Showing papers in "Clays and Clay Minerals in 2011"

Evidence for Low-Grade Metamorphism, Hydrothermal Alteration, and Diagenesis on Mars from Phyllosilicate Mineral Assemblages

Abstract: The enhanced spatial and spectral resolution provided by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO) has led to the discovery of numerous hydrated silicate minerals on Mars, particularly in the ancient, cratered crust comprising the southern highlands. Phases recently identified using visible/near-infrared spectra include: smectite, chlorite, prehnite, high-charge phyllosilicates (illite or muscovite), the zeolite analcime, opaline silica, and serpentine. Some mineral assemblages represent the products of aqueous alteration at elevated temperatures. Geologic occurrences of these mineral assemblages are described using examples from west of the Isidis basin near the Nili Fossae and with reference to differences in implied temperature, fluid composition, and starting materials during alteration. The alteration minerals are not distributed homogeneously. Rather, certain craters host distinctive alteration assemblages: (1) prehnite-chlorite-silica, (2) analcime-silica-Fe, Mg-smectite-chlorite, (3) chlorite-illite (muscovite), and (4) serpentine, which furthermore has been found in bedrock units. These assemblages contrast with the prevalence of solely Fe, Mg-smectites in most phyllosilicate-bearing terrains on Mars, and they represent materials altered at depth then exposed by cratering. Of the minerals found to date, prehnite provides the clearest evidence for subsurface, hydrothermal/metamorphic alteration, as it forms only under highly restricted conditions (T = 200-400°C). Multiple mechanisms exist for forming the other individual minerals; however, the most likely formation mechanisms for the characteristic mineralogic assemblages observed are, for (1) and (2), low-grade metamorphism or hydrothermal ( 400°C has not been found.

A comparison of point of zero charge measurement methodology

Abstract: Contaminant-transport modeling requires information about the charge of subsurface particle surfaces. Because values are commonly reused many times in a single simulation, small errors can be magnified greatly. Goethite (α-FeOOH) and pyrolusite (β-MnO2) are ubiquitous mineral phases that are especially contaminant reactive. The objective of the present study was to measure and compare the point of zero charge (PZC) using different methods. The pyrolusite PZC was measured with three methods: mass titration (MT) (PZC = 5.9±0.1), powder addition (PA) (PZC = 5.98±0.08), and isoelectric point, IEP (PZC = 4.4±0.1). The IEP measurement was in agreement with literature values. However, MT and PA resulted in a statistically larger PZC than the IEP measurement. The surface area of pyrolusite, 2.2 m2g−1, was too small to permit PZC determination by the potentiometric titration (PT) method. Goethite PZC values were measured using MT (7.5±0.1), PT (7.46±0.09), and PA (7.20±0.08). The present work presents the first reported instance where MT and PA have been applied to measure the point of zero charge of either pyrolusite or goethite. The results illustrate the importance of using multiple, complementary techniques to measure PZC values accurately.

Terrestrial perspective on authigenic clay mineral production in ancient martian lakes

Abstract: The discovery of phyllosilicates in terrains of Noachian age (>3.5 Ga) on Mars implies a period in the planet’s history that was characterized by wetter, warmer conditions that may have been more hospitable for life than the cold and dry conditions prevalent today. More specific information about the original locations and mechanisms of clay mineral formation on Mars is not as well constrained, however, in part because the origin of particular clay minerals is often non-unique. For example, Fe and Mg smectite-bearing deposits on Mars may have formed in various environments, including the weathering profiles of basic volcanic rocks, impact-induced hydrothermal sites, or in bodies of standing water. The identification of lacustrine deposits on Mars is of great interest due to their potential for the preservation of organic material, but identifying any given suite of sedimentary rocks as such is difficult when limited to mineralogy and morphology derived from orbital data. Here, the processes and conditions leading to clay mineral formation in lakes and evaporative marine basins on Earth are reviewed, with a focus on the spatial and stratigraphic distribution of clays in these settings. The goal is to provide criteria to determine if certain Martian clay deposits are consistent with such an origin, which in turn will aid in the identification of possible ancient habitable environments on Mars.

In situ x-ray diffraction study of the swelling of montmorillonite as affected by exchangeable cations and temperature

Abstract: The swelling property of smectite is dominated by the hydration of exchangeable cations in the interlayer spacing (‘interlayer hydration’). By investigating systematically the swelling behavior of various exchangeable cations with different valences and ionic radii, the interlayer hydration of smectite was explored. The swelling behavior of Li+-, K+-, Rb+-, Cs+-, Mg2+-, Sr2+-, Ba2+-, and La3+-montmorillonites in undersaturated conditions was measured precisely over the range 50–150°C by in situ X-ray diffraction (XRD) analyses. The systematic swelling behavior of ten homocationic montmorillonites, the aforementioned eight homoionic montmorillonites, plus Na+ and Ca2+ from a previous study, and the cation hydration energies were analysed by studying the changes occurring in the basal spacing and the 001 peak width. With decreasing cation hydration energy, swelling curves ( i.e . plots of basal spacing vs. relative humidity (RH)) change from continuous (Mg2+, La3+, and Ca2+) to stepwise (Sr2+, Li+, Ba2+, and Na+) to one-layer only (K+, Rb+, and Cs+). For the first two groups, the RH at the midpoint between the one- and two-layer hydration states increased as the cation hydration energy decreased. Under low RH, with increasing temperature, the basal spacings of Mg-, La-, Ca-, Sr-, Li-, and Ba-montmorillonites decreased continuously to the zero-layer hydration state, whereas Na-, K-, Rb-, and Cs-montmorillonites swelled from the zero-layer hydration state even at the lowest temperature (50°C). A decrease in the basal spacing at the same RH but at different temperatures suggests the existence of metastable states or that the layer-stacking structure changes with temperature. The systematics of the swelling behavior of various homocationic montmorillonites as functions of RH and temperature (<150°C) at 1 atm are reported here.

The formation of illite from nontronite by mesophilic and thermophilic bacterial reaction

Abstract: The formation of illite through the smectite-to-illite (S-I) reaction is considered to be one of the most important mineral reactions occurring during diagenesis. In biologically catalyzed systems, however, this transformation has been suggested to be rapid and to bypass the high temperature and long time requirements. To understand the factors that promote the S-I reaction, the present study focused on the effects of pH, temperature, solution chemistry, and aging on the S-I reaction in microbially mediated systems. Fe(III)-reduction experiments were performed in both growth and non-growth media with two types of bacteria: mesophilic (Shewanella putrefaciens CN32) and thermophilic (Thermus scotoductus SA-01). Reductive dissolution of NAu-2 was observed and the formation of illite in treatment with thermophilic SA-01 was indicated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A basic pH (8.4) and high temperature (65°C) were the most favorable conditions forthe formation of illite. A long incubation time was also found to enhance the formation of illite. K-nontronite (non-permanent fixation of K) was also detected and differentiated from the discrete illite in the XRD profiles. These results collectively suggested that the formation of illite associated with the biologically catalyzed smectite-to-illite reaction pathway may bypass the prolonged time and high temperature required for the S-I reaction in the absence of microbial activity.

