to draw a comparison with the behaviour of enargite, however this task is hampered by the fragmentary nature of the data. Author's personal copy 82 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 The case of pyrite enlightens the complexity of reactions involved in sulphide oxidation, so that, in spite of decades of research, several aspects of mechanisms and rates of the oxidation of pyrite remain open to debate (e.g., Rimstidt and Vaughan, 2003; Druschel and Borda, 2006, and references therein). Rimstidt and Vaughan (2003) interpret pyrite oxidation in terms of an electrochemical process whereby the rate-determining step is the cathodic reaction of Fe2+ oxidation by an aqueous electron acceptor. Depending on pH, the main oxidising species is either oxygen or ferric iron. The reaction rate is strongly dependent on the concentration of the oxidant, as well as on the presence of ironoxidising bacteria such as Acidithiobacillus ferrooxidans. The anodic process involves a complex multistep process of sulphur oxidation (typical of all sulphides). The behaviour of arsenopyrite is further complicated by the existence of a third element, arsenic, that undergoes significant multistep changes of the oxidation state during the oxidation/dissolution process (e.g., Rosso and Vaughan, 2006b). At low pH, also the dissolution of arsenopyrite is apparently strongly dependent on the concentration of oxidising species like dissolved oxygen or ferric iron, but at circumneutral pH (6.3–6.7) it is independent on dissolved oxygen (Walker et al., 2006). The latter authors suggest that at this pH the ratedetermining step is the anodic reduction of water. At all studied pH, arsenic is mobilised preferentially over iron, presumably because of the formation of an iron oxyhydroxide layer. By comparison, the multistep process of sulphur oxidation is documented for enargite as well, although much detail is missing. Moreover, the available data indicate that enargite is strongly sensitive to the presence of oxidising species, i.e. behaves like pyrite and arsenopyrite at low pH. Another similarity with arsenopyrite is, at alkaline pH, the preferential release of arsenic over the metal (Cu or Fe), because the latter forms an oxide– hydroxide layer. By contrast, at low pH copper mobility from enargite seems at least equal, if not greater, than that of arsenic. The formation of an iron oxide/hydroxide layer is widely cited also with respect to the oxidation/dissolution of chalcopyrite, at least at alkaline pH (e.g., Rosso and Vaughan, 2006b; and references therein). For instance, at pH= 9.2 it is suggested that Cu and S remain unoxidised to form a metastable phase indicated as CuS⁎2 (meaning a CuS2 composition, but not a defined, e.g. pyrite, structure). This phase decomposes at applied potentials higher than about 0.65 V vs. SHE. At pH 4, the main process is again the formation of iron oxides/hydroxides, but some copper leaching occurs. XPS detects a minor formation of surface polysulphide or sulphur. At more acidic pH, iron is preferentially released to solution with respect to copper, and elemental sulphur is formed at high redox potentials; there is, however, considerable uncertainty about the nature of the reacted chalcopyrite surface at low pH. An important result by Acero et al. (2007) is that, at pH 1–3, chalcopyrite dissolution rate is independent on the concentration of dissolved oxygen, and is only slightly dependent on pH. As previously noted, the reactivity of chalcopyrite compared with enargite at alkaline pH was reported as either higher (Guo and Yen, 2005) or lower (Fullston et al., 1999c). According to Watling (2006), at the high redox potentials (presence of ferric sulphate) required for bioleaching of enargite, chalcopyrite is less readily dissolved, because its bioleaching is best achieved at intermediate redox potentials. Tennantite is stable at lower sulphur fugacities than enargite; it is more widespread, and is probably the most abundant Cu–As sulphide. In spite of this importance, literature on tennantite oxidation is not extensive; recent studies include those of Mielczarski et al. (1996), Fullston et al. (1999a,b), Asbjornsson et al. (2004b), and Lin (2006). Because of the variability of composition of natural tennantite, in a conservative approach any conclusion of these studies applies only to the specific studied material, and generalisation is not warranted. However, even accounting for this limitation, it appears that the oxidation behaviour of tennantite is overall similar to enargite. For instance, the formation of elemental sulphur occurs upon electrochemical oxidation of tennantite at low pH, and the proposed reactions (Asbjornsson et al., 2004b; Lin, 2006) are similar to reaction (9) reported here. Tennantite, however, appears somewhat more reactive, at the least for the studied samples and under the specific experimental conditions. For instance, EOCP of tennantite in HCl solutions (ca. 0.17 V vs. SHE, Asbjornsson et al., 2004b) is lower than enargite (cf. Table 5), and more copper per surface unit is released from tennantite than from enargite in the course of the same experiment (Fullston et al., 1999a). Moreover, according to Fullston et al. (1999b), the oxidation layer formed upon conditioning at pH = 11 is thicker in tennantite than in enargite. 