Meteoritics & Planetary Science 1–26 (2014) doi: 10.1111/maps.12385 Shock metamorphism and impact melting in small impact craters on Earth: Evidence from Kamil crater, Egypt Agnese FAZIO1*, Luigi FOLCO1, Massimo D’ORAZIO1, Maria Luce FREZZOTTI2, and Carole CORDIER3,4 1 Dipartimento di Scienze della Terra, Universit
a di Pisa, Via S. Maria 53, 56126 Pisa, Italy Dipartimento di Scienze dell’ambiente e del Territorio e di Scienze della Terra (DISAT), Sezione di Scienze Geologiche Geotecnologie, Piazza della Scienza 4, 20126 Milano, Italy 3 Universite de Grenoble Alpes, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France 4 CNRS, ISTerre, BP 53, 38041 Grenoble CEDEX 9, France * Corresponding author. E-mail: agnesefazio@dst.unipi.it 2 (Received 20 March 2014; revision accepted 15 August 2014) Abstract–Kamil is a 45 m diameter impact crater identified in 2008 in southern Egypt. It was generated by the hypervelocity impact of the Gebel Kamil iron meteorite on a sedimentary target, namely layered sandstones with subhorizontal bedding. We have carried out a petrographic study of samples from the crater wall and ejecta deposits collected during our first geophysical campaign (February 2010) in order to investigate shock effects recorded in these rocks. Ejecta samples reveal a wide range of shock features common in quartz-rich target rocks. They have been divided into two categories, as a function of their abundance at thin section scale: (1) pervasive shock features (the most abundant), including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features (the least abundant) including high-pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films in shatter cones. In particular, Kamil crater is the smallest impact crater where shatter cones, coesite, stishovite, diamond, and melt veins have been reported. Based on experimental calibrations reported in the literature, pervasive shock features suggest that the maximum shock pressure was between 30 and 60 GPa. Using the planar impact approximation, we calculate a vertical component of the impact velocity of at least 3.5 km s
1. The wide range of shock features and their freshness make Kamil a natural laboratory for studying impact cratering and shock deformation processes in small impact structures. of the local target rocks. After its discovery in 2008, Kamil has been explored in 2010 by our first Italian– Egyptian geophysical campaign (Folco et al. 2010, 2011; D’Orazio et al. 2011; Urbini et al. 2012). During this campaign a series of target rock samples representing the crater wall and ejecta were collected with the aim of investigating the shock metamorphism and impact melting induced in the local rocks by the impact of the small (5–10 t) Gebel Kamil Ni-rich ataxite (D’Orazio et al. 2011). In this work, we present a detailed description of the shock features found within these samples. We will show that the target rocks from Kamil preserve an astonishingly wide record of shock conditions, and we will use petrographic observations INTRODUCTION Shock metamorphism and impact melting in terrestrial rocks during small-scale meteorite impacts is still not fully known. This is mainly due to the rarity of very small impact craters on Earth (only 17 terrestrial craters of 184 have diameters <300 m; Earth Impact Database), and, more importantly, to the generally low degree of preservation of very small craters. One exception is Kamil crater (southern Egypt). This 45 m diameter crater shows all its original features in an almost pristine state of preservation due to its very young age (most likely <5000 yr), the favorable climatic conditions of the Sahara, and the mechanical strength 1 © The Meteoritical Society, 2014. 2 A. Fazio et al. on these shock features to constrain peak shock pressure and impact velocity of the iron meteorite projectile. Because the target rocks of Kamil area are dominantly siliciclastic sedimentary rocks, our data will be useful for comparing the better known shock effects on crystalline rocks (e.g., St€ offler and Langenhorst 1994) with those affecting sedimentary rocks, which are characterized by highly variable porosity, type, and amount of matrix; grain size; water content; fabric; and mineralogical assemblage (e.g., Kowitz et al. 2013a, 2013b). GEOLOGICAL SETTING A detailed account of the geology and geophysics of Kamil is given in Urbini et al. (2012). The Kamil impact structure is situated in a rocky desert area in the East Uweinat district of southern Egypt (Fig. 1). It occurs in a simple geological context: flat, rocky desert surface, and target rocks comprising subhorizontally layered sandstones probably belonging to the Cretaceous Gilf Kebir Formation. The Gilf Kebir Formation unconformably overlies the Precambrian Basement, with a peneplaned surface marking a considerable stratigraphic gap. No rocks from the basement were observed in the crater and ejecta. The impact did not involve the crystalline basement. Kamil is a simple, approximately circular crater, with a diameter of 45  2 m (Figs. 1 and 2a), a depth of approximately 10 m, an upraised rim about 3  0.7 m above the pre-impact surface as modeled by Urbini et al. (2012). In situ, uplifted sandstone strata are observed along the upper part of the crater walls (e.g., Fig. 2b). Tear faults separate sectors of the crater wall with distinct inclinations of the upturned strata. The lower part of the crater walls are covered by slump debris (Fig. 2c). The bottom of the crater is occupied by a breccia lens with a maximum thickness of approximately 6 m. A wind-blown sand deposit covers part of the northern crater wall and crater floor. The overall stratigraphic sequence consists of pale (top) and reddish brown (bottom; Fig. 2b), coarse to very-coarse, gritty, and ferruginous quartzarenite. The topmost layers are whitish, kaolinitic, and fine-grained. The Kamil structure is characterized by a radial pattern of bright ejecta (Fig. 1). Three nearly straight major ejecta rays trend to the north (approximately 355°), southeast (approximately 130°), and southwest (approximately 210°). The ray trending to the southeast (Fig. 3a) is also the longest (approximately 300 m from the crater rim) and the largest one in terms of volume of ejected material, and gives rise to a positive topography close to the crater rim. A nearly continuous Fig. 1. Enhanced true color QuickBird satellite scene (22 October 2005; courtesy of Telespazio) of the Kamil area (Egypt; see inset) showing the location of the studied samples. ejecta blanket extends for approximately 50 m from the crater rim. Overall, the bulk of the ejecta material is preferentially concentrated between the two main ejecta rays trending to the north and southwest. Within approximately 50 m from the crater rim, ejecta consist of pale and reddish target rocks (Figs. 3b and 3d) plus impact melt lapilli and bombs (Fig. 3c). Farther away, the pale target rocks dominate. The masses of the ejected debris range from approximately 4 t to dust (Fig. 3b; Urbini et al. 2012). Thousands of shrapnel pieces up to 34 kg in mass, formed through fragmentation upon hypervelocity impact of the Gebel Kamil iron meteorite, were found concentrated due southeast of the crater with a broad concentration maximum at approximately 200 m from the crater rim (D’Orazio et al. 2011). With the exception of the eolian sand deposit that covers part of the northern crater wall, the Kamil structure is well preserved. This is evidenced—for instance—by the lack of erosion features on the crater rim, and the essentially pristine distribution of bright ejecta and meteorite fragments (D’Orazio et al. 2011; Urbini et al. 2012). The pristine state of preservation is in agreement with the young age of the crater (<5000 yr; Folco et al. 2011) estimated on the basis of archeological evidence. Due to the high depth-to-diameter ratio of the transient cavity, Urbini et al. (2012) suggested that Kamil formed by the impact of a single iron mass (or a tight cluster of fragments). Based on the geometry of the crater and asymmetries in shrapnel and ejecta distribution Folco et al. (2011), D’Orazio et al. (2011), Shock metamorphism and impact melting at Kamil crater 3 Fig. 2. Field photos of Kamil wall rocks. a) Photomosaic of the interior of the crater viewed from southwest. The location of sample L13 is shown. The white rectangle indicates the outcrop featured in (b). b) Upturned rim strata exposed on the north wall of the crater. Samples M26 and M27 were collected from the top pale layer and the underlying reddish layer, respectively. The thickness of the top pale layer is about 70 cm. c) Slump debris covering the lower part of the southern crater wall. The location of sample L14 is shown. Fig. 3. Field photos of Kamil ejecta. a) Panoramic view from the southeast side of the crater rim showing the main ejecta ray trending 130°E. The arrow indicates the largest ejecta boulder (sample L07; 2.1 9 0.7 9 1.1 m, equivalent to approximately 4 t), lying about 90 m outside from the crater rim. b) Close-up view of the same boulder. The boulder is overturned with pale (sample L07a) and reddish (sample L07b) sandstone layers at its stratigraphic top and bottom, respectively. c) Sample L08: impact melt bomb (long axis approximately 10 cm) consisting of white pumiceous glass partially coated by a magnetic dark glass. d) Sample R01 (long axis approximately 12 cm): ejected sandstone showing a shatter cone surface (see text for details). and Urbini et al. (2012) concluded that the Gebel Kamil iron meteorite arrived from the NW with an impact angle between 30° to 45°. The minimum projectile mass inferred from systematic meteorite search and geomagnetic data is approximately 5 t (Urbini et al. 2012). 