Reflectance spectroscopy of beidellites and their importance for mars

Abstract: Beidellites may exist on Mars and represent intermediate alteration products; their presence would indicate different alteration environments than previously identified because montmorillonite is a low-grade alteration mineral whereas beidellite is a higher-temperature alteration mineral, and often represents a step toward illite formation. The reflectance spectra of beidellites are under study to support their orbital detection on Mars, where spectral signatures of other Al-rich phyllosilicates have been observed. Reflectance spectra of ten Al-rich smectites are presented here which include pure beidellites and Al smectites having compositions between those of beidellite and montmorillonite, and emphasis is placed here on the OH combination bands near 4545 cm −1 (2.2 μm) as these vibrational features are commonly used in the identification of phyllosilicates on Mars. Shifts were observed in the Al 2 OH band centers, which occur near 4590 cm −1 (2.18 μm) in reflectance spectra of beidellite and near 4525 cm −1 (2.21 μm) in reflectance spectra of montmorillonite. These are compared with the Al 2 OH bending vibrations observed near 941–948 cm −1 (10.5–10.6 μm) for beidellite and near 918–926 cm −1 (10.8–10.9 μm) for montmorillonite. Although the octahedral site cation composition provides the greatest influence on the vibrational energies of the M 2 OH groups, the tetrahedral site cation composition also influences these vibrations. Shifts were observed in the Si–O–Al bending vibrations from 552 and 480 cm −1 (18.1 and 20.8 μm) in beidellite spectra to 544 and 475 cm −1 (18.4 and 21.0 μm) in montmorillonite spectra. Gaussian modeling of the 4545 cm −1 (2.2 μm) bands led to the discrimination of four overlapping bands in each of the ten Al smectite spectra examined in this study. Shifts in the band center and area of the primary spectral band are coordinated with substitution of Al for Si in the tetrahedral sheet. This is consistent with beidellites having a greater tetrahedral layer charge than montmorillonites. The observed spectral differences were sufficiently large that these Al-rich smectites can be differentiated in orbital data of Mars. A pure beidellite-type spectrum is observed in an isolated Al phyllosilicate-bearing outcrop in Libya Montes, a region where Fe-rich smectite is common but Al-rich smectite is rare. Beidellite-type reflectance spectra were also observed in one area of the Nili Fossae region. In contrast, a variety of Al phyllosilicates were found in the ancient rocks at Mawrth Vallis, including some smaller clay-bearing regions exhibiting spectral signatures more consistent with beidellite-like than montmorillonite-like chemistry.

Acidic dissolution of magnetite: experimental study on the effects of acid concentration and temperature

Abstract: Magnetite (Fe3O4) is a key economically valuable component in iron ore and is extracted by dissolution processes, but among the Fe (oxyhydr)oxides its solubility behavior is one of the least understood. The objective of this study was to improve understanding of magnetite dissolution mechanisms leading to thermodynamic equilibrium by comparing the dissolution of two solid samples, one synthetic and one industrial, using oxalic, sulfuric, and nitric acids at varying concentrations and temperatures. Of the three solid-liquid systems investigated, only the system consisting of magnetite and oxalic acid reached an equilibrium state within the duration of an individual experiment (6 h). In this system, increasing the acid concentration resulted in a significant increase in the equilibrium concentration of dissolved Fe. When dissolving synthetic and industrial magnetite, increasing the temperature not only increased the rate of reaction but also affected the concentration of dissolved Fe. Significant effects were observed when increasing the temperature from 15 to 35°C, but only slight differences were seen on further increases in temperature. Observations regarding the equilibrium state of the sulfuric and nitric acid systems could not be made because equilibrium was not reached. The most important individual observation regarding the equilibrium state of the nitric- and sulfuric-acid systems seems to be that in future studies a much longer reaction time is necessary, due to slow kinetics of the dissolution mechanism. A proton-based mechanism has been hypothesized as the one governing the dissolution of magnetite by these two acids, but only the dissolution of the industrial sample yielded results that were similar for these two acids and consistent with that hypothesis.

Interpretation of reflectance spectra of clay mineral-silica mixtures: implications for martian clay mineralogy at mawrth vallis

Abstract: The Al-clay-rich rock units at Mawrth Vallis, Mars, have been identified as mixtures of multiple components based on their spectral reflectance properties and the known spectral character of pure clay minerals. In particular, the spectral characteristics associated with the ~2.2 μm feature in Martian reflectance spectra indicate that mixtures of AlOH- and SiOH-bearing minerals are present. The present study investigated the spectral reflectance properties of the following binary mixtures to aid in the interpretation of remotely acquired reflectance spectra of rocks at Mawrth Vallis: kaolinite-opal-A, kaolinite-montmorillonite, montmorillonite-obsidian, montmorillonite-hydrated silica (opal), and glassillite-smectite (where glass was hydrothermally altered to mixed-layer illite-smectite). The best spectral matches with Martian data from the present study’s laboratory experiments are mixtures of montmorillonite and obsidian having ~50% montmorillonite or mixtures of kaolinite and montmorillonite with ~30% kaolinite. For both of these mixtures the maximum inflection point on the long wavelength side of the 2.21 μm absorption feature is shifted to longer wavelengths, and in the case of the kaolinite-montmorillonite mixtures the 2.17 μm absorption found in kaolinite is of similar relative magnitude to that feature as observed in CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) data. The reflectance spectra of clay mixed with opal and of hydrothermally altered glass-illite-smectite did not represent the Martian spectra observed in this region as well. A spectral comparison of linear vs. intimate mixtures of kaolinite and montmorillonite indicated that for these sieved samples, the intimate mixtures are very similar to the linear mixtures with the exception of the altered glass-illite-smectite samples. However, the 2.17 μm kaolinite absorption is stronger in the intimate mixtures than in the equivalent linear mixture. Modified Gaussian Modeling of absorption features observed in reflectance spectra of the kaolinite-montmorillonite mixtures indicated a strong correlation between percent kaolinite in the mixture and the ratio of the area of the 2.16 μm band found in kaolinite to the area of the 2.20 μm band found in montmorillonite.

