Arif Sikder

The Intertrapean beds are thin sedimentary sequences, ranging between tens of centimeters to meters in thickness. These beds were deposited in the intermittent, apparently quiescent periods between two successive volcanisms and are sporadically exposed in isolated patches at widely separated geographical areas in the peripheral regions of the Deccan volcanic province. The present study was conducted in one such newly uncovered and well preserved intertappen beds in Gandhwani of Madhya Pradesh. In contrary to the overall paleontological observation that the fossil species of the intertrapean varies from transitional marine to terrestrial, the geochemical signature of these newly exposed intertrappen beds show dysoxic to anoxic (i.e. Ni/Co>1, V/Cr>2 and U/Th>0.75) conditions prevailed with a strong marine influence (Sr/Ba ratio >0.5), during the formation of Gandhwani intertrapean sequence. Mineralogical analysis revealed an rapid upward diagenetic changes of Celadonite [K(Mg,Fe++)(Fe+++,Al)[Si4O10](OH)2] to Nontronite [Na0.3Fe+++2(Si,Al)4O10(OH)2•n(H2O)], an Fe3 rich member of smectite, Celadonite is almost exclusively found associated with basic-eruptive rocks in deep-marine environments due to low-grade metamorphism, <250°C. On the other hand Nontronite is usually forms above the basalt on the seafloor at temperatures <70°C where hydrothermal fluids mix with sea water. Such a rapid change indicates sudden changes in overall geologic setting. Finally the discovery of the Rhyolitic volcanic ash and Rhodonitic micro-lapillies in the middle of studied sections, which were concealed by several phases of alterations since it’s deposition in Late Cretaceous. indicates that the intertrapean may not represent a quiescent geologic period, but rather a violent episode full of proximal to distal explosive hydrovolcanism that was followed by basalt flooding, at the end of Cretaceous.

Abstract. The diamond in Paraguay was reported for the first time in 1825, and around 1960 the first artisanal exploitation was carried out in the city of Capiibary, Department of San Pedro, Eastern Paraguay. The first scientific analysis of Capiibary diamonds was conducted in 2012. A set of millimetersized new crystals of Capiibary diamonds from cratonic lamproite colluvium/alluvium with a higher abundance of inclusion were selected for Raman spectroscopic analysis of the diamonds and their mineral inclusions. The selected set of diamond crystals is transparent brown, light-brown, cognac brown, colorless to colorless with a faint yellow tint, and green to shades of green. The crystals are sharp axes octahedral (some with polycentric development of the crystalline faces) or with partially reabsorbed rounded octa-dodecahedral transition forms; pseudo hemimorphic form and also crystals with irregular rounded shapes; whole to fractured crystals. Raman spectra obtained from the majority of the studied crystals, except one, exhibit an explicit change of D-peak wavenumber (shifts from 1294 to 1353 cm-1) and variable FWHM numbers (>>3 to 197 cm-1) characteristic of diamond with internal defects, as observed in lonsdaleitic diamonds. The lonsdaleitic diamonds signature in the crystal structure of host diamonds associated with the suit of super deep inclusions (Ca-perovskite, bridgmanite, stishovite, and ferropericlase associations) support the conclusion that the Capiibary diamonds were probably derived from a source (subducted Nazca slab) situated in the lower mantle and transformed partially into lonsdaleite or mixed cubic hexagonal polytype structures of diamonds by a kinetic process of shear stress and the shock wave compression induced by provable very high temperature.
Keywords. Capiibary diamond Raman Spectroscopy, lonsdaleitic diamond, lower mantle diamond inclusions, diamond-bearing lamproite, Paraguay