6. Summary and conclusions 6.1. Oxidation to air Bulk oxidation of enargite in air can be regarded as a slow process. However, XPS studies can detect appreciable changes in the chemical states of arsenic and sulphur after only very short exposure times. The disappearance, in few minutes, of the low energy Author's personal copy P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 component in the As3d signal is consistent with the concept that the first reaction step in oxidising environments is the creation of As–O bonds with protruding surface As atoms. Grinding in air further induces the appearance of a S2p component that can be interpreted as due to elemental sulphur or to polysulphide species. XPS evidence of As–O bonds is clear in samples exposed to air for weeks or months. Formation of arsenic(III) oxides occurs upon grinding. The behaviour of some natural enargite samples in dissolution experiments was explained by the presence of an arsenic oxide alteration; indeed, arsenic oxides such as arsenolite or claudetite occur in some enargite-bearing deposits. The formation of As(III) oxides upon weathering of enargite is therefore possible. Oxidation to species containing As(V)–O bonds occurs at a later stage. Sulphur is less prone to form bonds with oxygen, and formation of sulphate occurs, at least at room temperature, only after long exposure times to air (presumably, years). High-temperature (80 °C) exposure to moist air causes the formation of small, but visible, amounts of copper (II) sulphate over a time period of weeks; copper sulphate is also formed upon grinding. In general, copper in enargite appears reluctant to form bonds with oxygen. In some museum samples, exposed to air presumably for years, evidence of partial establishment of Cu–O bonds was found, but other studies (in different samples) did not confirm this finding. Exposure to air for years results in the formation of a nanometric surface layer with a composition appreciably different from the bulk. 6.2. Reaction with aqueous solutions Thermodynamic data suggest that enargite should not be stable in acidic solutions. Moreover, EOCP data indicate that enargite is not a particularly “noble” sulphide. Nonetheless, bulk experiments indicate a sluggish oxidative dissolution of enargite, especially at low pH. This sluggishness is confirmed by electrochemical oxidation studies: current densities are typically low (in the order of 10− 5 A/cm2 for stationary polarization curves), indicating a slow process. This behaviour was ascribed to a moderate conductivity of enargite (lesser than chalcopyrite). Notice, however, that many electrochemical studies were performed on polished surfaces – as demonstrated by Velasquez et al. (2002) such surfaces are chemically modified with respect to an untreated surface. Significant oxidation requires strongly oxidising conditions (e.g., the presence of hypochlorite, or applied potentials higher than 0.5 V vs. SHE). At low pH, XPS evidence suggests that the first reaction step may be the release of Cu ions (and arsenic) to the solution, with formation of a copper-depleted 83 layer, and of polysulphide surface species. The subsequent steps involve formation of As–O bonds, oxidation of sulphur to elemental sulphur and eventually sulphate, and formation of surface Cu(II) species. There are discrepancies regarding bulk dissolution rates, some studies indicating stoichiometric copper/arsenic ratios in solution, and others suggesting an arsenic deficit. At high pH, oxidation is possibly faster, and implies formation of surface Cu–O species, in addition to the copper-depleted layer. Formation of cupric arsenate is possible, but apparently limited to a restricted range of conditions. In these conditions, arsenic is apparently more mobile than copper. 6.3. Implications for natural and mine environments In natural environments, enargite oxidation is likely to occur under low pH conditions, because of the nearly ubiquitous presence of acid-generating sulphides, such as pyrite. Moreover, oxidative dissolution of enargite may be an additional source of acidity. Only in carbonate-hosted deposits may oxidation occur at circumneutral pH. Under low pH conditions, it is unlikely that enargite oxidises in situ by a simple mechanism but, most probably, copper is leached and redeposited at some distance. In laboratory oxidation at low pH, elemental sulphur is commonly formed, but there is no explicit report that native sulphur is formed in nature during supergene alteration of enargite. This point deserves more attention, because the formation of elemental sulphur could 1) reduce the acidity of the environment – cf. reaction (11), and 2) slow down significantly the oxidation of enargite. On field and laboratory evidence, it is difficult to predict the final stable assemblage of enargite oxidation. Even considering the simple Cu–S–As–O–H system, there are at least 13 IMA-approved copper arsenates, 11 sulphates, 2 arsenates-sulphates, 2 copper oxides and one hydroxide, 2 arsenic oxides, and native sulphur. Very few of these phases have been positively identified in laboratory experiments (Table 2). Thermodynamic data are not available for most of these phases, indeed published phase diagrams only take into account simple compounds such as Cu2O, CuO, As2O3, CuSO4, Cu3 (AsO4)2. In natural systems, the occurrence of a specific phase(s) may depend upon slight changes in the environment and/or departures from equilibrium. Moreover, in most deposits the chemical environment is even more complex, including