4 A. Fazio et al. SAMPLES AND ANALYTICAL METHODS Geological samples studied in this work were collected during the first Italian–Egyptian geophysical campaign to Kamil in February 2010 (Tables 1 and 2; Fig. 1). Due to severe restrictions imposed by the local authorities, only a limited selection of rock samples from the crater wall and ejecta rays totaling less than 10 kg was allowed for export and research abroad. Nonetheless, these samples are a representative selection of the target enabling a first study of shock features at Kamil. Two to three thin sections from each sample were made for petrographic observations by optical and electron microscopy. An optical microscope of the Zeiss Axioplan type and scanning electron microscope (SEM) Philips XL30, operating at 20 kV coupled with an energy-dispersive X-ray detector (EDX), were used at Pisa University’s Dipartimento di Scienze della Terra. A field emission scanning electron microscope (FE-SEM) Jeol JSM 6500F (upgraded to 7000 series), operating at 10 kV, was used at the Istituto di Geofisica e Vulcanologia (INGV) of Rome. Modal abundances of phyllosilicate minerals and iron-oxide matrix, as well as the porosity of the rock samples from the crater rim and ejecta were determined by digital image analysis of backscattered electron images and X-ray element maps using the JMicroVision software. We determined the crystallographic orientations of planar deformation features (PDFs) in the quartz crystals of sample L23 following procedures described in Langenhorst (2002) using a five-axis U-stage mounted on a Zeiss Axioplan optical microscope. For PDF indexing, the automated numerical index executor (ANIE; Huber et al. 2011) was used. Quantitative chemical microanalyses were carried out with a Jeol JXA 8230 electron microprobe fitted with five wavelength dispersive spectrometers at the Institut des Sciences de la Terre (ISTerre) of Grenoble. Running conditions were 15 kV accelerating voltage, 12 nA beam current, and 1 lm nominal beam spot. The ZAF procedure was employed for raw data reduction. Standards used for instrumental calibration were SiO2rich glass (USNM 72854), ilmenite (USNM 96189), and hornblende (USNM 143965). Average detection limits are 0.04 wt% for Na2O and ZrO2; 0.02 wt% for Al2O3, Cr2O3, and V2O3; and 0.01 wt% for MgO, CaO, K2O, FeO, MnO, TiO2, NiO, and P2O5. Typical precisions are better than 1% (relative standard deviation = 1 9 standard deviation/average*100) for oxides concentration >1 wt% and between 1 and 10% for oxide concentrations between 1 and 0.1 wt%. The accuracy is typically better than 5%. In order to concentrate and identify high-pressure silica polymorphs through X-ray powder diffraction (XRPD), aliquots of samples L23 and M25 were powdered, treated with dilute (approximately 8 vol%) hydrofluoric acid for 12 h at room temperature, and then partitioned using a high-density liquid (2.675 g cm
3 sodium polytungstate aqueous solution). X-ray powder diffraction of particles denser that 2.675 g cm
3 was conducted using a Philips PW1830 instrument at Pisa University’s Dipartimento di Scienze della Terra operating with a Bragg-Brentano geometry and Ni filtered Cu Ka radiation. Each 2h step (0.2°) was counted for 1 s. In situ identification of silica phases was conducted through laser-induced Raman microspectroscopy using a Horiba LabRAM HRVIS instrument of the “Centro Scansetti” at the Torino University’s Dipartimento di Scienza della Terra. The studied sample was a doublepolished 100 lm thick section from sample L23. Spectra were excited using the 532.11 nm emission of an argonion laser. The Raman microprobe system (focal length 800 mm) was equipped with an Olympus BX41 optical microscope and Peltier-cooled charge-coupled device (CCD) detector and with an Olympus 1009 objective lens (numerical aperture 0.9); the focal spot was approximately 1–2 lm in diameter. Silica phase spectra accumulation times varied between 30 and 90 s, while diamond spectra were acquired in 1–5 s. The wave numbers of the Raman lines were calibrated daily using the silicon band at 520.6 cm
1. RESULTS Petrography of Crater Wall Rocks The layered rocks cropping out at the Kamil walls are sandstones ranging from very-coarse quartzarenite (sample M27; Fig. 4a) to coarse siltstone with intercalated levels of very fine wacke (sample L13; Fig. 4b). The location and description of crater wall samples are summarized in Table 1. The mineralogy of the crater wall rocks is dominated by subrounded quartz grains. Accessory minerals up to 400 lm in size include fine intergrowths of Fe-Ti oxides, besides zircon, tourmaline, and rutile (Table 3). The sum of accessory minerals usually constitutes about 1–2 vol% of crater wall rocks. They are more common in fine-grained rocks. The phyllosilicate matrix is dominated by kaolinite (Table 3). In some of the fine-grained samples, the matrix can reach up to 40 vol% (e.g., L13 and L14, Figs. 4b and 4c and Table 2). Minor amounts (approximately 2 vol%) of iron oxides occur in the matrix of sample M27 (Fig. 4a). Samples M26 and L02 are devoid of matrix material, and show polygonal texture due to extensive overgrowth of syntaxial quartz cement (Fig. 4d). Table 1. Location, description and shock features of Kamil Crater target rocks collected from the crater walls and ejecta. Shock features Latitude Sample N Crater wall L02 22°10 5.58″ 22°10 5.47″ L14 22°10 5.59″ 26°50 15.94″ S sector, in situ UL 26°50 15.80″ S sector, in situ UL 26°50 15.79″ S sector, slump debris Lithology M to C pale quartzarenite with pervasive syntaxial quartz. PI = 0 vol%. VF pale wacke with level of C pale siltstone, >20 vol% KM. PI = 0 vol%. M to C pale quartzarentite, ~6 vol% KM and ~3 vol% IM with levels of C siltstone to very F wacke, ~40 vol% KM. PI = 4 vol%. C pale quartzarenite with pervasive syntaxial quartz, <5 of both KM and IM. PI= 0 vol%. VC reddish quartzarenite, ~6 vol% KM and ~2 vol% IM. PI = 17 vol%. 26°50 15.45″ N sector, in situ, top of UL 0 0 M27 22°1 6.44″ 26°5 15.45″ N sector, in situ, bottom of UL Ejecta: sandstone blocks R01 22°10 8.02″ 26°50 17.28″ 50 m due M to F pale wacke, NE of CR ~30 vol% KM. PI = 0 vol%. 0 0 R02 22°1 7.13″ 26°5 18.16″ 54 m due VC to F pale quartzarenite. NE of the PI = 0 vol%. crater L07a M pale quartzarenite. L07 22°10 4.22″ 26°50 18.35″ 90 m due ~15 vol% KM, bottom of ESE the ejecta. PI= 15 vol%. of CR L07b M to VC reddish quartzarentite. Up to 5 vol% KM and IM, top of the ejecta. PI = 24 vol%. M24 22°10 9.89″ 26°50 16.04″ 100 m due M to F pale wacke, N of CR ~40 vol% KM with levels of M– to C siltstone, ~20 vol% KM. PI = 0 vol%. L23 22°10 1.23″ 26°50 30.13″ 400 m due F to C pale quartzarenite. SE of CR PI = 0 vol%. VF pale wacke to M pale M25 22°00 45.16″ 26°50 1.73″ 740 m due siltstone. PI = 0 vol%. SSW of CR M26 22°10 6.44″ Location Melt veins and films <1 – – – – – – – – – – <1 – – – – – – – – – – <1 – – – – – – – – – – <1 – – – – – – – – – – <1 – – – – – – – – – – <5 x x x – x n.a. x – – x <5 x x x – – – – – – – <1 – – – – – – – – – – <5 x x x – – n.a. – – – – 20–25 x x x x x x x x x x <5 x x x – – – – – – x 5 UL = upturned layer; CR = crater rim; VF = very fine; F = fine; M = medium; C = coarse; VC = very coarse; KM = clay minerals in matrix; IM = iron-oxide matrix; PI = porosity index; x = observed, – = not detected, n.a. = not analyzed. Shock metamorphism and impact melting at Kamil crater L13 Longitude E Incipient melting of Highpressure Decomposed Intergranular accessory Pressure Undulose Reduction of PF/PDF PDF in minerals zircon melt (GPa) Fractu-ring extinction birefrin-gence in quartz tourmaline phases 6 A. Fazio et al. Table 2. Location and description of impact melt lapilli and bombs from Kamil crater. Sample Latitude Longitude Location Description Ejecta: impact melt lapilli and bombs L15 22°010 6.05″ 26°050 15.77″ Inside the crater M23 22°010 5.69″ 26°050 16.27″ L08 22°010 4.05″ 26°050 16.53″ L09 22°010 4.01″ 26°050 16.57″ L05 22°010 5.29″ 26°050 18.17″ L06 22°010 5.27″ 26°050 18.14″ E30 22°010 3.39″ 26°050 16.44″ m.s. MNA07 m.s. 22°010 4.51″ 26°050 18.11″ L03 22°010 7.91″ 26°050 19.88″ L04 22°010 7.93″ 26°050 19.92″ Magnetic dark glass with inclusions of sandstone clasts, diaplectic glass, and lechatelierite. Metallic blebs are scattered in the dark glass. Inside the crater Gray lechatelierite. Vesicles are rounded and generally micrometric. Single grain of decomposed zircon. 52 m due ENE of the CR White lechatelierite partially coated by a <2 mm thick magnetic dark glass envelope. 52 m due ENE of the CR White lechatelierite with relicts of highly shocked quartz grains (melted PDFs, amorphization) stained by reddish iron oxi-hydroxides. 