The diamond in Paraguay was reported for the first time in 1825, and around 1960 the first artisanal exploitation was carried out in the city of Capiibary, Department of San Pedro, Eastern Paraguay. The first scientific analysis of Capiibary diamonds was conducted in 2012. A set of millimeter-sized new crystals of Capiibary diamonds from cratonic lamproite colluvium/alluvium with a higher abundance of inclusion were selected for Raman spectroscopic analysis of the diamonds and their mineral inclusions. The selected set of diamond crystals is transparent brown, light-brown, cognac brown, colorless to colorless with a faint yellow tint, and green to shades of green. The crystals are sharp axes octahedral (some with polycentric development of the crystalline faces) or with partially reabsorbed rounded octa-dodecahedral transition forms; pseudo hemimorphic form and also crystals with irregular rounded shapes; whole to fractured crystals. Raman spectra obtained from the majority of the studied crystals, except one, exhibit an explicit change of D-peak wavenumber (shifts from 1294 to 1353 cm-1) and variable FWHM numbers (>>3 to 197 cm-1) characteristic of diamond with internal defects, as observed in lonsdaleitic diamonds. The lonsdaleitic diamonds signature in the crystal structure of host diamonds associated with the suit of super deep inclusions (Ca-perovskite, bridgmanite, stishovite, and ferropericlase associations) support the conclusion that the Capiibary diamonds were probably derived from a source (subducted Nazca slab) situated in the lower mantle and transformed partially into lonsdaleite or mixed cubic hexagonal polytype structures of diamonds by a kinetic process of shear stress and the shock wave compression induced by provable very high temperature.

Detection of an unusual magnetic susceptibility (χ) in the sediments at some locations of karstic Dobra River by the Croatian co-authors, initiated the search for source of magnetic particles in sediments of upper course of the river and anticipated an exceptional origin (volcanic or meteorite impact) of the magnetic particles based on chemical signature (i.e. ratio of Ni:Fe vs. Cr:Fe). In the second phase of the study in collaboration with Virginia Commonwealth University (VCU), the sediment samples were collected from locations with higher χ. Magnetic particles were manually separated from thoroughly washed and dried sediment samples by a rare-earth magnet. Then the magnetic particles were carefully sorted via stereo-microscopy to avoid erosional interference. Phase identification was performed by x-ray diffraction (XRD) analysis using a Cu anode (Cu Kα ≈ 1.54 Ǻ) over a scan range from 5 to 70 degrees 2θ. Multiple phases were identified and estimated that include anorthite (51%), hypersthene (25%), clinopyroxene (11%), ferrous diopside (10%) and magnetite (3%). Polished thin sections were primarily investigated with optical microscopy in polarized transmitted and reflected light revealing that the magnetic spherules are mostly constituted of vesicular glass with quench texture and schlieren. Ballen textures were also observed in the lechatelierite and spectrums were acquired for the ballen textures by Raman microscopy giving the typical peaks for α-cristobalite at 111–113, 227–229, 413–417, and 780–782 cm-1. Further detailed analyses were performed with FIB-SEM (focused-ion beam) equipped with energy dispersive spectroscopy (EDS) and an electron back-scatter diffraction capabilities (EBSD) that revealed an immiscible phases of silicates and devitrification texture in the samples along with elongated laths of clinopyroxene embedded in anorthitic melts. The presence of clinopyroxene was ascertained by electron backscatter diffraction (EBSD). Furthermore, the presence of micro-spherules of iron sulfide with trace concentrations of Cr, Ni and Co along with iron sulfide spherules having compositional variability and Fe rich angular grains, likely troilite, suggest an meteorite impact origin of the magnetic spherules from the sediments of the karstic Dobra River.