Surface area and porosity of nanotubes obtained from kaolin minerals of different structural order

Abstract: Mesoporous materials with pore diameters in the range 2–50 nm forming tubular or fibrous structures are of great interest due to their unique properties. Because they are commonly used as sorbents and catalyst carriers, knowledge of their surface area and porosity is critical. A modified intercalation/deintercalation method was used to increase the efficiency of nanotube formation from kaolin-group minerals which differ in terms of their degree of structural order. Unlike previous experiments, in the procedure adopted in the present study, methanol was used instead of 1,3-butanediol for grafting reactions and octadecylamine intercalation was also performed. The samples were examined using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The specific surface area and porosity of previously described and newly formed materials were investigated by N2 adsorption/desorption. Compared to results described earlier, the percent yield of nanotubes obtained in the present study was significantly greater only in the case of ‘Maria III’ kaolinite, which has high structural order. This increase was obtained mainly by the grafting reaction with methanol. Highly ordered stacking of kaolinite-methanol intercalates was noticed and, thus, the amine intercalation was more efficient. In particular, the use of long-chain octadecylamine significantly increased the nanotube yield. The grafting reaction with methanol procedure yielded fewer nanotubes, however, when applied to poorly ordered samples (‘Jaroszow’ kaolinite and ‘Dunino’ halloysite). In the case of the ‘Maria III’ kaolinite, the diameter of the rolled layers observed by TEM was ~30 nm and corresponded to average diameters of newly formed pores (DmN) determined using N2 adsorption/desorption, confirming that nanotubes contributed to an increase in surface area and total pore volume. In the case of ‘Jaroszow’ kaolinite and ‘Dunino’ halloysite mainly macropores (DmN > 100 nm) and mesopores (20 nm > DmN > 40 nm) were formed. The pores were attributed to interparticle and interaggregate spaces in the stacks of platy particles and to the small relative number of nanotubes.

Smectite formation in submarine hydrothermal sediments: samples from the hms challenger expedition (1872–1876)

Abstract: Clay processes, mineral reactions, and element budgets in oceans continue to be important topics for scientific investigation, particularly with respect to understanding better the roles of chemistry, formation mechanism, and input from hydrothermal fluids, seawater, and non-hydrothermal mineral phases.To that end, the present study was undertaken.Thre e samples of submarine metalliferous sediments of hydrothermal origin were studied to investigate the formation of smectite, usually Fe-rich, which takes place in such environments.The samples are from the historical collection returned by the British HMS Challenger expedition (1872–1876) and kept at the Natural History Museum in London.The samples were collected from the vicinity of the Pacific–Antarctic Ridge and the Chile Ridge.The samples were analyzed by means of X-ray diffraction (XRD), chemical analysis, scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM-EDX), infrared (IR), and transmission electron microscopyanalytical electron microscopy (TEM-AEM).After removal of biogenic calcite the samples appeared to consist mainly of two low-crystallinity phases mixed intimately: Fe/Mn (oxyhydr)oxides and a Si-Al-Mg- Fe phase of similar chemical characteristics to smectite and with variable proportions of the above elements, as indicated by XRD, IR, and SEM-EDX.In particular, analysis by XRD revealed the presence of highly disordered δ-MnO2.The TEM-AEM analysis showed that Fe/MnOOH particles have Fe/Mn ratios in the range 25–0.2 and textures changing from granular to veil-like as the proportion ofMn increased. The smectite-like material has the morphology and chemistry of smectite, as well as 10–15 A lattice fringes. Selected area electron diffraction (SAED) patterns indicated a very poorly crystalline material: in some cases distances between diffraction rings corresponded to d values of smectite.The smectite composition indicated a main Fe-rich dioctahedral component with a substantialMg-rich trioctahedral component (total octahedral occupancy between 2.02 and 2.51 atoms per O10[OH]2). The (proto-) smectite is interpreted to have formed within the metalliferous sediment, as a slow reaction between Fe/MnOOH, seawater (providing Mg), detrital silicates from the continent (providing Si and Al), and X-ray amorphous silica of hydrothermal origin that adsorbed on Fe/MnOOH phases and deposited with them.This material is possibly in the process of maturation into well crystallized smectite.

SOLUBILITY AND THERMODYNAMIC PROPERTIES OF CARBONATE-BEARING HYDROTALCITE–PYROAURITE SOLID SOLUTIONS WITH A 3:1 Mg/(Al+Fe) MOLE RATIO

Abstract: The naturally occurring layered double hydroxides (LDH, or anionic clays) are of particular interest in environmental geochemistry because of their ability to retain hazardous cations and especially anions. However, incorporation of these minerals into predictive models of water-rock interaction in contaminant environments, including radioactive-waste repositories, is hampered by a lack of thermodynamic and stability data. To fill part of this gap the present authors have derived properties of one of the complex multicomponent solid solutions within the LDH family: the hydrotalcite-pyroaurite series, Mg3(Al1−xFex)(OH)8(CO3)0.5·2.5H2O. Members of the hydrotalcite-pyroaurite series with fixed MgII/(AlIII+FeIII) = 3 and various FeIII/(FeIII+AlIII) ratios were synthesized by co-precipitation and dissolved in long-term experiments at 23±2°C and pH = 11.40±0.03. The chemical compositions of co-existing solid and aqueous phases were determined by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, and liquid scintillation counting of 55Fe tracers; X-ray diffraction and Raman were used to characterize the solids. Based on good evidence for reversible equilibrium in the experiments, the thermodynamic properties of the solid solution were examined using total-scale Lippmann solubility products, ΣΠT. No significant difference was observed between values of SPT from co-precipitation and from dissolution experiments throughout the whole range of Fe/Al ratios. A simple ideal solid-solution model with similar end-member ΣΠT values (a regular model with 0 < WG < 2 kJ mol −1 sufficient to describe the full range of intermediate mineral compositions. In turn, this yielded the first estimate of the standard Gibbs free energy of the pyroaurite end member, G 298,Pyr o = −3882.60±2.00 kJ/mol, consistent with G 298,Htlc o = −4339.85 kJ/mol of the hydrotalcite end member, and with the whole range of solubilities of the mixed phases. The molar volumes of the solid-solution at standard conditions were derived from X-ray data. Finally, Helgeson’s method was used to extend the estimates of standard molar entropy and heat capacity of the end members over the pressure-temperature range 0−70°C and 1–100 bar.

Adsorption of low-concentration ammonium onto vermiculite from hebei province, china

Abstract: Vermiculite is a common layered silicate clay mineral which has good adsorption and ion-exchange properties, and which is used to remove pollutants from groundwater. The adsorption by vermiculite from Heibei Province, China, of low-concentration ammonium in water was assessed here to evaluate the effects of adsorption time, particle size, adsorbent dose, pH, and temperature. Using Fourier-transform infrared spectroscopy, the concentration of NH 4 + at 1430 cm−1 was evaluated after ammonium was adsorbed by vermiculite. Based on Langmuir-model analysis, the adsorption capacity ofthe Chinese vermiculite (in the particle-size range 0.025–0.075 mm) for ammonium was 18 mg/g after 3 h of equilibration. Optimal adsorption occurred at pH 6–7 and 60°C, which is different from that at high ammonium concentrations. Smaller particle-size fractions showed greater degrees of adsorption. Increase in Mg2+, K+, or Na+ concentrations influenced ammonium adsorption and, therefore, indicated that cation exchange was the mechanism for ammonium uptake from low-concentration solutions.