57 m due E of the CR Magnetic dark glass with inclusions of sandstone clasts, diaplectic glass, and lechatelierite. Metallic blebs are scattered in the dark glass. 57 m due E of the CR Magnetic dark glass with inclusions of sandstone clasts, diaplectic glass, and lechatelierite. Metallic blebs are scattered in the dark glass. Rare occurrence of fragments of the iron projectile Gebel Kamil. 57 m due ESE of the CR Magnetic dark glass with inclusions of sandstone clasts, diaplectic glass, and lechatelierite. Metallic blebs are scattered in the dark glass. Rare occurrence of fragments of the iron projectile Gebel Kamil. Found stuck to the surface of iron-shrapnel specimens. 61 m due SSE of the CR Magnetic dark glass found stuck to the surface of a shrapnel of the iron projectile Gebel Kamil. Inclusions of sandstone clasts, diaplectic glass, and lechatelierite are abundant. Metallic blebs are scattered in the dark glass. Rare occurrence of meteorite fragments. 120 m due ENE of the CR White lechatelierite coated by a <2 mm thick magnetic dark glass envelop. 120 m due ENE of the CR Pumiceous white lechatelierite. Vesicles are elongated and can be up to 3 mm in size. CR = crater rim; m.s. = meteorite shrapnel. Petrography of Ejecta most fine-grained lithologies, totaling up to 40 vol%. The matrix of the very-coarse sandstone L07b contains abundant iron oxides, constituting up to 5 vol% of the bulk rock (Fig. 5b). The porosity is negligible in all samples but L07, in which it ranges from 15 vol% (L07a) to 24 vol% (L07b) (Table 1; Figs. 5a and 5b). Sandstone Blocks Six ejected sandstone samples were collected at different distances from the crater in order to explore possible variations of the degree of shock metamorphism with increasing ejection distance (Table 1). In terms of overall texture and mineral composition, the ejected sandstone samples are indistinguishable from those of the crater wall rocks (Table 1; Fig. 5). They range from very-coarse pale quartzarenite to very-fine pale wacke. Quartz is the most abundant mineral, followed by heavy accessory minerals (1–2 vol%) including fine intergrowths of Fe-Ti oxides, zircon, tourmaline and rutile. The kaolinite-rich matrix is more abundant in the Impact Melt Lapilli and Bombs Eight impact melt lapilli and bombs, ranging from a few centimeters to a few tens of centimeters in size, were collected within 100 m from the crater and inside the crater (Table 2; Fig. 1). They are divided into two endmember compositions: a white silica-rich glass and a Fe-Ni-rich dark glass (Fig. 6). White glass lapilli and bombs may be coated by a more or less continuous rim of dark glass up 2 mm thick (Fig. 3c; Table 2). Clasts of dark glass have also been discovered stuck to the surface of iron-shrapnel specimens (D’Orazio et al. 2011; Table 2). All samples are well preserved and they are not affected by secondary alteration, with the only The porosity of these rocks is generally lower than 4 vol% (Figs. 4b and 4c), and close to zero in matrixfree samples M26 (Fig. 4d) and L02. The exception is sample M27 where porosity is 17 vol% (Fig. 4a). Shock metamorphism and impact melting at Kamil crater 7 Fig. 4. Photomicrographs of target rocks collected from the crater wall. a) Sample M27: very-coarse quartzarenite collected in situ from the upturned layers in the northern sector of the crater wall (crossed polarizers, XPL). The white arrows show the iron-oxide matrix. b) Sample L13: coarse siltstone to very fine wacke collected in situ from the southern sector of the crater wall (plane polarized light, PPL). c) Sample L14: layered sandstone collected from the slump debris on the southern sector of the crater wall (PPL). d) Sample M26: coarse quartzarenite collected in situ from the upturned layers in the northern sector of the crater wall (XPL). The inset shows a detail of the extensive overgrowth of syntaxial quartz cement (black arrows). Table 3. Representative electron microprobe analyses (oxide wt%) of the matrix clay minerals and accessory minerals of unshocked samples. Phyllosilicate mineral matrix SiO2 Al2O3 Na2O MgO CaO K2 O FeOtot MnO TiO2 Cr2O3 NiO V2O3 P2O5 ZrO2 Total 42.0 33.1 b.d.l. 0.10 0.04 0.05 2.43 b.d.l. 0.46 0.03 b.d.l. 0.08 b.d.l. b.d.l. 78.5 45.9 36.8 0.07 0.13 0.19 0.05 1.51 b.d.l. 1.93 0.03 b.d.l. 0.09 0.18 b.d.l. 87.0 46.1 36.6 b.d.l. 0.12 0.17 0.05 1.44 b.d.l. 2.17 b.d.l. b.d.l. 0.09 0.16 0.05 87.0 Fe-Ti oxides 38.9 30.2 0.11 0.07 0.01 0.03 1.73 b.d.l. 0.76 b.d.l. b.d.l. 0.04 0.08 0.11 72.4 1.38 1.97 0.06 0.07 0.30 0.02 12.8 0.06 73.6 0.83 0.07 3.22 0.22 0.44 95.1 0.57 1.27 b.d.l. 0.19 0.12 b.d.l. 25.1 0.12 64.0 0.37 b.d.l. 2.76 0.09 0.21 94.8 Rutile Zircon Tourmaline 0.03 0.04 b.d.l. b.d.l. b.d.l. b.d.l. 0.05 b.d.l. 93.5 0.24 b.d.l. 4.98 0.01 0.11 99.0 34.3 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 66.8 101.1 36.8 35.2 1.46 2.98 0.05 0.03 9.63 0.10 0.10 b.d.l. 0.03 b.d.l. b.d.l. b.d.l. 86.4 b.d.l. = below detection limit. Detection limits are 0.04 wt% for Na2O and ZrO2, 0.02 wt% for Al2O3, Cr2O3, and V2O3, and 0.01 wt% for MgO, CaO, K2O, FeO, MnO, TiO2, NiO, and P2O5. FeOtot: Total iron as FeO. exceptions being some portions of sample L09 that are stained by reddish iron oxi-hydroxides (Tables 2 and 4). White glasses typically show pumiceous texture with vesicle sizes ranging from a few tens of micrometers (Fig. 7a) to several millimeters (Fig. 6a). Sample L09 (Table 2) contains millimetric relicts of quartz grains. Silica content in white glass ranges from 99.2 wt% to 96 wt%. The glass is, thus, referred as lechatelierite. 8 A. Fazio et al. Fig. 5. Photomicrographs of four ejected target rock samples (L07, M24, L23, and M25), ordered according to their distance from the crater rim (same magnification as in Fig. 4). a) Sample L07a: undeformed pale quartzarenite (PPL). L07b: undeformed reddish quartzarenite (PPL). c) Sample M24: pale wacke with highly fractured quartz grains (PPL). L23: shocked pale quartzarenite (PPL). e) Sample L23: same image of (d) in XPL. f) Sample M25: pale wacke (PPL). arrow points to a melt vein. See Table 1 for detailed sample description. increasing b) Sample d) Sample The black Fig. 6. Impact melt bombs (cut surfaces). a) Sample L04: white, pumiceous, lechatelierite. b) Sample L06: dark, magnetic glass with abundant sandstone clasts, lechatelierite clasts, and meteorite fragments. MF = meteorite fragment; SC = shocked sandstone clast; LG = lechatelierite. Shock metamorphism and impact melting at Kamil crater 9 Table 4. Representative electron microprobe analyses (oxide wt%) of glass of impact melt lapilli and bombs. White glass (lechatelierite) SiO2 Al2O3 Na2O MgO CaO K2 O FeOtot MnO TiO2 NiO P2O5 ZrO2 Total Dark glass L09 L03 L04 L06* L04 L09 L03 L06 L06 L06 L06 99.2 0.03 b.d.l. b.d.l. 0.01 b.d.l. 0.03 0.00 b.d.l. b.d.l. b.d.l. 0.04 99.4 98.5 0.02 0.04 0.01 b.d.l. 0.01 0.04 0.01 0.02 b.d.l. b.d.l. b.d.l. 98.7 98.5 0.03 b.d.l. 0.01 b.d.l. b.d.l. 0.02 0.00 b.d.l. b.d.l. b.d.l. b.d.l. 98.6 98.5 0.03 b.d.l. 0.03 0.01 0.02 0.02 0.00 b.d.l. 0.05 b.d.l. b.d.l. 98.6 98.2 0.26 b.d.l. b.d.l. b.d.l. b.d.l. 0.01 0.00 b.d.l. 0.01 0.02 b.d.l. 98.5 96.0 0.11 0.03 b.d.l. 0.01 0.01 3.93 0.00 b.d.l. 0.01 b.d.l. 0.04 100.2 60.2 14.8 b.d.l. 0.29 0.25 0.18 19.9 0.27 0.57 0.40 0.17 0.22 97.2 57.4 11.9 0.04 0.19 0.87 0.20 25.4 0.37 0.93 0.43 b.d.l. b.d.l. 97.6 54.3 12.5 0.12 0.20 1.04 0.22 28.6 0.66 1.13 0.26 0.01 b.d.l. 99.0 54.3 11.4 0.07 0.22 0.74 0.28 29.7 0.55 0.87 b.d.l. 0.02 b.d.l. 98.0 49.8 12.4 0.05 0.17 0.76 0.11 32.2 0.66 1.09 2.51 0.18 b.d.l. 100.0 b.d.l. = below detection limit. *White glass in inclusion in dark glass lapilli and bombs. Detection limits are 0.04 wt% for Na2O and ZrO2; 0.02 wt% for Al2O3; and 0.01 wt% for MgO, CaO, K2O, FeO, MnO, TiO2, NiO, and P2O5. FeOtot: Total iron as FeO. FeO content is lower than 0.04 wt%, with the exception of some portions of sample L09 where FeO content can reach approximately 4 wt%. The Al2O3 content is lower than 0.26 wt%. Dark glass lapilli and bombs show scoriaceous texture. They contain several inclusions (Fig. 7b); in order of abundance they are: clasts up to 5 mm in size of shocked sandstone (grain size up 700 lm) with planar deformation features, fragments up to 3 mm in size of vesicular white glass (lechatelierite), fragments up to 1 mm in size of diaplectic glass, lm-sized metal blebs and fragments up to 200 lm in size of the Gebel Kamil iron meteorite (Figs. 6b and 7b). Dark glass has higher contents of Al2O3 and FeO than white glass (Table 4). The average of Al2O3/SiO2 and FeO/SiO2 ratios are 0.23 and 0.50, respectively. The maximum NiO content of dark glass is 2.51 wt%, but it generally ranges from 0.2 to 0.4 wt%. The inclusions of lechatelierite glass have significantly higher NiO content (0.05 wt%) than other lechatelierite glasses (≤0.01 wt%; Table 4). Metal blebs are significantly enriched in Ni (from 56.1 wt% to 69.8 wt%) and Co (from 1.56 wt% to 2.07 wt%) relative to the Gebel Kamil meteorite (Ni 20.6 wt% and Co 0.76 wt%; D’Orazio et al. 2011). Shock Metamorphic Features Fracturing, Reduction in Birefringence, and Undulose Extinction Fracturing, undulose extinction, and reduction in birefringence in quartz are common to all ejecta sandstone samples with the exception of sample L07. Concussion fractures, i.e., fractures due to the collision of two or more quartz grains, are common in samples containing relatively large quartz grains up to hundreds of micrometers in size embedded in a more fine-grained mineral matrix (samples M24 and R02; Fig. 8). The number of irregular fractures per millimeter (f mm
