Fe-bearing Phyllosilicates Research Articles

Cysteine-Facilitated Cr(VI) reduction by Fe(II/III)-bearing phyllosilicates: Enhancement from In-Situ Fe(II) generation

Structural Fe in phyllosilicates represents a crucial and potentially renewable reservoir of electron equivalents for contaminants reduction in aquatic and soil systems. However, it remains unclear how in-situ modification of Fe redox states within Fe-bearing phyllosilicates, induced by electron shuttles such as naturally occurring organics, influences the fate of contaminants. Herein, this study investigated the processes and mechanism of Cr(VI) reduction on two typical Fe(II/III)-bearing phyllosilicates, biotite and chlorite, in the presence of cysteine (Cys) at circumneutral pH. The experimental results demonstrated that Cys markedly enhanced the rate and extent of Cr(VI) reduction by biotite/chlorite, likely because of the formation of Cr(V)-organic complexes and consequent electron transfer from Cys to Cr(V). The concomitant production of non-structural Fe(II) (including aqueous Fe(II), surface bound Fe(II), and Cys-Fe(II) complex) cascaded transferring electrons from Cys to surface Fe(III), which further contributed to Cr(VI) reduction. Notably, structural Fe(II) in phyllosilicates also facilitated Cr(VI) reduction by mediating electron transfer from Cys to structural Fe(III) and then to edge-sorbed Cr(VI). 57Fe Mössbauer analysis revealed that cis-coordinated Fe(II) in biotite and chlorite exhibits higher reductivity compared to trans-coordinated Fe(II). The Cr end-products were insoluble Cr(III)-organic complex and sub-nanometer Cr2O3/Cr(OH)3, associated with residual minerals as micro-aggregates. These findings highlight the significance of in-situ produced Fe(II) from Fe(II/III)-bearing phyllosilicates in the cycling of redox-sensitive contaminants in the environment.

Syndepositional and diagenetic processes in the pigmentation of Middle Ordovician carbonate red beds in South China

Marine red beds (MRBs) are often attributed to specific redox environments during syndepositional and early diagenetic phases. During the Middle Ordovician, a succession of reddish, deeper-water nodular argillaceous limestones (i.e., Zitai and Kuniutan formations) were deposited along the margin of Yangtze Platform in South China. However, the origin of their color remains enigmatic. In this study, we investigated the Middle Ordovician MRBs from a borehole core newly drilled in the Lower Yangtze area of South China whose stratigraphy frameworks are constrained by carbon isotope and biostratigraphy. This study investigates the pigmentation of these MRBs by integrating petrographic observations, elemental geochemistry, diffusive reflectance spectrometry (DRS), and scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometry (EDS). DRS results show that the red pigment is caused by hematite particles in submicron- to micron-level size. SEM demonstrates that the hematite grains are either detrital grains with traces of physical transport from terrestrial source, or flaky amorphous hematite aggregates situated within the calcite crystal interstices, implicating both syndepositional and early diagenetic origins, respectively. In terms of the geochemical result of the bulk rocks, the close positive correlation between Al2O3 and Fe2O3 indicates that the iron pigment materials may mainly originate from terrestrial Fe-bearing phyllosilicates. These observations are also consistent with the distribution patterns of rare earth elements (REEs) in carbonate leachates. The MRB limestones are characterized by MREE-bulged patterns and close to ~1 Ce anomalies, suggesting active reductive dissolution and subsequent reprecipitation of iron oxides during diagenesis in pore systems. This study proposes that the coloration of Middle Ordovician MRBs in Lower Yangtze Platform was linked to the enhanced input of terrestrial clay minerals rich in iron. The reductive dissolution released iron ions from terrestrial detrital and allowed subsequent reprecipitation of iron-oxides in pore water system during the fluid-buffered diagenesis. In this light, hematites formed during both the syndepositional and diagenetic processes thus could have involved the coloration of the Middle Ordovician carbonate red beds in this case.

Anaerobic methane-oxidizing activity in a deep underground borehole dominantly colonized by Ca. Methanoperedenaceae.

The family Ca. Methanoperedenaceae archaea mediates the anaerobic oxidation of methane (AOM) in different terrestrial environments. Using a newly developed high-pressure laboratory incubation system, we investigated 214- and 249-m deep groundwater samples at Horonobe Underground Research Laboratory, Japan, where the high and low abundances of Ca. Methanoperedenaceae archaea have been shown by genome-resolved metagenomics, respectively. The groundwater samples amended with 13 C-labelled methane and amorphous Fe(III) were incubated at a pressure of 1.6MPa. After 3-7 days of incubation, the AOM rate was 45.8 ± 19.8nM/day in 214-m groundwater. However, almost no activity was detected from 249-m groundwater. Based on the results from 16S rRNA gene analysis, the abundance of Ca. Methanoperedenaceae archaea was high in the 214-m deep groundwater sample, whereas Ca. Methanoperedenaceae archaea was undetected in the 249-m deep groundwater sample. These results support the in situ AOM activity of Ca. Methanoperedenaceae archaea in the 214-m deep subsurface borehole interval. Although the presence of Fe-bearing phyllosilicates was demonstrated in the 214-m deep groundwater, it needs to be determined whether Ca. Methanoperedenaceae archaea use the Fe-bearing phyllosilicates as in situ electron acceptors by high-pressure incubation amended with the Fe-bearing phyllosilicates.

Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

Biosignatures in Subsurface Filamentous Fabrics (SFF) from the Deccan Volcanic Province, India

The morphology, chemical, and mineralogical composition of subsurface filamentous fabrics (SFF) from the Deccan Volcanic Province (DVP) were investigated to determine the origin of these spectacular aggregates. SFF occur in a wide variety of morphologies ranging from pseudo-stalactites to irregular fabrics and are classified as SFFIr (irregular) or SFFMa (matted). The SFF samples exhibit a thread-like (or filament-like) center from which mineral precipitation starts to form the final macroscopic morphologies. Detailed investigations revealed organic material (fungal chitin) in the innermost filamentous core, which may have acted as an initial nucleus for the mineralization processes. The morphometric characteristics of certain filamentous fabrics are very similar to those of microbial filaments and the fabrics formed from them but are clearly distinct from similar types of non-biological precipitates (fibrous minerals, speleothems, and “chemical gardens”). These features indicate that the filamentous cores might be products of microbial communities that were active in the basaltic cavities. The SFF cross-sections display similar concentric layers of the mineral succession and reach thicknesses of several centimeters with spectacular lengths up to 100 cm and constant diameters. The typical mineralization sequence points to temporal variation in the chemical composition of the mineralizing fluids from Fe(Mg)-rich (Fe-oxides/-hydroxides, Fe-rich sheet silicates such as celadonite and di-/tri-smectite) to Ca-dominated (Ca-rich zeolites) and finally pure SiO2 (opal-CT, chalcedony, and macro-crystalline quartz). Assuming biological activity at least during the early mineralization processes, circumneutral pH conditions and maximum temperatures of 100–120 °C were supposed. The formation of filamentous cores including Fe-bearing phyllosilicates probably occurred near the surface after cooling of the lava, where the elements necessary for mineral formation (i.e., Si, Mg, Al, Fe) were released during alteration of the volcanic host rocks by percolating fluids.

Contribution of bacterially-induced oxidation of Fe-silicates in iron-rich ore to laterite formation, Salobo IOCG mine, Brazil

A diverse array of microorganisms, found within the uppermost lithosphere, can mediate the dissolution and precipitation of minerals and therefore contribute to the formation of laterites. The Salobo iron-oxide copper gold (IOCG) mine in Brazil is an ideal environment to examine the specific interaction between iron-oxidising bacteria and ferrous iron-bearing minerals during formation of a ~60 m thick laterite weathering profile. We identified bacteria using DNA extracted from samples in active weathering zones. Many of the identified species are capable of oxidising the ferrous iron and/or reduced sulphur that occurred in minerals associated with the unweathered rocks of the deposit. Fe-bearing phyllosilicates have been variably altered to clays along cleavage planes by bacterial iron oxidation. Accelerated weathering of fresh rocks in laboratory-scale leaching column experiments was conducted using an endemic Acidithiobacillus ferrooxidans ssp. previously cultured from the Salobo mine. There were strong similarities between field samples from the Salobo laterite zone, and experimental leachate chemistry, associated precipitates, and fossilised bacteria remnants, particularly with respect to ferric (oxyhydr)oxide formation. Groundwaters in the Salobo laterite zone have circumneutral pH, whereas some iron-oxidising bacteria thrive in, and locally create, more acidic conditions (~pH 3). The leaching experiments showed that bacterially-facilitated silicate weathering, and bornite (Cu5FeS4) oxidation, can consume acid generated by bacterial oxidation reactions, creating an effective equilibrium with ferric (oxyhydr)oxide precipitation. However, the current acid neutralisation capacity of the ferricrete horizon at the top of the laterite zone was minimal. While bacterial activity promoted mineral oxidation and decomposition within the thick laterite at the Salobo mine, related iron mobility is restricted to the micrometre scale by essentially instantaneous precipitation of ferric (oxyhydr)oxide that eventually transforms via inorganic dehydration to goethite and hematite. Similar processes to those described in this study have likely occurred during the formation of many other iron-rich laterites.

The Role of Fe-Bearing Phyllosilicates in DTPMP Degradation under High-Temperature and High-Pressure Conditions.