The Dispersive Effect of Sodium Silicate on Kaolinite Particles in Process Water: Implications for Iron-Ore Processing

Abstract: Kaolinite is a common gangue mineral in iron ore and sodium silicate has been used widely as a dispersant of silicate gangue minerals including kaolinite in various iron-ore flotation methods over a wide range of pH. Its actual dispersive effect on kaolinite under iron-ore flotation conditions has received very limited attention, however. The presence of hydrolyzable metal cations in process water further complicates sodium silicate—kaolinite interactions. In the present study, the dispersive effect of sodium silicate on kaolinite particles in distilled water as well as in CaCl2 and MgCl2 solutions was investigated systematically through electrophoretic mobility and colloid-stability studies. The studies were based on controlled pH, which eliminated the dispersive effect of sodium silicate induced by increasingpulp pH, in order to simulate the conditions of iron-ore processing. With pH controlled at constant levels, sodium silicate dispersed kaolinite only when positively charged sites were present on kaolinite surfaces and the zeta potential of kaolinite was more negative than ~−30 mV. Over the pH range from 5 to 10.5, a significant dispersive effect of sodium silicate was only observed at pH 7. In process water, when Ca and Mgwere present, the strong coagulation of kaolinite particles caused by the hydrolyzable metal cations could not be dispersed effectively with sodium silicate.

Layer charge density of smectites – closing the gap between the structural formula method and the alkyl ammonium method

Abstract: The layer charge density (LCD) of montmorillonite represents the permanent negative charge, its most important property. The LCD can be determined by two different methods, the structural formula method(SF M) and the alkylammonium method(AAM). Other methods of determining the LCD are calibrated against one or the other of these. The results of the two methods differ systematically: SFM values are larger than AAM values and the difference increases with increasing layer charge density. In the present study, the critical parameters of both methods were considered quantitatively in order to identify the most likely reason for the systematic difference. One particularly important argument against the validity of the SFM is that typical SFM values correspond to unrealistically large CEC values that have never been reported. In addition, SFM does not consider the variable charge which causes cations to be adsorbed to the outer surface (at pH >4). In contrast to minor constituents, which can of course also affect SFM values, the variable charge can explain only part of the systematic difference. The exchange of pure smectite samples with both Cu-trien andalkyla mmonium revealedthe presence of non-exchangeable, nonstructural cations (Na, K, Ca). These cations, together with 10% (or more) variable charge, may explain the differences in LCD values. The non-exchangeable, non-structural cations could stem from undetected traces of feldspar or volcanic glass. The present samples indicated that the systematic difference in LCD values between the two methods is related to the amount of non-exchangeable, non-structural cations only, indicating that the two LCD methods probe different features of smectites. Using the SFM on pure smectite provides a value for the total number of charges (permanent with andwithout fixed(= non-exchangeable, non-structural) cations plus variable charge). The AAM, on the other hand, provides the charge density of the exchangeable cations (without variable charge).

Mineralogical and Geochemical Characteristics and Genesis of Hydrothermal Kaolinite Deposits within Neogene Volcanites, Kütahya (Western Anatolia), Turkey

Abstract: The Kutahya kaolinite deposits are the most important source of raw materials for the ceramics industry in Turkey. To date, no detailed mineralogical or geochemical characterizations of these materials have been carried out; the present study aims to fill that gap. The Kutahya kaolinite deposits formed by alteration of dacite and andesite tuffs related to Neogene volcanism whichwas associated withe xtensional tectonics. The kaolinite deposits contain silica and Fe- and Ti-bearing phases (pyrite, goethite, and rutile) in vertical and subvertical veins that diminish and then disappear upward. Mineralogical zonation outward from the main kaolinite deposit is as follows: kaolinite ± smectite + illite + opal-CT + feldspar; feldspar + kaolinite + quartz + smectite + illite; quartz + feldspar + volcanic glass. The veins and mineral distributions demonstrate that hydrothermal alteration was the main process in the development of the kaolinite deposits of the area. The very sharp, intense, diagnostic basal reflections at 7.2 and 3.57 A, as well as non-basal reflections, well defined pseudohexagonal to hexagonal crystallinity with regular outlines, ideal differential thermal analysis-thermal gravimetric curves, and ideal, sharp, infrared spectral bands indicate well crystallized kaolinite. Micromorphologically, the development of kaolinite plates at the edges of altered feldspar and devitrified volcanic glass indicates an authigenic origin. Lateral increase in (SiO2+Fe2O3+MgO+Na2O+CaO+K2O)/(Al2O3+TiO2) from the center of the kaolinite deposit outward also indicates hydrothermal zonation. Enrichment of Sr in altered and partially altered rocks relative to freshvolca nic-rock samples demonstrates retention of Sr and depletion of Rb, Ba, Ca, and K during hydrothermal alteration of sanidine and plagioclase within the volcanic units. In addition, depletion of heavy rare earth elements (HREE) relative to light rare earth elements (LREE) in the kaolinized materials may be attributed to the alteration of hornblende. The negative Eu anomaly suggests the alteration of feldspar by hydrothermal fluids. The isotopic data from kaolinite and smectite indicate that hydrothermalalteration processes developed at 119.1–186.9°C and 61.8–84.5°C, respectively. Thus, the kaolinite deposits formed by hydrothermal alteration of volcanic glass, feldspar, and hornblende by a dissolutionprecipitation mechanism which operated under acidic conditions within Neogene dacite, andesite, and tuffs.

Fine-Grained Serpentine in CM2 Carbonaceous Chondrites and Its Implications for the Extent of Aqueous Alteration on the Parent Body: A Review