Weathering rinds are common in various terrestrial environments and have been used for decades as relative age indicators in Quaternary sequences. However, little study has been conducted on the development and preservation of alteration rinds on metamorphic and igneous clasts in older sedimentary sequences. Metamorphic clasts with conspicuous alteration rinds sporadically occur in a lower Cretaceous sandy gravel of the Potomac Formation exposed in the Puddledock quarry (Vulcan Materials Co). Polished thin sections of clasts were prepared for petrographic and elemental mapping by FIB-SEM with EDS. Major elemental compositions of bulk samples were determined by XRF. The clasts vary in lithology and color, and surfaces range from smooth to irregular due to flaking. Variable major element compositions (wt%), SiO2 (59.2-87.81), Al2O3 (5.76-15.42), Fe2O3 (1.54-19.214) and K2O (.075-12.1), reflect a variety of metamorphic rock types. The alteration rinds are generally uniform in thickness, typically 4 to 9 mm, irrespective of the size and type of the gravels. Most of the clasts have a single outer band of alteration, but some have multiple bands. The latter alteration rinds typically have an outer light-gray to white band and an inner darker band. Some clasts have a thin (typically 2 to 4 mm) outer coating of crystalline silica. The outer rinds (white bands) appear to reflect strong dissolution, probably due to hydrolysis, and are enriched in silica. The inner darker bands are enriched in aluminum (Al), and correspond to the highest level of Chemical Index of Alteration [molar (Al2O3/Al2O3+Na2O+K2O+CaO)*100]. Both outer and inner bands are devoid of significant concentrations of iron (Fe), although cores of some clasts are rich in Fe. Preliminary results are not entirely consistent with typical physical and chemical characteristics of weathering rinds, suggesting that the mechanism of formation may be more complex, such as: (1) formation of the darker Al-rich inner bands and outer light gray to white bands under oxidizing conditions concurrent with transport from a proximal source and deposition in the gravel bed, (2) preservation of weathering rinds in a reducing environment facilitated by burial diagenesis, and (3) further alteration by burial diagenesis with precipitation of silica from pore water on clast surfaces.

To understand the mechanisms of formation and preservation of alteration rinds on the clasts in older sedimentary sequences, pebble-size clasts from the Lower Cretaceous Potomac Fm. were collected from the Puddledock quarry (Vulcan Materials Co.). Sandy gravel in the quarry contains siliceous metavolcanic clasts (rounded pebbles) with conspicuous alteration rinds. Rinds and cores of the clasts were subjected to mineralogical analysis by X-ray diffraction, elemental mapping by micro XRF, and optical petrographic analysis. Irrespective of the size and composition of the clasts, the alteration rinds are generally uniform in thickness (~4 to 9 mm). In most cases, the clasts have a single outer band of alteration, but some have multiple bands. The alteration rinds typically have an outer light-gray to white band and surround a darker core. Some clasts also have a thin (~2 to 4 mm) outer coating of crystalline silica. The cores of the clasts contain quartz (~54.5%), feldspar (~22.8%), kaolinite (~23%), hematite (~1.5%) and traces of anatase (~2.4%), by weight. The rinds (white bands) contain comparatively more quartz (~74%) and feldspar (~16.7%), and lesser amounts of kaolinite (~9.4%), hematite (~0.3%), and anatase (~1%). The enrichment of quartz and depletion of kaolinite, hematite and anatase in the rinds appear to reflect dissolution of feldspar and removal of clay, probably due to continued hydrolysis. Optical petrographic analysis and SEM-EDS revealed hydrous aluminum silicates that fill fractures and fissures in the cores but are depleted in the rinds of most specimens. Concentrations of transition and post-transition metals (i.e. Fe, Al and Ti), along with sulfur, in cores of the clasts exceed those in the rinds. Alkalis do not show any trend in most clasts, but a few show an increased concentration in the rind, perhaps due to incomplete kaolinization. Results indicate that the alteration rinds formed primarily by (1) partial removal of transition and alkali metals by hydrolysis due to chemical weathering during transport and/or after deposition, accompanied by (2) preferential removal of fracture-fill clay minerals in the rinds, followed by (3) preservation in a reducing environment and precipitation of crystalline silica from pore water during burial diagenesis.