1) estimated according to Kowitz et al. (2013a) decreases in the following order: sample L23 (450 f mm
1), R02 (approximately 11 f mm
1), R01 (approximately 110 f mm
1), M25 (approximately 100 f mm
1), and M24 (approximately 60 f mm
1). Note that sample R01 bears striated shattered surfaces decorated by discontinuous lm thick silica-rich melt films (Fig. 3d; see the Melt in Shatter Cones section for further details). In this sample, fractures are generally parallel to the melt films and their abundance increases to approximately 170 f mm
1 toward the lm thick silica-rich melt films. Planar Deformation Features Quartz: Planar deformation features (PDFs) in quartz grains occur in sample L23 and in several sandstone clasts embedded in the dark impact melt glasses (hereafter sandstone clasts). Irrespective of the grain size of the sandstone, PDFs occurring within individual crystals develop in domains up to 200 lm across (Fig. 9a). A maximum of four sets of PDFs has been recognized in a single quartz grain (Fig. 9a). Overall, an average of 2.1 sets of PDFs per quartz grain is observed; PDFs occur in about 70% of quartz grains. The spacing between PDFs belonging to the same set in 10 A. Fazio et al. Fig. 7. Backscattered electron (BSE) images of sectioned impact melt bombs. a) White, pumiceous, lechatelierite (sample L04). b) Dark, magnetic, impact melt glass (sample L06). The latter is laden with clasts of target and projectile materials. Abbreviations: MF = meteorite fragment; SC = shocked sandstone clast; LG = lechatelierite; DG = diaplectic glass; MB = Ni-Fe metallic bleb. a quartz grain is not uniform and it ranges from 0.2 lm to 2 lm. Their thickness is rather uniform, varying from 0.08 lm to 0.15 lm (Fig. 9a). We measured the crystallographic orientations of 149 sets of PDFs in 70 quartz grains in two representative thin sections of sample L23, and of 59 sets of PDFs in 29 quartz grains in three thin sections of dark impact melt glass containing sandstone clasts. The results of PDF indexing are summarized in Table 5 and in Fig. 10. The histograms in Figs. 10a and 10b report the frequency distribution of PDF poles to c-axis angle within 5° bins (i.e., with a 5° error) for sample L23 and sandstone clasts, respectively. The histograms in Figs. 10c and 10d show the absolute frequency Fig. 8. BSE images of ejected sandstone. a) Kaolinite-rich matrix between quartz grains in sample M24. b) Concussion fractures in quartz grains in sample R02. percent of indexed PDFs within a 5° error for sample L23 and sandstone clasts, respectively. The crystallographic orientations of PDFs in sample L23 and sandstone clasts are quite similar; however, sandstone clasts show a larger number of PDFs with poles to c-axis angles > 40°. In sample L23, the most frequent orientations are {1013} 23%, {1012} 14%, {1014} 13%, {1011} 11%, and {1122} 7% (Table 5; Fig. 10c). Instead, in sandstone clasts the orientations {1013} and {1122} are most common at 17%, followed by {1011}, 15%, {1012}, 12%, and {1014}, 10% (Table 5; Fig. 10d). The unindexed planes represent the 15.4% and 15.3% of the total number of measured sets of PDFs in sample L23 and sandstone clasts, respectively. Amorphous planar to subplanar and subparallel lamellae, thicker than common PDFs, were found in some quartz grains abutting melt pockets and melt veins Shock metamorphism and impact melting at Kamil crater 11 Fig. 9. BSE images of features in sandstone ejecta sample L23. a) Portion of a quartz grain showing four sets of PDFs. The four PDF orientations have been highlighted with four white-dashed lines. b) Quartz grain showing two main sets of enlarged PDFs, which underwent preferential melting. The inset shows an enlarged PDF containing a tiny elongated vesicle (approximately 0.3 lm). The bright aggregate on the left side of the image is a fine intergrowth of baddeleyite (ZrO2) and a SiO2 phase, resulting from the decomposition of a zircon crystal. c) Tourmaline grain surrounded by vesicles up to 50 lm in diameter as evidence of incipient melting at the contact with the surrounding quartz grains. White rectangle indicates the area of the close-up view of Fig. 9d. d) Close-up view of the rectangular area highlighted in (c) showing PFDs in tourmaline. e) Intergranular SiO2 melt (lechatelierite) with a highly vesicular central region. f) Close-up view of the glass area in (e) showing the textural relationships between the intergranular lechatelierite and PDFs in adjacent quartz grains. (Fig. 9b). They show undulose margins with the host grain. They are in optical and compositional continuity with the surrounding melt and they can contain tiny vesicles (see inset in Fig. 9b). We interpret them as original PDFs that enlarged as consequence of the ongoing quartz melting along them. Actually, they represent an intermediate stage of the melting of quartz grains. We refer to these lamellae as enlarged PDFs. A similar occurrence of enlarged PDFs in quartz grains has been reported by Ebert et al. (2013; Fig. 5a, Qtz-B). Quartz grains with enlarged PDFs were not used for the PDF crystallographic orientation measurements and statistics. Tourmaline: Planar deformation features were also observed in tourmaline (Figs. 9c and 9d). They are generally short, namely <10 lm in length, and thin, 12 Table 5. Crystallographic orientations of PDFs in quartz from sample L23 and from the sandstone clasts embedded in dark impact melt lapilli and bombs. PDF Reference # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 e 2+e Miller-Bravais Indices (0001) {1013} {10 12} {10 11} {10 10} {11 22} {11 21} {21 31} {51 61} {11 20} {22 41} {31 41} {40 41} {51 60} {10 14} Unindexed 0 0 35 23% 21 14% 17 11% 0 0 11 7% 0 0 3 2% 1 1% 2 1% 3 2% 7 5% 1 1% 0 0 19 13% 23 15% 6 5% 0 0 10 17% 7 12% 9 15% 0 0 10 17% 0 0 2 3% 0 0 0 0 1 2% 1 2% 0 0 1 2% 6 10% 9 15% 3 6% A. Fazio et al. Sample L23 Total number of sets Absolute frequency Sandstone clasts Total number of sets Absolute frequency Shock metamorphism and impact melting at Kamil crater 13 Fig. 10. Histograms showing the results of PDF indexing for quartz in quartzarenite sample L23 (a) and in sandstone clasts occurring in dark glass (b). Indexed PDFs are marked in gray, while unindexed PDFs are marked in black. (c-d) Histograms for the same samples with polar angle values of PDFs binned at 5° intervals (i.e., the estimated measurement error). The measured sets of PDFs that are within the area of overlap between the index {10 13} and the index {10 14} are shown in gray. namely <0.1 lm in thickness. At least three PDF sets were identified in a single tourmaline grain. Some of them are decorated by tiny vesicles. High-Pressure Phases We found three high-pressure phases in sample L23: coesite, stishovite, and diamond. Coesite is the most abundant. It was positively identified by micro-Raman and X-ray powder diffraction (XRPD) and imaged in the field emission scanning electron microscope (FESEM); stishovite was identified only by XRPD; diamond only by micro-Raman. Figure 11 shows the XRPD spectrum of the denser than 2.675 g cm
3 powder of sample L23. Besides the peaks of quartz, two additional peaks occur at 2h = 28.9° and 30.4°. They are the 100 intensity peaks of coesite (040) and stishovite (110), respectively. No other peaks related to these phases are detectable, due to their low intensities and overlap with the peaks of quartz. Coesite was confirmed both as single phase and associated with diaplectic glass/SiO2 melt by Raman microspectroscopy (Fig. 12a). Coesite peaks are all slightly shifted toward higher Raman shifts by approximately 2 cm
1 with respect to literature values Fig. 11. X-ray powder diffraction pattern for sample L23 (fraction denser than 2.675 g cm
3). All X-ray peaks are from quartz, with the exception of the peaks at 2h values of 28.9° and 30.4°, which are the 100 intensity peaks of coesite (040) and stishovite (110), respectively. (e.g., Frezzotti et al. 2012). Full width at half maximum of the coesite peak at 523 cm
1 ranges from 6.2 cm
1 to 11.6 cm