Abstract: Outer main-belt asteroids are predominantly of the C-type (carbonaceous), suggesting that they are likely parent bodies of carbonaceous chondrites. Abundant phyllosilicates in some classes of carbonaceous chondrites have chemical compositions, mineral associations, and textures that preserve direct evidence of the processes by which carbonaceous chondrites and their parent asteroids originated and evolved to their present state. Serpentine is the dominant hydroxyl-bearing mineral in the most abundant (CM) group of carbonaceous chondrites. Serpentine may have formed as a direct nebular condensate during cooling of the solar nebula, or by aqueous alteration of anhydrous Mg,Fe-silicate precursors. Such alteration of anhydrous precursors may have occurred in the solar nebula prior to assembly of the meteorites’ parent bodies or on the parent bodies. The relative proportions of Fe and Mg in fine-grained CM2 serpentines have been used to compare the degree of aqueous alteration of different CM2 chondrites with one another. The Mg content of serpentine increases with increasing overall degree of aqueous alteration, so CM2 chondrites with Mg-rich serpentines experienced a more advanced degree of aqueous alteration than CM2 chondrites with Fe-rich serpentines. Attempts to quantify aqueous alteration of CM chondrites by interpreting electron microprobe analyses in terms of charge-balance and site-occupancy constraints from serpentine stoichiometry have met with mixed success. Despite its imperfections, one widely used alteration index based on serpentine stoichiometry is strongly correlated with the elapsed time since the fall and recovery of witnessed CM chondrite falls. Additionally, volatile organic contaminants introduced during sample processing in the laboratory are associated with serpentine and other matrix phyllosilicates. Together, these post-recovery changes in scientifically important sample attributes imply that oxidation-reduction and other types of weathering and contamination affect these meteorites even during curatorial storage and laboratory processing. The same phyllosilicates that make their carbonaceous-chondritic host rocks scientifically important research targets also render those same rocks extraordinarily vulnerable to terrestrial contamination of some of their most scientifically important attributes. This has possible implications for reconstructing pre-terrestrial (parent body) aqueous alteration phenomena from carbonaceous chondritic meteorites and eventually from samples returned by future missions to asteroids with spectral reflectance properties similar to carbonaceous chondrites.

Clay-Mineral Formation in Soils Developed in the Weathering Zone of Pyrite-Bearing Schists: A Case Study from The Abandoned Pyrite Mine in Wieściszowice, Lower Silesia, SW Poland

Abstract: Intense mineral transformations that produce acid soils from weathering zones of pyritebearing rocks, including alterations of layer silicates, are of critical importance to agricultural and environmental interests in various regions of the world. To date, the transformations of layer silicates in these soils have not been studied in detail. The aim of the present investigation was to examine the weathering pathways controlling processes of clay-mineral formation in acidic soils developed near the abandoned pyrite mine in Wieściszowice (Lower Silesia, SW Poland). A sequence of soils, from weakly developed technogenic soils (located on the mine dumps) to well developed natural soils, was selected. Bulk soil material and separated clay fractions were analyzed using X-ray diffractometry, Fouriertransform infrared spectroscopy, and scanning electron microscopy-energy dispersive spectrometry. The profiles analyzed were developed on pyrite-bearing schists containing trioctahedral Mg,Fe-chlorite and dioctahedral micas (muscovite and paragonite). Because of pyrite weathering, all the soils studied were strongly acidic (pH 2.8–4.4). Inherited chlorite and micas (K- and Na-mica) predominated in the clay fractions of soils developed on the mine dumps, whereas clays from natural soils were rich in pedogenic minerals (i.e. smectite, vermiculite, and mixed-layer minerals containing hydrated interlayers). The formation of both vermiculite and smectite at the expense of chlorite was observed in the soils studied. The transformation probably led to smectite formation via intermediate stages of mixed-layer minerals (i.e. chlorite-vermiculite, chlorite-smectite, and/or vermiculite-smectite). The process of chlorite dissolution took place simultaneously with the transformation. Micas were mainly transformed to smectite and mixedlayer mica-smectite. Neoformation of kaolinite occurring in A horizons of the soils investigated was also documented. Metal-hydroxy interlayers in Bw horizons of well developed soils were found. The process of interlayer development appeared to be pH dependent and took place at pH ⩾4.2. The processes of claymineral formation in soils developed in the weathering zone of a pyrite-bearing schist are similar to those occurring in podzols (Spodosols).

A study of the removal of copper ions from aqueous solution using clinoptilolite from serbia

Abstract: Toxic metal contamination of waste waters can be mitigated by metal adsorption to clay and zeolitic minerals, but in developing countries such environmental remediation can be cost prohibitive if these minerals are not readily available. Because of its abundance, low cost, and excellent selectivity for several toxic metal ions, clinoptilolite from the Zlatokop deposit in Serbia was investigated for its ability to remove copper ions from aqueous solutions and serve as an effective local resource for this purpose. The sorption capacity of the clinoptilolite at 298 K varied from 8.3 mg Cu g−1 (for C = 100 mg Cu dm−3) to 16.8 mg Cu g−1 (for C = 400 mg Cu dm−3). The sorption data were best described by the Freundlich isotherm and the sorption kinetics followed the pseudo-second-order model. Intra-particle diffusion of Cu2+ was present but it is not the rate-limiting step. The sorption of Cu2+ on the clinoptilolite occurred spontaneously, the free energy change decreasing with temperature. The sorption was endothermic and was accompanied by an increase in entropy. Dehydration of the Cu-loaded clinoptilolite at 540°C led to the formation of nanocrystalline Cu(I) oxide particles with an average size of ~2 nm, suggesting possible novel applications for the Cu-loaded clinoptilolite.

Mineralogical and Geochemical Properties of the Na- And Ca-bentonites of Ordu (Ne Turkey)

Abstract: A number of different types of bentonite deposits formed by hydrothermal alteration and diagenetic processes are to be found in the Ordu area of the Eastern Black Sea region. The Ca- and Na-bentonite deposits are related to Upper Cretaceous tholeitic to calc-alkaline volcanites, predominantly dacite and andesite, and also include rhyodacite with lesser basalt and their pyroclastic equivalents. In the present study, dacite (PR1), perlite (PR2), moderately altered rocks (MPR), and Na- and Ca-bentonites were studied to describe and compare their mineralogical and geochemical properties and their conditions of formation by means of X-ray diffraction, optical microscopy, scanning electron microscopy, and chemical analytical techniques. Ca-bentonites, except for smectite, contain opal-CT, feldspar, biotite, and rarely pyrite, while Na-bentonites contain smectite and less feldspar, opal-CT, kaolinite, and illite. Progressive alteration of the PR2 caused depletion in K2O and Na2O and enrichment in MgO and CaO in all of the Ca-bentonite samples. Na2O was depleted in all of the Na-bentonites and in most of the MPR. The medium and heavy rare earth elements (MREE and HREE) show mass gain or mass loss in the Na-bentonites. The HREE show nearly immobile behavior in the Ca-bentonites. The rare earth elements (REE) and transition elements (TRE) mostly gained mass in the Ca-bentonites in contrast to Na-bentonites. Large-ion lithophile elements (LILE) are strongly depleted in all of the bentonites. The LREE, MREE, and HREE were strongly depleted in most of the MPR samples. TiO2, Lu, Tm, and Tb show immobile behavior in all samples. PR1 exhibits a slightly positive Eu anomaly. Two MPR samples show slightly positive Eu anomalies (1.03, 1.13), and one Na-bentonite sample displays a slightly positive Eu anomaly (1.04). Most of the Nabentonites have weakly negative Eu anomalies, whereas perlite and the Ca-bentonite have a strongly negative Eu anomaly. The PR1, PR2, MPR, and Na-bentonite present a positive Ce anomaly, and the Ca-bentonite shows a moderately negative Ce anomaly. The Ca-montmorillonites are mainly hydrothermal in origin and derived from alteration of volcanoclastic material in situ and/or in the subaerial environment. The Na-montmorillonites formed by alteration and diagenesis of volcanoclastic material in the sedimentary basin.