ABSTRACT Some years ago an unusual and unexpected anomaly of magnetic susceptibility (MS) was observed in sediments of the upper course of the karstic Dobra River (Croatia) during preliminary measurements of MS throughout the Kupa River drainage basin [1, 2]. In that study, following techniques have been used: magnetic susceptibility and Curie temperature, isothermal remnant magnetism (IRM), mineralogical analysis (using XRD) and chemical analysis (using ICP-MS). Thermomagnetic curves showed a distinctive Curie-point of magnetite at 580 °C, while additional transformation observed at 520–560°C derives from titanomagnetite. Magnetic particles from the Dobra River sediments contain pyroxene, plagioclase, hematite and quartz, in addition to magnetite. The ratio Ni/Fe versus Cr/Fe suggests that the magnetic spherules are most probably impactites. New and exciting finding of magnetic spherules in sediments of this karstic river deserved further field and laboratory research, which continued in collaboration with Virginia Commonwealth University (VCU), USA. Petrographic analysis of polished thin sections in polarized transmitted light revealed that the magnetic particles are mostly made of vesicular glass with quench texture. Ballen textures are also observed in the lechatelierite and consisting of α-cristobalite. Scanning electron microscopy and elemental mapping revealed immiscible phase of silicates and devitrification texture in the glasses. Presence of micro-spherules of iron sulfide with traces of Cr, Ni and Co, along with iron sulfide spherule with immiscible compositional variability and Fe rich angular grains, probably Troilite, further supports already suggested impact origin of the magnetic spherules from the sediments of the karstic Dobra River.

Lead is a potent neurotoxin that is particularly detrimental to children's cognitive development. Batteries account for at least 80% of global lead use and unsafe battery recycling is a major contributor to childhood lead poisoning. Our objectives were to assess the intensity and nature of child lead exposure at abandoned, informal used lead acid battery (ULAB) recycling sites in Kathgora, Savar, Bangladesh, as well as to assess the feasibility and effectiveness of a soil remediation effort to reduce exposure. ULAB recycling operations were abandoned in 2016 due to complaints from residents, but the lead contamination remained in the soil after operations ceased. We measured soil and blood lead levels (BLLs) among 69 children living within 200 meters of the ULAB recycling site once before, and twice after (7 and 14 months after), a multi-part remediation intervention involving soil capping, household cleaning, and awareness-raising activities. Due to attrition, the sample size of children decreased from 69 to 47 children at the 7-month post-intervention assessment and further to 25 children at 14 months. We conducted non-parametric tests to assess changes in soil lead levels and BLLs. We conducted baseline surveys, as well as semi-structured interviews and observations with residents throughout the study period to characterize exposure behaviors and the community perceptions. We conducted bivariate and multivariate regression analyses of exposure characteristics to determine the strongest predictors of baseline child BLLs. Prior to remediation, median soil lead concentrations were 1,400 mg/kg, with a maximum of 119,000 mg/kg and dropped to a median of 55 mg/kg after remediation (p<0.0001). Among the 47 children with both baseline and post-intervention time 1 measurements, BLLs dropped from a median of 21.3 μg/dL to 17.0 μg/dL at 7 months (p<0.0001). Among the 25 children with all three measurements, BLLs dropped from a median of 22.6 μg/dL to 14.8 μg/dL after 14 months (p<0.0001). At baseline, distance from a child's residence to the nearest abandoned ULAB site was the strongest predictor of BLLs and baseline BLLs were 31% higher for children living within 50 meters from the sites compared to those living further away (n=69, p=0.028). Women and children spent time in the contaminated site daily and relied on it for their livelihoods and for recreation. Overall, this study highlights the intensity of lead exposure associated with the ULAB recycling industry. Additionally, we document the feasibility and effectiveness of a multi-part remediation intervention at a contaminated site embedded within a residential community; substantially reducing child BLLs and soil lead concentrations.