1. Optical microscope observations indicate that coesite is present within roundish domains of silica glass set between highly deformed quartz grains 14 A. Fazio et al. Fig. 12. High-pressure silica phases occurring in quartzarenite sample L23. a) Raman spectra for coesite and for coesite + diaplectic glass/SiO2 melt of sample L23. Intensity of the spectrum for coesite + diaplectic glass/SiO2 melt was enhanced five times with respect to the spectrum for coesite. b) Photomicrograph of intergranular colorless SiO2 melt surrounded by brownish cryptocrystalline and amorphous material (PPL). c) BSE image of the area of photomicrograph (b). The arrows in (b) and (c) indicate the same vesicles within the colorless SiO2 melt. d) Detail of the outer zone (white rectangle in [c]) made up of submicrometric coesite grains (C) embedded in a glassy matrix (G). (Fig. 12a). They were observed in approximately 60 different occurrences for each thin section of sample L23. The average size of these coesite-bearing domains is 400 lm in diameter. Their cores are colorless in transmitted light and may host a few vesicles. Their peripheries are turbid brown in transmitted light. Backscattered FE-SEM images show that these areas are composed of myriads of cryptocrystals (from 0.1 to 2 lm in size) of coesite, some of them showing resorbed crystal boundaries (Fig. 12d). They are embedded in a diaplectic glass/SiO2 melt. The abundance of diaplectic glass/SiO2 melt increases toward the colorless vesiclebearing core. The chemical composition of one of these roundish aggregates is reported in Table 6. Micro-Raman spectra of a melt pocket within sample L23 further revealed the occurrence of a peak centered at 1336 cm
1 (Fig. 13), and although shifted toward higher Raman shifts (approximately 4 cm
1 respect to literature values; e.g., Frezzotti et al. 2012), it corresponds to diamond. Full width at half maximum of the diamond peak ranges from 3.2 cm
1 to 6.4 cm
1. Diamonds are up to 1 lm in size and are commonly associated with diaplectic glass/SiO2 melt. Zircon Decomposition Zircon is one of the most common accessory phases in the sandstones of Kamil. Some of the zircon grains in the shocked sandstone L23 and in shocked sandstone clasts are decomposed into baddeleyite (ZrO2) + SiO2 fine intergrowths (Fig. 9b). A single decomposed zircon was also found in the impact melt bomb M23. Intergranular Melt Thin films (from a few micrometers up to 200 lm) of glass occupy the intergranular space between some quartz grains showing PDFs and between quartz and accessory minerals in sample L23. The composition of these intergranular melts varies from nearly pure lechatelierite (SiO2 = 98.6 wt%; Table 6) to a “kaolinite” glass (Al2O3/SiO2 = 0.8; Table 6). Figures 9e and 9f show details of an intergranular glass film with nearly pure SiO2 composition (SEM-EDS analysis) characterized by a highly vesicular central portion. In quartz crystals adjacent to glass films, planar amorphous lamellae occur; they could be interpreted as enlarged PDFs or as injected melt in fractures. They are Table 6. Representative electron microprobe analyses (oxide wt%) of glasses and mineral phases of ejecta sandstone samples L23 and M25. SiO2 Al2O3 Na2O MgO CaO K2O FeOtot MnO TiO2 Cr2O3 NiO V2O3 P2O5 ZrO2 Total M25 Coesite aggregate Intergranular glass Intergranular glass Fe-Ti oxides Glass 1a Glass 2b Melt veins 98.7 b.d.l. b.d.l. b.d.l. 0.02 b.d.l. 0.01 b.d.l. 0.01 n.d. b.d.l. n.d. b.d.l. b.d.l. 98.8 98.6 0.05 b.d.l. b.d.l. 0.02 0.01 0.03 b.d.l. b.d.l. n.d. b.d.l. n.d. b.d.l. b.d.l. 98.7 45.2 36.1 0.25 0.07 0.18 0.12 0.99 0.01 3.13 n.d. b.d.l. n.d. 0.14 0.14 86.3 0.49 0.53 b.d.l. 0.36 0.20 0.01 23.1 1.02 68.5 0.21 0.02 3.02 0.13 b.d.l. 97.6 1.34 0.52 b.d.l. 0.41 0.14 b.d.l. 21.5 1.00 67.9 n.d. b.d.l. n.d. 0.09 b.d.l. 93.0 74.7 0.68 b.d.l. 0.07 0.06 0.03 4.21 0.18 12.1 n.d. b.d.l. n.d. 0.06 b.d.l. 92.1 98.7 b.d.l. b.d.l. b.d.l. 0.02 b.d.l. 0.01 b.d.l. 0.01 n.d. b.d.l. n.d. b.d.l. b.d.l. 98.8 46.1 27.8 0.15 0.09 0.08 0.10 1.22 b.d.l. 1.62 n.d. b.d.l. n.d. 0.25 0.14 87.3 53.7 32.9 0.67 0.17 0.23 0.18 1.39 0.04 2.06 n.d. 0.02 n.d. 0.17 0.01 91.5 46.9 39.3 0.26 0.11 0.20 0.41 1.27 b.d.l. 3.16 n.d. 0.02 n.d. 0.08 0.11 91.9 b.d.l. = below detection limit. Detection limits are 0.04 wt% for Na2O and ZrO2; 0.02 wt% for Al2O3 Cr2O3, and V2O3; and 0.01 wt% for MgO, CaO, K2O, FeO, MnO, TiO2, NiO, and P2O5; n.d. = not determined. a Analysis of the glass from approximately 10 lm from the fine intergrowths of Fe-Ti oxides (see text for explanation). b Analysis of the glass from approximately 130 lm from the fine intergrowths of Fe-Ti oxides (see text for explanation). FeOtot: Total iron as FeO. Shock metamorphism and impact melting at Kamil crater L23 15 16 A. Fazio et al. minerals occur in the veins. Quartz is the most common and sometimes it shows planar or subplanar amorphous lamellae (Fig. 14b). These structures resemble the enlarged PDFs occurring in shocked quartz grains of sample L23 (Fig. 9b). The chemical composition of glass in melt veins is the result of the mixing between kaolinite-derived, quartzderived, and Fe-Ti oxide-derived melts. The Al2O3/SiO2 ratio of the glasses of melt veins varies between 0 and 0.84 (Table 6; Fig. 14c; Al2O3/SiO2 ratio of pure kaolinite endmember = 0.85), with most values above 0.5 indicating that the kaolinite fraction in the melt is generally above 50%. The veins brighter in the SEM-BSE images contain up to 3.16 wt% and 1.39 wt% of TiO2 and FeO, respectively, as a result of the melting of Fe-Ti oxides (Table 6; Figs. 14a and 14b). These compositional variations give rise to schlieren, which emphasize flow textures (Figs. 14a and 14b). No high-pressure phases were identified by Raman microspectroscopy in the melt veins. Fig. 13. Raman spectra for diamond and for diamond + diaplectic glass/SiO2 melt found in diaplectic glass/SiO2 melt pocket of sample L23. Diaplectic glass/SiO2 melt peaks are those with the asterisk. Intensity of the spectrum for diamond + diaplectic glass/SiO2 melt was enhanced five times respect the spectrum for diamond. in optical and compositional continuity evidencing an intimate genetic relationship. Figure 9c shows a tourmaline crystal whose boundaries abutting quartz grains underwent melting. Melting is documented by the occurrence of vesicles up to 50 lm in diameter. The glass surrounding a Fe-Ti oxide crystal shows a compositional variation that we interpret as the mixing between a Fe-Ti-rich melt and a SiO2 melt (Table 6). The contents of FeO and TiO2 are, respectively, 23.1 wt% and 68.5 wt% in the unmelted oxide, 21.5 wt% and 67.9 wt% in the glass analyzed at approximately 10 lm from the oxide, and 4.21 wt% and 12.1 wt% in the glass analyzed at approximately 130 lm from the oxide. The TiO2/FeO ratio is, however, almost unchanged (3.0 in the unmelted portion, 3.1 in the glass at approximately 10 lm from the oxide boundary, and 2.9 in the glass at approximately 130 lm from the oxide boundary). Intergranular melts have been also observed between quartz and zircon grains in sample L23. Melt Veins A few melt veins crosscut sample M25. They are typically a few tens of micrometers wide with some up to 0.2 mm (Figs. 5f and 14). The contact with the host rock is sharp and no shock features occur in the latter (Fig. 14a). Occasionally, orthogonal or quasi-orthogonal injection veins propagate from a main vein (Fig. 14a). Several relict Melt in Shatter Cones The external surfaces of the pale wacke sample R01 show mesoscopic fractures with striae that are arranged in poorly defined cm-scale horse-tail patterns in hierarchical branched structures (Figs. 3d, 15a, and 15b). Some striated surfaces have semiconical morphology (Fig. 15b). Overall, we identify these structures as shatter cones. The striated surfaces of the cones are coated by thin films (<200 lm in thickness) of glass (white films indicated by black arrows in Fig. 15b). Sections orthogonal to the cones (Figs. 15c and 15d) show undulating sample morphology. In thin section (Fig. 15e), the thin films of glass are nearly opaque. Backscattered electron images reveal that the glass is highly vesicular and clast-laden (Figs. 15f and 15g). Clasts include mainly relicts of silica mineral grains. Backscattered electron images of sections broadly parallel to the direction of the striae show the fracture number in the host wacke increases by about 1.59 on approaching the contact with the melt (see Fracturing, Reduction in Birefringence and Undulose Extinction section), and that vesicles in the melt are coherently stretched defining a shear fabric (Fig. 15g). The major axes of the vesicles form an angle of approximately 45° relative to the contact with the host wacke. The glass is silica- and alumina-rich with Al2O3/SiO2 = 0.48 (SEMEDS analyses), very close to the Al2O3/SiO2 ratio (0.46; SEM-EDS analyses) of the whole rock. DISCUSSION All samples from the crater wall of Kamil and the largest ejecta boulder (sample L07) do not show any shock feature; these rocks, thus, experienced shock Shock metamorphism and impact melting at Kamil crater 17 Fig. 14. BSE images of ejected wacke sample M25. a) Finely vesicular melt vein. The bright material is enriched in Fe and Ti. Note the straight contact with the undeformed host rock, and the arrowed injection vein on the right side of the vein. b) Close-up view of the white rectangular area in (a) showing a finely vesicular portion with schlieren and relict quartz grains with planar amorphous lamellae. c) Highly vesicular silicaand alumina-rich melt forming a melt vein. It is the result of the preferential melting of the phyllosilicate mineral matrix and quartz of the host rock. All other ejecta samples exhibit a wide range of shock features from fracturing to impact melting. Shock features have been divided into two categories, as a function of their abundance at the thin section scale (1) pervasive shock features are the most abundant and can represent up to the 100 vol% of the sample: they include fracturing, PDFs, and impact melt lapilli and bombs, and (2) localized shock features occur in <1 vol% of the sample: they include high-pressure phases and localized impact melting in the form of intergranular melt, melt veins, and melt films on shatter cones. Pervasive shock features also allow us to estimate shock pressures suffered by the target rocks, whereas localized shock features are the consequence of local enhancement of shock pressure and temperature in correspondence to heterogeneities of the target rock. Pervasive Shock Features and Shock Pressures pressures <1 GPa (Kieffer 1971; Table 1). The only impact-related macroscopic structures observed on crater wall samples are striated surfaces on tear fault planes described by Urbini et al. (2012). According to Kowitz et al. (2013a) the fracture density is directly proportional to the shock pressure suffered by rocks. As the porosity of sandstone used for the experimental calibration by Kowitz et al. (2013a) is higher than the porosity of sandstone rocks of Kamil area, we cannot apply their calibration to our sample. We can only constrain the shock pressure suffered by sample M24 (approximately 60 f mm