Study of the interaction between bentonite and a strain of bacillus mucilaginosus

Abstract: Mineral-microbe interactions are widespread in a number of environmental processes such as mineral weathering, decomposition, and transformation. Both clay minerals and silicate-weathering bacteria are widely distributed in nature, and the latter contribute to weathering, diagenesis, and mineralization of major rock-forming minerals. The purpose of this study was to observe changes in the chemical composition and structure, especially the phase transformation, of smectite after processing by a silicate-weathering bacterium. The interaction between Bacillus mucilaginosus and bentonite was studied using custom culture media. Results from Inductively Coupled Plasma-Atomic Emission Spectrometry revealed that the bacterium promoted release of Si and Al from solid bentonite to solution. Concomitantly, the K and Fe contents of the mineral increased as shown by X-ray photoelectron spectroscopy results. After interaction with the bacterium, the montmorillonite underwent a possible structure transformation to smectite, as indicated by the emergence of a new weak peak ( d = 9.08 A ° ) shown by X-ray diffraction patterns. The mineralogical changes were also demonstrated by the decrease in the specific surface area of the mineral from 33.0 to 24.0 m2/g (these lower values for SSA of bentonite are related to the particle size of the smectite examined (120–160 mesh) and the weakened absorption bands in Al–O–H and Si–O–Si vibrations by Micro Fourier-transform infrared spectroscopy. The morphology changes in the bacteria observed by environmental scanning electron microscopy and atomic force microscopy revealed an obvious growth of the flagella in the presence of bentonite.

REMOVAL OF Hg(II) FROM AN AQUEOUS MEDIUM BY ADSORPTION ONTO NATURAL AND ALKYL-AMINE MODIFIED BRAZILIAN BENTONITE

Abstract: Mercury ion removal from waste-waters has been the subject of extensive research. The aim of the present investigation was to report the incorporation of the n-alkylamine molecules onto a bentonite surface and the capacities of these new chelating moieties on this modified bentonite surface for mercury removal from water. Bentonite collected from the Amazon region, Brazil, was used in an intercalation process with polar n-alkylamine molecules of general formula H3C(CH2)n-NH2 (n = 1 to 4) in 1,2-dichloroethane. The natural and modified bentonite samples were characterized by elemental analysis, Xray diffraction, helium picnometry, mercury porosimetry, and 29Si, 27Al, and 13Cnuclear magnetic resonance spectroscopy. Because of the increasing size of the molecules attached to the pendant chains, the metal-adsorption capability of the final chelating materials was measured in each case. The adsorption of Hg(II) on natural and modified bentonites was determined under different conditions. The effects of concentration of Hg(II), contact time, and pH were investigated; batch and dynamic adsorption experiments of Hg(II) were conducted on bentonite samples under various conditions. The ability of these materials to remove Hg(II) from aqueous solution was assessed by means of a series of adsorption isotherms at room temperature and pH 4.0. In order to evaluate the bentonite samples as adsorbents in a dynamic system, a glass column was filled with clay samples (1.0 g each) and fed with 1.8610−4 mol dm−3 Hg(II) at pH 4.0. The energetic effects caused by adsorption of metal cations were determined by means of calorimetric titrations. Thermodynamics indicated the existence of favorable conditions for such Hg(II)-nitrogen interactions.

Hydration behavior of mx80 bentonite in a confined-volume system: implications for backfill design

Abstract: Bentonites are considered suitable backfill material for planned underground nuclear-waste repositories because of an inherent capacity to self-seal and retain contaminants when hydrated. Barrier effectiveness, however, depends on the physical properties of bentonite after placement in a repository site, where hydration state and bulk density can vary. The objective of the present study was to investigate commercial bentonite MX80 hydration rates and mechanisms during water infiltration into dry, moist, and wet samples using the ‘wet-cell’ X-ray diffraction technique. During experimentation, water enters a small flow-through cell and induces swelling within a confined reaction volume, analogous to clay barriers in excavated underground sites. Results demonstrated the importance of using dry, well compacted (>1.4 g/cm3) bentonite, which became saturated slowly (<2.0610−9 m/s) with minimal water in noninterlayer sites (external-surface sites, or within pores). The significant degree of interlayer expansion dominated by the formation of two and eventually three water layers developed as hydration clusters with greater probabilities for the same thickness to lie in adjacent interlayer sites. The relatively thicker particles and the less accessible surface area of hydrated, initially dry bentonite probably resulted in less pore-controlled diffusion, but also less potential radionuclide adsorption by surface complexation. Moist MX80 had the greatest water uptake, the smallest (1.23 g/cm3) dry bulk density, and the greatest proportion of water in pores and on external surfaces. Water that initially accumulated in pore spaces subsequently acted as a reservoir for interlayer hydration and probable gel formation in trapped voids, which is expected to occur in more loosely filled gaps within an excavated repository.

Polymer model of zeolite thermochemical stability

Abstract: The polymer model provides a relatively simple and robust basis for estimating the standard Gibbs free energies of formation (ΔG f o ) and standard enthalpies of formation (ΔH f o ) of clay minerals and other aluminosilicates with an accuracy that is comparable to or better than can be obtained using alternative techniques. The model developed in the present study for zeolites entailed the selection of internally consistent standard thermodynamic properties for model components, calibration of adjustable model parameters using a linear regression technique constrained by ΔG f o and ΔH f o values retrieved from calorimetric, solubility, and phase-equilibrium experiments, and assessments of model accuracy based on comparisons of predicted values with experimental counterparts not included in the calibration dataset. The ΔG f o and ΔH f o predictions were found to average within ±0.2% and ±0.3%, respectively, of experimental values at 298.15 K and 1 bar. The latter result is comparable to the good accuracy that has been obtained by others using a more rigorous electronegativity-based model for ΔH f o that accounts explicitly for differences in zeolite structure based on differences in framework density and unit-cell volume. This observation is consistent with recent calorimetric studies indicating that enthalpies of transition from quartz to various pure-silica zeolite frameworks (zeosils) are small and only weakly dependent on framework type, and suggests that the effects on ΔH f o of differences in framework topology can be ignored for estimation purposes without incurring a significant loss of accuracy. The relative simplicity of the polymer model, together with its applicability to both zeolites and clay minerals, is based on a common set of experimentally determined and internally consistent thermodynamic properties for model components. These attributes are particularly well suited for studies of the effects of water-rock-barrier interactions on the long-term safety of geologic repositories for high-level nuclear waste (HLW).