Lead is a potent neurotoxin that is particularly detrimental to children's cognitive development. Batteries account for at least 80% of global lead use and unsafe battery recycling is a major contributor to childhood lead poisoning. Our objectives were to assess the intensity and nature of child lead exposure at abandoned, informal used lead acid battery (ULAB) recycling sites in Kathgora, Savar, Bangladesh, as well as to assess the feasibility and effectiveness of a soil remediation effort to reduce exposure. ULAB recycling operations were abandoned in 2016 due to complaints from residents, but the lead contamination remained in the soil after operations ceased. We measured soil and blood lead levels (BLLs) among 69 children living within 200 meters of the ULAB recycling site once before, and twice after (7 and 14 months after), a multi-part remediation intervention involving soil capping, household cleaning, and awareness-raising activities. Due to attrition, the sample size of children decreased from 69 to 47 children at the 7-month post-intervention assessment and further to 25 children at 14 months. We conducted non-parametric tests to assess changes in soil lead levels and BLLs. We conducted baseline surveys, as well as semi-structured interviews and observations with residents throughout the study period to characterize exposure behaviors and the community perceptions. We conducted bivariate and multivariate regression analyses of exposure characteristics to determine the strongest predictors of baseline child BLLs. Prior to remediation, median soil lead concentrations were 1,400 mg/kg, with a maximum of 119,000 mg/kg and dropped to a median of 55 mg/kg after remediation (p<0.0001). Among the 47 children with both baseline and post-intervention time 1 measurements, BLLs dropped from a median of 21.3 μg/dL to 17.0 μg/dL at 7 months (p<0.0001). Among the 25 children with all three measurements, BLLs dropped from a median of 22.6 μg/dL to 14.8 μg/dL after 14 months (p<0.0001). At baseline, distance from a child's residence to the nearest abandoned ULAB site was the strongest predictor of BLLs and baseline BLLs were 31% higher for children living within 50 meters from the sites compared to those living further away (n=69, p=0.028). Women and children spent time in the contaminated site daily and relied on it for their livelihoods and for recreation. Overall, this study highlights the intensity of lead exposure associated with the ULAB recycling industry. Additionally, we document the feasibility and effectiveness of a multi-part remediation intervention at a contaminated site embedded within a residential community; substantially reducing child BLLs and soil lead concentrations.

Fly ash is one of the key pathways for toxic metals from coal combustion power plants to be introduced into the environment. The presence of fly ash in the nearby soil layer indicates the escape of these contaminants from power plants. Discovering direct evidence of contamination from fly ash in the soil and sediments is not easy as fly ash particles are altered due to weathering and biological interference, after deposition. Soil samples were collected from a wetland in the proximity of a coal combustion power plant located in Chesterfield County, VA. X-ray Fluorescent (XRF) analysis of the bulk soil samples shows high concentrations of iron (13.45 – 9.07%), aluminum (10.33 – 8.09%), and sulfur (0.74 – 0.046%). Most of the samples contained high concentrations of lead (510 - 130 ppm) and chromium (178 – 164 ppm). The samples were analyzed further with a high sensitivity SEM-EDS to search for further physical evidence and to identify preserved micron-sized fly ash in the soil samples. The following concentrations were found: silicon (14.4- 0.7%), aluminum (19.9-17.59%), iron (14.57-4.9%) and titanium (4.9-0.64%). The micron-sized fly ash particles were probably transported to the nearby wetlands due to washing out from the fly ash dump adjacent to the power plant.

... Arsenic pos-ing in the Ganges Delta, Nature 401: 545. Ahmed, KM, Bhattacharya, P., Hasan, MA, Akhter, SH, Alam, SMM, Bhuyian, MAH, Imam, MB, Khan, AA & Sracek, O. 2004. ... 39 Page 57. Ahmed, KM, Imam, B., Akhter, SH, Hasan, MA & Khan, AA (2001). ...

High arsenic (As) concentrations in groundwater pose a serious threat to the health of millions of people in Bangladesh. Reductive dissolution of Fe(III)-oxyhydroxides and release of its adsorbed As is considered to be the principal mechanism responsible for mobilisation of As. The distribution of As is extremely heterogeneous both laterally and vertically. Groundwater abstracted from oxidised reddish sediments, in contrast to greyish reducing sediments, contains significantly lower amount of dissolved arsenic and can be a source of safe water. In order to study the sustainability of that mitigation option, this study describes the lithofacies and genesis of the sediments within 60 m depth and establishes a relationship between aqueous and solid phase geochemistry. Oxalate extractable Fe and Mn contents are higher in the reduced unit than in the oxidised unit, where Fe and Mn are present in more crystalline mineral phases. Equilibrium modelling of saturation indices suggest that the concentrations of dissolved Fe, Mn and PO(4)(3-)-tot in groundwater is influenced by secondary mineral phases in addition to redox processes. Simulating As(III) adsorption on hydroferric oxides using the Diffuse Layer Model and analytical data gave realistic concentrations of dissolved and adsorbed As(III) for the reducing aquifer and we speculate that the presence of high PO(4)(3-)-tot in combination with reductive dissolution results in the high-As groundwater. The study confirms high mobility of As in reducing aquifers with typically dark colour of sediments found in previous studies and thus validates the approach for location of wells used by local drillers based on sediment colour. A more systematic and standardised colour description and similar studies at more locations are necessary for wider application of the approach.