1), M25 (approximately 100 f mm
1), R01 (approximately 110 f mm
1), and R02 (approximately 115 f mm
1) to below 5 GPa and suggest that pressure increased from sample M24 to sample R02. Shock pressure below 5 GPa for sample R01 showing shatter cones is consistent with the most common low-pressure formation regime for shatter cones, ranging from approximately 2 to approximately 10 GPa (French 1998). Sample L23, which has a fracture density of approximately 450 f mm
1 suggests the highest shock pressures (>>5 GPa), that is confirmed also by the occurrence of PDFs. The overall distribution of crystallographic orientations of PDFs in quartz crystals of sample L23 and of the sandstone clasts embedded in impact melt is similar. They are both dominated by {10 13} 18 A. Fazio et al. Fig. 15. Mesoscopic and microscopic features of wacke sample R01. a) Shatter cone structures with striae arranged in a horsetail patterns. b) Close-up view of the rectangular area in image (a). Striations on the shatter cone surface radiate from a common apex. They are discontinuously coated with by a white film (100s of lm thick) of silica-rich glass (black arrows). The white arrow indicates where images (c) and (d) were taken, and where the thin section featured in (e) was cut. c) A cross sectional view of the shatter cone surface coated by silica-rich glass. d) Close-up view of the same feature shown in (c). e) Mosaic of photomicrographs of a thin section of sample R01 cut perpendicular to the shattered surface. The silica-rich melt appears as a discontinuous brown coating. The black arrow indicates where the BSE image shown in (f) was taken. f) BSE image of the silicarich glass coating. The glass is highly vesicular and contains quartz relics (see white arrows) of the host rock. g) BSE image of the silica-rich glass coating of a section broadly parallel to the direction of the striae; vesicles in the melt are coherently stretched defining a shear fabric forming an angle of approximately 45° (see dashed white lines) relative to the contact with the host wacke. orientations and by the lack of (0001) orientations (Table 5; Fig. 10). In sedimentary targets, the formation of PDFs with (0001) and {10 13} orientations is usually prevented because the shock energy is initially used to close the pores of the target rocks (Robertson 1980). As a result, PDFs are generally rare in sedimentary targets (e.g., 5 vol% in Coconino Sandstone, Barringer crater, Arizona, USA; Robertson 1980) or their orientations are dominated by high angles to the c-axis (e.g., {1122}, {1011}; Grieve and Therriault 1995; Grieve et al. 1996). Some target rocks at Kamil have low porosity (e.g., porosity in crater wall sandstones is <17 vol% and typically <4 vol%; Table 1). It is thus likely that only a small fraction of the shock energy was used for pore collapse, thereby preventing formation of PDFs with (0001) orientation and allowing formation of PDFs with {1013} orientation. In other words, due to their low porosity, some of the Kamil target rocks behaved like crystalline rocks. This behavior allows us to infer shock pressures using experimental calibrations on single Shock metamorphism and impact melting at Kamil crater quartz crystals reported in the literature (e.g., St€ offler and Langenhorst 1994). The distribution of PDF orientations in sandstone clasts exhibits a higher abundance of high angles to c-axis orientations (>40°) relative to sample L23 (Table 5; Fig. 10). The distribution in sandstone clasts is dominated by the {1013}, {11 22}, and {10 11} orientations, indicating shock pressures between 10 and 20 GPa. The distribution in sample L23 is dominated by index {1013} and {10 12}, indicating shock pressures between 20 and 25 GPa. Planar deformation features are distributed in domains within individual crystals; their maximum length is approximately 200 lm; their spacing ranges from 0.2 lm to 2 lm; the maximum number of PDF sets occurring within a single crystal domain is four. In nonporous crystalline rocks, melting of individual minerals starts at approximately 40 GPa, whereas whole-rock melting starts at approximately 60 GPa (St€ offler 1971). In sandstones (e.g., Coconino Sandstone, porosity: 10–20 vol%), shock melting of individual quartz grains starts at pressures as low as approximately 5 GPa and whole-rock melting occurs above approximately 30–35 GPa (Kieffer et al. 1976; Kowitz et al. 2013b). Recent experiments on porous sandstones (porosity: 25–30 vol%) show that approximately 80 vol% of the target material melts or transforms into high-pressure phases at 17.5 GPa (Kowitz et al. 2013b). Whole rock melting at Kamil is documented by impact melt lapilli and bombs (Figs. 3c, 6, and 7; Table 2). These consist of white lechatelierite and black Fe-Ni-rich glass (Figs. 6 and 7; Table 4). The former is the result of bulk melting of the target rocks. The latter testifies to melting of the iron meteorite projectile and mixing with the targetderived melts (see also Folco et al. 2011). In particular, the nearly pure SiO2 composition (SiO2 > 96 wt% and Al2O3 <0.24 wt%; Table 4) of the white glass indicates that the most likely precursor material among the target rocks studied in this work was similar to the quartzarenite almost free or devoid of kaolinite-rich matrix (e.g., samples L02 and M26 collected from the crater wall at the top of the stratigraphic sequence). The porosity of these quartzarenite rocks is extremely low (typically <4 vol%; Table 1) due to pervasive growth of syntaxial quartz (e.g., Fig. 4d). Sedimentary target rocks of this nature are expected to behave like crystalline rocks. We thus infer that shock pressures experienced by the precursor rocks of the white glass were between approximately 30 and approximately 60 GPa. More likely, shock pressures were >50 GPa, on the basis numerical modeling by W€ unnemann et al. (2008) for rocks with a very low initial porosity (approximately 4 vol%). 19 Impact Velocity The planar impact approximation (Melosh 2013) provides a theoretical basis for constraining a fundamental parameter for the definition of the Kamil impact scenario: the impact velocity at the contact with the target. The vertical component of the impact velocity was calculated using the linear shock-particle velocity equation of state parameters of an iron meteorite projectile and a sedimentary target with the physical properties of the Coconino Sandstone (Melosh 2013). Input shock pressures were 30 GPa and 60 GPa, i.e., the range of maximum shock pressures recorded by the target rocks at Kamil that generated impact melt lapilli and bombs. The results of the planar impact approximation indicate that the vertical component of the impact velocity is 3.5 km s
1 for 30 GPa and of 5.5 km s
1 for 60 GPa. Assuming an impact angle of 45°, as inferred by Urbini et al. (2012), the Gebel Kamil iron projectile impacted the ground with a face-on impact velocity between 5.0 km s
1 (30 GPa) and 7.5 km s