Upper cretaceous clayey levels from western portugal (aveiro and taveiro regions): clay mineral and trace-element distribution

Abstract: Clay-rich deposits of Upper Cretaceous levels in the Taveiro (Reveles and S. Pedro) and Aveiro (Bustos) regions of west-central Portugal are economically and environmentally important, but detailed chemical and mineralogical characterization is lacking. The purpose of this study was to partially fill that gap by correlating the trace-element geochemistry (particularly the rare earth elements, REE) with the mineralogy of both the whole rock and of the <2 μm fraction along selected stratigraphic levels of the formations. The results will help the ceramics industry in the region and will be important in paleoreconstruction environmental studies. Mineralogical and chemical characterizations were carried out using X-ray diffraction (XRD), X-ray fluorescence (XRF), and instrumental neutron activation analysis (INAA). The following clay-mineral associations were identified: (1) at Reveles — smectite, illite, and kaolin minerals; (2) at S. Pedro — kaolin minerals and illite; and (3) at Bustos — illite, kaolin minerals, and mixed-layer illite-smectite. The distribution of trace elements in the <2 μm fraction depended on the clay mineralogy, suggesting that the trace elements were incorporated in, adsorbed to, or even replaced major elements in the clays, as follows: (1) first-row transition elements, particularly Zn and Ga, were enriched when smectite predominated; (2) As, Rb, and Cs were enriched in this fraction of the S. Pedro deposit, the only one with Fe (oxyhydr)oxides and a high proportion of illite; and (3) REE were more concentrated when kaolin minerals predominated. Eu was enriched in the <2 μm fraction, which was due to preferential incorporation in the Fe (oxyhydr)oxides and/or carbonates.

Character and Amount of I-S Mixed-Layer Minerals and Physical-Chemical Parameters of Two Ceramic Clays from Westerwald, Germany: Implications for Processing Properties

Abstract: The industrial assessment of ceramic clays commonly consists of the determination of just two parameters, the particle-size distribution and the chemical composition; other parameters may also be important, however The aim of the present study was to show that a careful determination of the mineralogical phase content provides valuable additional information on the processing behavior of ceramic clays Two ceramic clays (W1 andW2) from theWesterwald area, Germany, were evaluated as being the same with respect to industrial screening criteria, but showed different processing properties In order to elucidate the different behaviors, both clays were investigated comprehensively using a multi-method approach combining physical-chemical and mineralogical methods Different aggregation characteristics for the two clays were revealed by determining the grain-size distribution with and without Na-pyrophosphate as a dispersant In addition, W1 showed a greater electrical conductivity and soluble-salt concentration which promoted dispersion behavior The phase content was identified both for bulk materials and for several grain-size fractions by X-ray diffraction (XRD) and Rietveld analysis The quantitative phase content was crosschecked with the chemical composition by X-ray fluorescence (XRF) analysis Additional information was gathered by thermal analysis, cation exchange capacity (CEC) measurements, Mossbauer spectroscopy, and optical microscopy While bulk samples of W1 and W2 showed nearly the same mineralogical and chemical compositions, investigation of the clay-size fractions (06–2 μm, <06 mm) revealed differences in the composition of the 2:1 layer silicates The percentages of smectite in the mixed-layer I-S, as well as the amount of kaolinite, discrete illite, and smectite were determined by one-dimensional XRD profile fitting (ODPF) Best-fitting results for W1 were achieved for a physical mixture of an illite-rich I-S mixed-layer mineral (R3 I(09)-S) with discrete smectite, whereas W2 was characterized by a greater proportion of smectite in the mixed-layer (R1 I(08)-S), without discrete smectite Based on the different structural features of the swellable clays, a qualitative delamination model for the 2:1 layer silicates during processing of the clays was derived The model provides a further approach, aside from aggregation characteristics, to help understand the clay-processing behavior, which was found to be different for the two ceramic clays investigated

The effect of surface charge and wettability on h2o self diffusion in compacted clays

Abstract: Organo-clays are of interest in the modification of anion transport properties in engineered barriers. In the present study, surface charge and wettability were assessed for tracking changes in the effective diffusion coefficient (Deff) by the formation or suppression of bound H2O layers on the external surfaces of clays. Bentonite samples modified with three different organic cations in amounts of 0 to 400% of the cation exchange capacity were used. Diffusive transport was determined in H2O→D2O exchange experiments in a newly constructed cell adapted to the attenuated total reflectance (ATR) accessory of a Fourier-transform infrared (FTIR) spectrometer at two different dry bulk densities and various degrees of water saturation. All organo-clay combinations showed changes in surface charge after the addition of organic cations, from a negative value of 99 mmolc/kg for the original bentonite to a maximum positive value of 230.5 mmolc/kg for hexadecylpyridinium (HDPy)-montmorillonite. The positive charge resulted from adsorption of the organic cation in excess of the CEC. Hydrophobic surface properties with contact angles >90° were obtained for HDPy-montmorillonite samples with monolayers of organic cations on the external surfaces only. Here, where hydrophobicity suppressed the formation of bound H2O layers, the largest Deff o f 2.7×10−10 m2/s was observed in the high dry bulk density range (1.0–1.5 g/cm3) under water-saturated conditions. In the low dry bulk density range (0.6–0.9 g/cm3) this effect was weakened significantly because, with increasing pore size, the effect of bound H2O layers was reduced. In the high dry bulk density range at partial water saturation (40%), diffusive transport was hindered by the small water volume. Previous work found that, in the high dry bulk density range and water-saturated state, Deff was 2.4×10−11 m2/s for the original bentonite. Deff for all hydrophilic organo-clay samples was ⩽2.1×10−11 m2/s, somewhat less than for the hydrophobic sample. In hydrophilic organo-clay samples, retardation factors that retard the value for Deff, up toa magnitude of 0.5, include an increase in dry bulk density and a decrease in water saturation. In the water-saturated state at high dry bulk densities, hydrophobic surface properties suppressing the formation of bound H2O layers can increase Deff by one order of magnitude.