Weathering rinds are common in various terrestrial environments and have been used for decades as relative age indicators in Quaternary sequences. However, little study has been conducted on the development and preservation of alteration rinds on metamorphic and igneous clasts in older sedimentary sequences. Metamorphic clasts with conspicuous alteration rinds sporadically occur in a lower Cretaceous sandy gravel of the Potomac Formation exposed in the Puddledock quarry (Vulcan Materials Co). Polished thin sections of clasts were prepared for petrographic and elemental mapping by FIB-SEM with EDS. Major elemental compositions of bulk samples were determined by XRF. The clasts vary in lithology and color, and surfaces range from smooth to irregular due to flaking. Variable major element compositions (wt%), SiO2 (59.2-87.81), Al2O3 (5.76-15.42), Fe2O3 (1.54-19.214) and K2O (.075-12.1), reflect a variety of metamorphic rock types. The alteration rinds are generally uniform in thickness...

Introduction: The pseudotachylite breccia deposits of Musgrave Province of Australia, are up to about 5 km wide and run intermittently for 300 km with approximately 10% pseudotachylite veining. The veins are range in width from a few centimeters up to 4 m and can be traced for up to 10 m. The orientation of the veins appears to be random. Pseudotachylites occur only in the granulite facies rocks. Rotated blocks of ultramylonite are present in some of the Pseudotachylites, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semiductile and brittle behavior. [1][2][3][4][5] The early researchers interpreted the pseudotachylite breccia from the Musgrave Province as generated by normal seismic processes despite the enormous volume of the melt.[1][2][3][4] This seismic interpretation continues even though some pseudotachylite deposits are radial to a suggested impact center (MAPCIS) and occur 40km away from the Woodroofe Thrust Fault. [6] Goal...

An unexpected anomaly of magnetic susceptibility (MS) was observed in stream sediments of the upper course of the karstic Dobra River (Croatia). Preliminary results pointed to a possible impactite, formed by a shock event caused by a meteorite impact or by volcanic processes [1]. In addition to geophysical experiments, petrological and geochemical studies are reported [2, 3]. The multidisciplinary work for identification and confirmation of impact structure is still in progress. Results will be presented and the difficulties due to weathering and transport processes will be discussed and compared with recent literature [4, 5]. In reported results numerous evidences exist, which are in support of impact origin, such as vesicular glass with quench texture, ballen textures in the lechatelierite, presence of Troilite, etc. We suggest that the Dobra River from its source to the abyss in Ogulin (Upper Dobra) is a possible natural laboratory for studying processes of mixing between impacti...

Some years ago an unusual and unexpected anomaly of magnetic susceptibility (MS) was observed in sediments of the upper course of the karstic Dobra River (Croatia) during preliminary measurements of MS throughout the Kupa River drainage basin [1, 2]. In that study, following techniques have been used: magnetic susceptibility and Curie temperature, isothermal remnant magnetism (IRM), mineralogical analysis (using XRD) and chemical analysis (using ICP-MS). Thermomagnetic curves showed a distinctive Curie-point of magnetite at 580 °C, while additional transformation observed at 520–560°C derives from titanomagnetite. Magnetic particles from the Dobra River sediments contain pyroxene, plagioclase, hematite and quartz, in addition to magnetite. The ratio Ni/Fe versus Cr/Fe suggests that the magnetic spherules are most probably impactites. New and exciting finding of magnetic spherules in sediments of this karstic river deserved further field and laboratory research, which continued in co...