1 (60 GPa). Localized Shock Features The closure of the pores upon shock loading of porous and heterogeneous target rocks can generate local amplification of shock pressure and temperature (e.g., Grady 1980; W€ unnemann et al. 2006, 2008; Schade and W€ unnemann 2007; Ogilvie et al. 2011; Kowitz et al. 2013b). For instance, experimental work and modeling by G€ uldemeister et al. (2013) showed that the initial shock pressure in porous and heterogeneous sandstones may experience a four-fold increase. Pressure amplification due to pore collapse can, thus, determine the localized occurrence of high-pressure phases and melting in rocks that underwent low to intermediate shock pressures (5 GPa–20 GPa). A wide range of localized shock features are recorded in the sandstone target rocks at Kamil (Table 1). They can be divided into two main groups (1) high-pressure phases, namely coesite, stishovite, and diamond; (2) localized shock melt, namely intergranular shock melt, melt veins, and shock melt films on shatter cones. Some of these localized shock features have never been reported before from small impact structures (Table 7). Three high-pressure mineral phases were identified in sample L23 (Table 1): coesite, stishovite, and diamond. Coesite occurs in a roundish area, described in Figs. 12b and 12c. Figure 12d shows that coesite occurs as irregularly shaped submicrometric crystals more abundant toward the periphery of intergranular diaplectic glass/SiO2 melt pockets. Silica glass pockets 20 Table 7. Summary table of shock features in small terrestrial impact craters with diameters ranging from 13.5 m to 1.5 km. Data about Kamil Crater are highlighted in bold. PF and/ or PDF HP phases Diaplec-tic glass Melt veins Interstitial glass Whole-rock impact glassa Projectile impact meltb Shock in other minerals Crater Location Target Projectile Age (Ma) Shatter cones Notes Ref. Carancas Haviland Peru Kansas, USA Australia 0.0135 0.015 S S Chondrite pallasite 0.000007 0.001 n.r. n.r. x deb. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. Melt deb. (1), (2), (3) (1) 0.024 C stony-iron 0.02 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. dens. of Qtz (r.a.f.) (1), (4) Russia Alberta, Canada Egypt 0.027 0.036 C S iron-IIAB iron-IIIAB 0.000067 <0.0011 n.r. n.r. n.r. x deb. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. x n.r. n.r. 0.045 S iron-ung. <0.005? x x x* x x x x L and B x Argentina 0.05 C <0.004 n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. Russia Estonia Australia Poland Estonia 0.053 0.08 0.08 0.1 0.11 C S S S S n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. x (r.a.f.) n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. (1) (1) (1) (1) (1), (7), (8) Saudi Arabia 0.116 S <0.001 0.0066 <1 <0.1 0.004  0.01 0.00014 n.r. n.r. n.r. n.r. x Wabar iron-IABcomplex iron unk. iron-IIAB iron iron-IABcomplex iron-IIIAB x(-Fe-Ti oxide, Tur, Zrn) n.r. n.r. x x coe n.r. n.r. n.r. x L and B x n.r. Henbury Australia 0.157 S iron-IIIAB n.r. n.r. n.r. n.r. n.r. n.r. xL x n.r. Odessa 0.168 S n.r. n.r. n.r. n.r. n.r. x (r.a.f.) x n.r. (1), (15) 0.17 C iron-IABcomplex iron-IIAB n.r. Boxhole Texas, USA Australia 0.0042  0.019 <0.0635 (1), (9), (10), (11), (12), (13) (1), (13), (14) n.r. n.r. n.r. n.r. n.r. n.r. n.r. x n.r. (1), (16) Macha Russia 0.3 S iron n.r. x x Stv n.r. n.r. n.r. Only one glassy spherule (0.12 mm) n.r. (1), (17) Aouelloul Mauritania 0.39 S iron n.r. x n.r. n.r. n.r. n.r. x 5 a-iron particles, only one spherule x x (Zrn) (1), (18) Amguid Algeria 0.45 S unk. n.r. x (planar elements; r.a.f.) n.r. n.r. n.r. n.r. n.r. n.r. n.r. (1), (19) Dalgaranga Sikhote Alin Whitecourt Kamil 0.0054  0.0015 <0.007 3.0  0.3 <0.1 (1) (1), (5) *Coe, Stv, and Dia (1), (6) (1) A. Fazio et al. Campo del Cielo Sobolev Ilumets€a Veevers Morasko Kaalij€arv Diameter (km) Table 7. Continued. Summary table of shock features in small terrestrial impact craters with diameters ranging from 13.5 m to 1.5 km. Data about Kamil Crater are highlighted in bold. HP phases Diaplec-tic glass Melt veins Interstitial glass Whole-rock impact glassa Projectile impact meltb Shock in other minerals Crater Location Target Projectile Age (Ma) Shatter cones Monturaqui Chile 0.46 C <1 n.r. x x Coe x n.r. n.r. x x x (Pl, Bt, Ap) Kalkkop South Africa Australia 0.64 S iron-IABcomplex unk. x s.c.-like x n.r. x n.r. n.r. x n.r. n.r. 0.875 S iron-IIIAB 0.25  0.05 <0.3 n.r. x (r.a.f.) n.r. n.r. n.r. n.r. n.r. n.r. n.r. South Africa 1.13 C chondrite 0.220  0.052 n.r. x n.r. x n.r. n.r. x x sulfides spherules X (Afs) Arizona, USA Mongolia 1.19 S x n.r. x x x x n.r. n.r. x Coe Stv n.r. n.r. C n.r. n.r. n.r. n.r. n.r. Finland Finland 1.5 1.5 C C unk. unk. 0.049  0.003 150  20 230 >600 n.r.* 1.3 iron-IABcomplex unk. x (r.a.f.) x x (r.a.f.) x n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. n.r. Wolfe Creek Tswaing (ex-Pretoria Saltpan) Barringer TabunKhara-Obo Karikkoselk€a Saarij€arvi Notes Ref. (1), (13), (20) suevite breccia dens. of Qtz (r.a.f.) suevite breccia (1), (21) *suspected n.r. (1), (9), (24), (25), (26) (1) x (Bt, Afs) (r.a.f.) x (Bt, Chl) (1), (27) (1), (28) (1), (22) (1), (23) Ref. = references; S = sedimentary; C = crystalline; ung. = ungrouped; unk. = unknown; x = observed; n.r. = not reported; r.a.f. = reported in abstract form; deb. = debated; dens. = densification; L = lapilli; B = bombs; s.c. = shatter cones; Coe = coesite; Stv = stishovite; Dia = diamond; Qtz = quartz; Tur = tourmaline; Zrn = zircon; Pl = plagioclase; Bt = biotite; Ap = apatite; Afs = alkalifeldspar; Chl = chlorite. a Whole-rock impact glass indicates a glass resulting only by the melting of the target rocks. b Projectile impact melt includes both whole-rock impact glass contaminated by meteoritic components and metal spherules. (1) http://www.passc.net/EarthImpactDatabase/saarijarvi.html; (2) Tancredi et al. (2009); (3) Kenkmann et al. (2009); (4) Miura and Kato (1991); (5) Kofman et al. (2010); (6) This work; (7) Dietz (1968); (8) Smith and Hodge (1993); (9) Bunch and Cohen (1964); (10) Chao et al. (1961); (11) H€ orz et al. (1989); (12) Mittlefehldt et al. (1992); (13) Gibbons et al. (1976); (14) Taylor (1967); (15) Smith and Hodge (1997); (16) Hodge and Wright (1970); (17) Gurov and Gurova (1998); (18) Koeberl et al. (1998); (19) McHone et al. (1990); (20) Bunch and Cassidy (1972); (21) Reimold et al. (1998); (22) Miura (1995); (23) Reimold and Koeberl (2014); (24) Mark (1995); (25) Kieffer et al. (1976); (26) Mittlefehldt et al. € (2005); (27) Personen et al. (1999); (28) Ohman and Preeden (2013). Shock metamorphism and impact melting at Kamil crater PF and/ or PDF Diameter (km) 21 22 A. Fazio et al. are often vesicular toward the center (Figs. 12b and 12c). The microstructural setting of coesite in sample L23 is very similar to that of coesite found by Kieffer et al. (1976; fig. 7a, p. 56) in Coconino Sandstones shocked between 20 and 30 GPa and described as symplectic regions. Kieffer et al. (1976) proposed that these microstructures were formed by a complex multistage process of phase changes (see fig. 25, p. 89, in Kieffer et al. 1976), summarized as follows (1) a hot, amorphous water-bearing silica material (called “jet”) is injected into the pores during the shock wave passage; (2) during the initial decompression crystals of coesite nucleate and grow, water present in the initial jet concentrates in the residual melt; (3) upon further pressure release, crystals of coesite in the central region of the original pore begin to melt; (4) subsequently, water vapor exsolves from the melt forming vesicles within the glass. Due to the similarity between the coesite-melt microstructures observed in shocked quartzarenite rocks from Kamil and in shocked Coconino Sandstone from Barringer crater, we suppose that the above-described process probably occurred also during the shock metamorphism of Kamil target rocks. The full width at half maximum for the coesite main peak and diamond peak indicate that these phases are crystalline. Peaks for coesite and diamond are slightly shifted toward higher Raman wave numbers, suggesting a moderate confining pressure (e.g., Hemley 1987) probably exerted by the diaplectic glass/SiO2 melt that has a higher volume than other SiO2 phases (Figs. 12a and 13). Films of intergranular melt occurring in quartzarenite sample L23 at the contact between quartz grains have a variable composition from pure SiO2 to pure kaolinite. This indicates that the precursor material of sample L23 contained some (likely small) amount of phyllosilicate mineral matrix. The films of silica glass are often in continuity with enlarged PDFs (Fig. 9b). The sequence of images shown in Figs. 9a and 9b features some analogies with the PDF formation model proposed by Langenhorst (1994). According to this model, PDFs start to form by solid-state amorphization at pressures between 5 GPa and 10 GPa, i.e., at a shock pressure regime unable to produce shock temperatures and postshock temperatures above the melting temperature of quartz (Fig. 9a). Between 25 GPa and 35 GPa small bands of melt form in correspondence to PDFs, because shock temperature exceeds melting temperature. Over 35 GPa, both shock and postshock temperatures are higher than melting temperature and the quartz crystals are more extensively consumed by melting (Fig. 9b). Finally, over 50 GPa lechatelierite forms (see inset Fig. 9b). Localized shock melting due to amplification of shock pressures and temperatures associated to pore collapse is also documented at the crystal boundaries between accessory mineral and quartz in sample L23 (Fig. 9c). Note that when hydrated minerals like tourmaline are involved, the melt volume and the vesicle size are greater than that produced by anhydrous minerals, documenting a role of volatiles in shock melting. The chemical composition of the melt veins of sample M25 (Table 6) indicates that they derived from the preferential melting of the phyllosilicate mineral matrix of the host rock. Phyllosilicate minerals have lower melting and breakdown temperatures (up to 650 °C), Mohs number (up to 4), indentation hardness (up to 2 GPa), yield strength (up to 0.66 GPa), and shear yield strength (up to 30 