elements such as Fe, Al, Cl, that can be combined with Cu and/or As in a plethora of phases. Finally, it should be noted that in many deposits the reaction with oxidising environments is preceded by hypogene or supergene alteration of enargite to copper- Author's personal copy 84 P. Lattanzi et al. / Earth-Science Reviews 86 (2008) 62–88 rich, arsenic-free sulphides. Hence, the final products observed in these deposits could reflect the oxidation of such sulphides, and not of primary enargite. Notwithstanding these complexities, a number of observations and considerations appear relevant: 1) as observed in the drainages of these deposits, copper shows a higher mobility with respect to arsenic 2) copper arsenates are widespread, but never occur in large amounts 3) the most common secondary copper minerals are highly soluble sulphates 4) arsenic oxides are comparatively rare, and never abundant, as expected from their high solubilities 5) scorodite is the most widespread secondary arsenic mineral. A simple mechanism such as depicted by reaction (3) can account for all these facts. There is, on the other hand, both laboratory and field evidence that in some circumstances copper arsenates may directly form from enargite. For chenevixite, a possible reaction scheme (4) is very similar to that for scorodite. With respect to environmental issues, there is an obvious threat represented by the release of toxic elements and of acidity, that can be accelerated in the presence of ferric iron and/or bacteria. However, compared to other copper sulphides the reactivity of enargite is apparently moderate, especially in acidic conditions. The persistence of enargite grains in soils of the Butte, Montana, area may suggest that alteration is modest even on the time scale of years, or perhaps decades. Moreover, the formation of scorodite and/or other arsenates may further slow down, at least temporarily, the release of arsenic. Therefore, a proper management of open pit exposures and of waste and tailings dumps can minimise the environmental impact of this sulphide during mine operations. Establishment of alkaline conditions is not likely in enargitebearing rocks, but is typical of some ore-dressing processes (e.g., flotation and cyanidation). In these situations, Cu mobility may be depressed by formation of Cu oxides, whereas a comparatively higher mobility of arsenic is predicted. A further point that requires consideration is the significant enhancement of enargite reactivity in grinding and milling operations. 6.4. Suggestions for future research Data on enargite oxidation are relatively abundant, even if they cannot match the extensive literature on more common sulphides. However, quantitative studies are comparatively scarce, especially in conditions representative of natural and mining environments. For instance, most kinetic studies of enargite dissolution were conducted either at high temperatures, or in the presence of reactants (e.g., hypochlorite) unlikely to occur in natural systems. There is a lack of detailed laboratory studies on the dependence of dissolution rates from parameters such as pH or dissolved oxygen. In addition, there is a scarcity of systematic studies documenting the alteration steps of enargite in the field. Important progresses in understanding enargite oxidation may be gained from a) careful observation of alteration textures and products in mineralised bodies and waste dumps, b) bulk experiments of enargite oxidation/dissolution at room temperature at different pH and dissolved oxygen concentrations, coupled with documentation of chemical states/products at the surface by techniques such as photoelectron and Auger spectroscopies. The use of methods taking advantage of highenergy sources, such as synchrotron-based XPS and/or SEXAFS, could lead to significant advances. Moreover, there are no details of the nature of enargite surface at the molecular scale. Exploratory studies indicate that atomic force microscope observation of the evolution of the enargite surface upon reaction with aqueous solutions may be feasible. Another field that is totally unexplored is the application of ab initio calculations to fully understand chemical bonding and to model crystal surfaces. Finally, future research should be directed also to establish a) the influence of minor elements (especially the almost ubiquitous antimony) on enargite reactivity, b) a clear distinction between the behaviour of enargite and luzonite. Acknowledgments The research was supported by the Italian Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR), in the framework of the national project (PRIN – Cofin 2004) “Meccanismi di interazione superficiali in fasi minerali” (coordinator G. Artioli). Pilar Costagliola, Giovanni De Giudici, Francesco Di Benedetto and Luca Fanfani offered useful comments on earlier drafts of the manuscript. The paper underwent a rather complex editorial history. Originally submitted to Chemical Geology, it was redirected to Earth Science Reviews upon recommendation of Chemical Geology's Editor David Rickard. In this process, the paper benefited from the criticism and suggestions by David J. Vaughan and three other anonymous reviewers, and by ESR Editor D. Kirk Nordstrom. While this course obviously implied a delay in publication, it certainly resulted in a significant improvement of the paper. Author's personal copy P. 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