To understand the mechanisms of formation and preservation of alteration rinds on the clasts in older sedimentary sequences, pebble-size clasts from the Lower Cretaceous Potomac Fm. were collected from the Puddledock quarry (Vulcan Materials Co.). Sandy gravel in the quarry contains siliceous metavolcanic clasts (rounded pebbles) with conspicuous alteration rinds. Rinds and cores of the clasts were subjected to mineralogical analysis by X-ray diffraction, elemental mapping by micro XRF, and optical petrographic analysis.
Irrespective of the size and composition of the clasts, the alteration rinds are generally uniform in thickness (~4 to 9 mm). In most cases, the clasts have a single outer band of alteration, but some have multiple bands. The alteration rinds typically have an outer light-gray to white band and surround a darker core. Some clasts also have a thin (~2 to 4 mm) outer coating of crystalline silica.
The cores of the clasts contain quartz (~54.5%), feldspar (~22.8%), kaolinite (~23%), hematite (~1.5%) and traces of anatase (~2.4%), by weight. The rinds (white bands) contain comparatively more quartz (~74%) and feldspar (~16.7%), and lesser amounts of kaolinite (~9.4%), hematite (~0.3%), and anatase (~1%). The enrichment of quartz and depletion of kaolinite, hematite and anatase in the rinds appear to reflect dissolution of feldspar and removal of clay, probably due to continued hydrolysis. Optical petrographic analysis and SEM-EDS revealed hydrous aluminum silicates that fill fractures and fissures in the cores but are depleted in the rinds of most specimens.
Concentrations of transition and post-transition metals (i.e. Fe, Al and Ti), along with sulfur, in cores of the clasts exceed those in the rinds. Alkalis do not show any trend in most clasts, but a few show an increased concentration in the rind, perhaps due to incomplete kaolinization.
Results indicate that the alteration rinds formed primarily by (1) partial removal of transition and alkali metals by hydrolysis due to chemical weathering during transport and/or after deposition, accompanied by (2) preferential removal of fracture-fill clay minerals in the rinds, followed by (3) preservation in a reducing environment and precipitation of crystalline silica from pore water during burial diagenesis.

Session No. 18
T17. Geologic Studies of the U.S. Atlantic Coastal Plain Province
Friday, 31 March 2017: 8:00 AM-12:00 PM

To understand the mechanisms of formation and preservation of alteration rinds on the clasts in older sedimentary sequences, pebble-size clasts from the Lower Cretaceous Potomac Fm. were collected from the Puddledock quarry (Vulcan Materials Co.). Sandy gravel in the quarry contains siliceous metavolcanic clasts (rounded pebbles) with conspicuous alteration rinds. Rinds and cores of the clasts were subjected to mineralogical analysis by X-ray diffraction, elemental mapping by micro XRF, and optical petrographic analysis.
Irrespective of the size and composition of the clasts, the alteration rinds are generally uniform in thickness (~4 to 9 mm). In most cases, the clasts have a single outer band of alteration, but some have multiple bands. The alteration rinds typically have an outer light-gray to white band and surround a darker core. Some clasts also have a thin (~2 to 4 mm) outer coating of crystalline silica.
The cores of the clasts contain quartz (~54.5%), feldspar (~22.8%), kaolinite (~23%), hematite (~1.5%) and traces of anatase (~2.4%), by weight. The rinds (white bands) contain comparatively more quartz (~74%) and feldspar (~16.7%), and lesser amounts of kaolinite (~9.4%), hematite (~0.3%), and anatase (~1%). The enrichment of quartz and depletion of kaolinite, hematite and anatase in the rinds appear to reflect dissolution of feldspar and removal of clay, probably due to continued hydrolysis. Optical petrographic analysis and SEM-EDS revealed hydrous aluminum silicates that fill fractures and fissures in the cores but are depleted in the rinds of most specimens.
Concentrations of transition and post-transition metals (i.e. Fe, Al and Ti), along with sulfur, in cores of the clasts exceed those in the rinds. Alkalis do not show any trend in most clasts, but a few show an increased concentration in the rind, perhaps due to incomplete kaolinization.
Results indicate that the alteration rinds formed primarily by (1) partial removal of transition and alkali metals by hydrolysis due to chemical weathering during transport and/or after deposition, accompanied by (2) preferential removal of fracture-fill clay minerals in the rinds, followed by (3) preservation in a reducing environment and precipitation of crystalline silica from pore water during burial diagenesis.