GPa) than other common rock-forming minerals (Spray 2010). We thus conclude that coarse siltstone levels in sample M25 played an important role in the formation of these melt veins. Sample L13 show alternating sandstone and siltstone levels and it could represent the protolith of sample M25 (Table 1; Fig. 4b). The hundred micrometers thick glass films observed on ejecta sandstone sample R01 on shatter cone surfaces (Fig. 15) is characterized by a highly vesicular texture and high silica and alumina composition (Al2O3/ SiO2 = 0.48) consistent with localized melting of the host wacke (Al2O3/SiO2 = 0.46). Melt films at the surface of shatter cones were also reported from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures (e.g., Gay 1978; Gibson and Spray 1998; Nicolaysen and Reimold 1999) and, possibly, from the Santa Fe (New Mexico, USA) impact structure (Fackelman et al. 2008). Although it is generally accepted that shatter cones typically develop at low shock pressures between approximately 2 and approximately 10 GPa (French 1998), the precise mechanism of shatter cone formation is still debated (e.g., Sagy et al. 2002, 2004; Baratoux and Melosh 2003; Wieland et al. 2006). Nonetheless, evidence from Kamil sample R01 confirms that melting conditions can be attained at the interface between adjacent shatter cone surfaces. Furthermore, the micrometric-scale shear fabric observed in the melt films (namely, parallel to the major axis of the shatter cones; Fig. 15g) suggests that frictional melting contributed, at least in part, to their formation, as previously suggested by Nicolaysen and Reimold (1999) based on the occurrence of microdisplacements on the surface of some shatter cones from the Vredefort impact structure. Comparison of Small (<1.5 km in Diameter) Impact Structures Kamil is the sixth smallest impact structure (45 m in diameter) known on Earth. In common with most of very small (<300 m in diameter) and small (<1.5 km in Shock metamorphism and impact melting at Kamil crater diameter) impact structures, Kamil was formed by the impact of an iron or stony-iron meteorite (19 of 27) on a sedimentary target (17 of 27; Table 7). The age of the impact is still uncertain. It is, however, likely younger than 5000 yr based on archeological evidence (Folco et al. 2011) and, therefore one of the ten youngest impact craters of Earth. As discussed in the previous sections, ejecta at Kamil show a wide range of shock features, some of them never reported for impact craters with sizes comparable to Kamil (Table 7). Urbini et al. (2012) estimated that the volume of rock excavated by the impact of the Gebel Kamil meteorite was about 3800 m3. Not all the excavated rocks suffered shock pressure sufficient to produce shock effects, for example the large quartzarenite boulder L07. During impact cratering, the volume of rock that suffered shock pressures between 1 and 5 GPa is much larger than the volume of rock shocked over 5 GPa (French 1998). Moreover, incoherent deposits are easily eroded and weathered. Thus, the common occurrence of shock metamorphosed rocks and impact melt lapilli and bombs is a further proof of the pristine state of preservation of Kamil. There are three reasons for the exceptional state of preservation of Kamil (1) the young age of the impact (most likely <5000 yr; Folco et al. 2011); (2) the mechanical strength of the target rocks, namely quartzarenite (Table 1); and (3) the low erosion rates of arid desert areas (e.g., 1–16 m Ma
1 for the Namib Desert; Bierman and Caffee 2001). Table 7 shows that shock features have been reported only for 19 small impact craters of 27, and often in abstract form only. In very small impact craters <300 m diameter, observations about shock features are reported almost exclusively for impact craters younger than 5000 yr. Besides Kamil, significant shock features have been reported only from Wabar (Saudi Arabia). Extending the comparison to impact craters up to 1.5 km diameter, extensive shock features have been reported for Aouelloul (Mauritania), Monturaqui (Chile), and Barringer (Arizona, USA). The most common shock features in small impact craters are PDFs and impact melt lapilli and bombs. Planar deformation features are, however, rare in very small impact craters (5 of 17 very small impact craters). Shatter cones and high-pressure phases have been reported only from 4 small impact craters (including Kamil). Melt veins have been reported only from Kamil (Table 7), although, in Kalkkop a possible pseudotachylite veinlet was found (Reimold et al. 1998). The shock features found in ejecta from the Kamil indicate that the formation of these features is possible also in very small impact structures formed by metersized projectiles impacting the ground with a high 23 velocity, i.e., >3 km s
1 (hypervelocity impact). Their detection requires a combination of special circumstances like the young age of the crater, the high mechanical strength of target rocks, and minimal burial and low erosion rates. CONCLUSIONS This is a detailed report of the petrography and some chemical observations of samples from the crater wall and ejecta deposits from Kamil crater (Egypt) collected during the first Italian-Egyptian geophysical expedition in February 2010. Data allow us to draw the following conclusions in terms of shock effects recorded by target rocks, impact scenario, and unique state of preservation of the ejecta rocks: 1. A broad set of shock features is recorded in the target rocks at Kamil (45 m in diameter), ranging from fracturing to whole-rock impact melting. 2. Shock features are classified into two categories (1) pervasive shock features, including fracturing, planar deformation features, and impact melt lapilli and bombs, and (2) localized shock features including high-pressure phases and localized impact melts occurring as intergranular melt, melt veins, and melt films enveloping shatter cones. 3. Kamil is the smallest impact structure where shatter cones, coesite, stishovite, diamond, and impact melt (target and projectile) have been reported. 4. Because of the occurrence of impact melt lapilli and bombs, the maximum shock pressure during the impact of the iron meteorite Gebel Kamil was between 30 GPa and 60 GPa. 5. The maximum shock pressures recorded at Kamil can be achieved through face-on impact velocities from 5.0 km s
1 (30 GPa) and of 7.5 km s
1 (60 GPa), assuming an impact angle of 45°. 6. Localized shock features, namely high-pressure phases (coesite, stishovite, and diamond) and localized impact melts (intergranular melt, melt veins, and melt films on shatter cones), are the result of enhanced shock pressure and temperature related to pores collapse and heterogeneities of the target rocks. 7. Shatter cones from Kamil are coated by thin (<200 lm) films of glass, confirming that melting conditions can be attained at the surfaces of shatter cones, and evidence for shear shows that frictional melting contributed, at least in part, to their formation. 8. The hypervelocity impact of meter-sized iron meteorite projectiles can produce shock effects similar to those observed in high velocity, larger impacts. The young age of the crater (most likely <5000 yr), the mechanical strength of target rocks, 24 A. Fazio et al. and the low erosion rates of the hot-desert area played a crucial role in the preservation of all these shock features. Acknowledgments—This work was supported by the Italian Ministero degli Affari Esteri—Progetti di Grande Rilevanza, Protocollo Esecutivo ITALIA-EGITTO. The 2010 geophysical expedition work was carried out within the framework of the 2009 Italian–Egyptian Year of Science and Technology. We thank Prof. M. Alsherbiny (former president of the Egyptian National Academy for Scientific Research and Technology) and Prof. F. Porcelli (Scientific Attache, Italian Embassy, Egypt) for diplomatic and institutional support; the Egyptian Army for logistical support. Agnese Fazio was supported by the PhD School Scuola di Dottorato in Scienze di Base Galileo Galilei program on Earth Science of the University of Pisa, and by the Barringer Family Fund for Meteorite Impact Research 2014. Luigi Folco and Massimo D’Orazio are also supported by the University of Pisa Fondi di Ateneo. Institut des Sciences de la Terre (ISTerre) of Grenoble is part of Labex OSUG@2020 (ANR10 LABX56). Astrid Kowitz and John G. Spray are thanked for constructive reviews, and Uwe Reimold for editorial handling. The authors are grateful to Valentina Batanova, Andrea Cavallo, and Cristian Biagioni for their assistance during electron microprobe analysis at ISTerre (Grenoble), field emission scanning electron microscope analysis at Istituto di Geofisica e Vulcanologia (Rome), and X-ray powder diffraction analysis at Pisa University’s Dipartimento di Scienze della Terra, respectively. Raman facilities by the Center “G. Scansetti” for studies on asbestos and other toxic particulates were funded by the Compagnia di San Paolo, Torino. PNRA is acknowledged for preliminary Raman microspectroscopy analyses. Editorial Handling—Dr. Uwe Reimold REFERENCES Baratoux D. and Melosh H. J. 2003. 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