Application of electron energy-loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) to the study of mineral transformation associated with microbial Fe-reduction of magnetite

Abstract: Electron energy-loss spectroscopy (EELS), energy-filtered transmission electron microscopy (EFTEM), and high-resolution transmission electron microscopy (HRTEM) have been applied in mineralogy and materials research to determine the oxidation states of various metals at high spatial resolution. Such information is critical in understanding the kinetics and mechanisms of mineral–microbe interactions. To date, the aforementioned techniques have not been applied widely in the study of such interactions. In the present study, the three techniques above were employed to investigate mineral transformations associated with microbial Fe(III) reduction in magnetite. Shewanella putrefaciens strain CN32, a dissimilatory metal-reducing bacterium, was incubated with magnetite as the sole electron acceptor and lactate as the electron donor for 14 days under anoxic conditions in bicarbonate buffer. The extent of bioreduction was determined by wet chemistry and mineral solids were investigated by HRTEM, EFTEM, and EELS. Magnetite was partially reduced and biogenic siderite formed. The elemental maps of Fe, O, and C and red-green-blue (RGB) composite map for residual magnetite and newly formed siderite were contrasted by the EFTEM technique. The HRTEM revealed nm-sized magnetite crystals coating bacterial cells. The Fe oxidation state in residual magnetite and biogenic siderite was determined using the EELS technique (the integral ratio of L3 to L2). The integral ratio of L3 to L2 for magnetite (6.29) and siderite (2.71) corresponded to 71% of Fe(III) in magnetite, and 24% of Fe(III) in siderite, respectively. A chemical shift (~1.9 eV) in the Fe-L3 edge of magnetite and siderite indicated a difference in the oxidation state of Fe between these two minerals. Furthermore, the EELS images of magnetite (709 eV) and siderite (707 eV) were extracted from the electron energy-loss spectra collected, ranging from 675 to 755 eV, displaying different oxidation states of Fe in the magnetite and siderite phases. The results demonstrate that EELS is a powerful technique for studying the Fe oxidation-state change as a result of microbial interaction with Fe-containing minerals.

Modification of montmorillonite with poly(oxypropylene) amine hydrochlorides: basal spacing, amount intercalated, and thermal stability

Abstract: Few studies have explored the change in thermal stability of poly(oxypropylene) (POP) ammonium ions after intercalation, even though several studies have focused on the modification of montmorillonite (Mt) with POP amine hydrochloride. The purpose of the present study was to understand the effect of chain length of POP amine hydrochlorides on the basal spacing of modified Mt, and the amount and thermal stability of the ammonium ions intercalated. The relations between basal spacing, organic fraction, and thermal stability of the ammonium ions intercalated were also explored. Series of modified Mt were prepared via ion-exchange between Na-montmorillonite (Na + -Mt) and POP diammonium ions or POP triammonium ions with different chain lengths, and were then characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, and simultaneous differential scanning calorimetry-thermogravimetric analysis. The results revealed that the basal spacing of modified Mt increased with the hydrophobic chain length of the POP ammonium ions. The amount of triammonium ions intercalated was close to the theoretical amount, while the organic fraction of modified Mt was directly proportional to the basal spacing of modified Mt. The intercalated ammonium ions were, therefore, contained within the interlayer space of Mt. After intercalation, the thermal stability of the POP ammonium ions with various chain lengths was reduced; i.e. T onset was reduced by 7–60oC for short-chain POP ammonium ions (D400 and T403) and by 177–192oC for long-chain ions (D2000, D4000, T3000, and T5000).

Mineralogical and geochemical characteristics and genesis of the sepiolite deposits at polatli basin (ankara, turkey)

Abstract: The Middle—Upper Miocene—Pliocene sediments near Polatli contain commercial sepiolitic clay deposits. The sepiolite-rich Polatli basin sediments were studied to describe the sepiolitic clay deposits of the area and to assess the environments of formation using X-ray diffraction, optical and scanning electron microscopy, and chemical analysis. The Polatli basin is an elongated, rift-related graben trending NE—SW in central Turkey, filled with continental Late Miocene to Early Pliocene sediments. The sediments which comprise claystone, marl and limestone, dolostone, and evaporites are characteristic deposits of low-salinity, playa-lake depositional environments. These sepiolite-rich deposits include sepiolite, dolomite, and calcite, with minor amounts of palygorskite, quartz, moganite, amorphous silica (opal-CT), and feldspar. The sepiolite shows all the characteristic X-ray diffraction reflections of that mineral, whereas amorphous silica containing sepiolite shows some of the characteristic reflections of sepiolite, but with somewhat broader and less intense basal reflections. In the siliceous deposits, the long, fibrous, and filamentous aggregates of the sepiolite were converted to thick, short fibers, low in Mg, and showing transition to amorphous silica. Major and trace elements (e.g. Si, Al, Fe, Mg, Sr, Ba, etc.) were found almost exclusively in Mg-rich smectitic claystone and detrital silicate-rich rocks, whereas Mg, Ca, and some Si were concentrated in the neoformed minerals in the basin. The rare-earth elements (REE) and some of the high-field strength elements (HFSE), large ion lithophile elements (LILE), and transition elements (TRE) patterns were similar for detrital silicate-rich rocks and formed from neoformed mineral lithologies. The REE, TRE, LILE, and some of the HFSE contents of limestone, dolostone, and sepiolitic claystone were similar while those of detrital silicate-rich rocks and Mg-rich smectitic claystones were similar to each other. PAAS-normalized REE and other trace-element patterns were typically subparallel and depleted in neoformed minerals. All sample groups had positive Eu* anomalies, except Mg-rich smectite (0.80). Limestone, dolostone, and amorphous silica compounds showed slightly negative Ce* anomalies, whereas sepiolitic claystones, Mg-rich smectitic claystones, and detrital silicate-rich rocks had a slightly positive Ce* anomaly.

Volcanism in association with the prelude to mass extinction and environment change across the permian-triassic boundary (ptb), southern china

Abstract: In order to better understand the provenance of the sediments and environmental change associated with the Permian-Triassic (P/T) biotic crisis, a comparative clay mineralogical study of the Permian-Triassic boundary (PTB) sediments between the Meishan section (the Global Stratotype Section and Point of the PTB) and the Xiakou section, southern China, was undertaken using X-ray diffraction and differential scanning calorimetry (DSC). The results showed that clay minerals of the packstone bed 24e, in which the preludial mass extinction occurred at Meishan, consist of 56% mixed-layer illite-smectite (I-S), 39% illite, and 5% kaolinite. A dehydroxylation effect was measured at 652°C, indicating that I-S and illite of this bed contain mainly cis -vacant ( cv ) layers related to volcanic origin. The dehydroxylation event correlates with bed P257 at Xiakou. The white clay bed 25 also corresponding to the main extinction event at Meishan contains 95% I-S and 5% kaolinite, with a strong endothermic effect at 676°C and a weaker one at 514°C in the DSC curve. These results are attributed to dehydroxylation of cv layers in I-S clays, suggesting that I-S in the white clay bed was derived from marine alteration of volcanic ash, in agreement with the conodont-correlated clay (P258) at Xiakou. (Conodonts are tooth-like microfossils and are usually used as an indicator of age in PTB stratigraphy.) Increases in chlorite and illite contents in the black clays (bed 26) at Meishan and the conodont-correlated black clay layer (P259b) at Xiakou probably indicate stronger erosional processes under cooler and more arid conditions. Volcanic materials found in a bed which marked the prelude to the main episode of mass extinction reinforce the temporal link between volcanism and the mass extinction.