To ensure safer and more efficient unconventional oil/gas recovery and other energy-related subsurface operations, it is important to understand the effects of abundant Fe-bearing phyllosilicates on the degradation of phosphonates, which are applied to inhibit scale formation. In this study, under subsurface relevant conditions (i.e., slightly oxic owing to oxygen-containing injection, 50-95 °C, and 102 atm CO2), we reacted 0.5 mM DTPMP (diethylenetriaminepenta(methylene)phosphonate, a model phosphonate) with three phyllosilicates: an Fe-poor muscovite, an Fe(II)-rich biotite, and an Fe(III)-rich nontronite. The three phyllosilicates induced different effects on DTPMP degradation, with no distinguishable effect by muscovite, slight promotion by nontronite, and remarkable promotion by biotite. We found that Fe associated with phyllosilicates is key to the redox degradation of DTPMP: reactive oxygen species (ROS) were generated through the reduction of molecular oxygen by Fe(II) adsorbed on the mineral surface or in the mineral structure, and the hydroxyl radicals further degraded DTPMP to form phosphate, formate, and DTPMP residuals. In addition, DTPMP degradation was favored at higher temperatures, probably resulting from more exposed reactive Fe(II) sites created by enhanced biotite dissolution and also from faster electron transfers. Dissolved Fe and Al precipitated with phosphate or degraded DTPMP and formed secondary minerals. This study provides new information about how DTPMP degradation is affected by the presence of Fe-bearing phyllosilicates under high-temperature and high-pressure conditions and has implications for engineered subsurface operations.

What is controlling the reflectance spectra (0.35–150 µm) of hydrated (and dehydrated) carbonaceous chondrites?

In order to determine the controls on the reflectance spectra of hydrated carbonaceous chondrites, reflectance spectra were measured for a series of samples with well-determined mineralogy, water-content, and thermal history. This includes 5 CR chondrites, 11 CM chondrites, and 7 thermally metamorphosed CM chondrites. These samples were characterized over the 0.35–150 µm range by reflectance spectroscopy in order to cover the full spectral range accessible from ground based observation, and that will be determined in the near-future by the Hayabusa-2 and Osiris-REx missions. While spectra show absorption features shortward of 35 µm, no strong absorption bands were identified in this suite of samples longward of 35 µm. This work shows that the 0.7-µm band observed in hydrated carbonaceous chondrites is correlated with the total water content as well as with the band depth at 2.7 µm, confirming the suggestion that they are related to Mg-rich, Fe-bearing phyllosilicates. A feature at 2.3 µm, diagnostic of such phyllosilicates was found for all samples with a detectable 0.7-µm band, also indicative of Mg-rich phyllosilicates.A strong variability is found in the shape of the 3-µm band among CM chondrites, and between CM, CR and thermally metamorphosed CM chondrites. Heavily altered CM chondrites show a single strong band around 2.72 µm while more thermally metamorphosed CM samples show an absorption band at higher wavelength. The CR chondrite GRO 95577 has a 3-µm feature very similar to those of extensively altered CM chondrites while other CR chondrite rather shows goethite-like signatures (possibly due to terrestrial weathering of metals). Thermally metamorphosed CM chondrites all have 3-µm features, which are not purely due to terrestrial adsorbed water. The band shape ranges from heavily altered CM-like to goethite-like.The overall reflectance was found to be significantly higher for CR chondrites than for CM chondrites. This is also true for the hydrated CR chondrite GRO 95577 whose reflectance spectrum is almost identical to spectra obtained for CM chondrites except that it is brighter by about 40% in the visible. Another possibility to distinguish hydrated CM from hydrated CR chondrites is to use the combination of band depths at 0.7 and 2.3 µm.When comparing the spectra obtained with Cg and Cgh spectral end member, it is found that the band depth determined for hydrated chondrites (0.7 and 2.3 µm) are always higher than calculated for these spectral endmembers. If one considers only asteroids with unambiguous hydration detection, band depth at 0.7 µm is of similar value to those measured for hydrated carbonaceous chondrites.

THE Ch-CLASS ASTEROIDS: CONNECTING A VISIBLE TAXONOMIC CLASS TO A 3μm BAND SHAPE

Asteroids belonging to the Ch spectral taxonomic class are defined by the presence of an absorption near 0.7 {\mu}m, which is interpreted as due to Fe-bearing phyllosilicates. Phyllosilicates also cause strong absorptions in the 3-{\mu}m region, as do other hydrated and hydroxylated minerals and H2O ice. Over the past decade, spectral observations have revealed different 3-{\mu}m band shapes the asteroid population. Although a formal taxonomy is yet to be fully established, the "Pallas-type" spectral group is most consistent with the presence of phyllosilicates. If Ch class and Pallas type are both indicative of phyllosilicates, then all Ch-class asteroids should also be Pallas-type. In order to test this hypothesis, we obtained 42 observations of 36 Ch-class asteroids in the 2- to 4-{\mu}m spectral region. We found that 88% of the spectra have 3-{\mu}m band shapes most consistent with the Pallas-type group. This is the first asteroid class for which such a strong correlation has been found. Because the Ch class is defined by the presence of an absorption near 0.7 {\mu}m, this demonstrates that the 0.7-{\mu}m band serves not only as a proxy for the presence of a band in the 3-{\mu}m region, but specifically for the presence of Pallas-type bands. There is some evidence for a correlation between band depth at 2.95 {\mu}m and absolute magnitude and/or albedo. However, we find only weak correlations between 2.95-{\mu}m band depth and semi-major axis. The connection between band depths in the 0.7- and 3-{\mu}m regions is complex and in need of further investigation.

Spectral parameters for Dawn FC color data: Carbonaceous chondrites and aqueous alteration products as potential cerean analog materials

We identified a set of spectral parameters based on Dawn Framing Camera (FC) bandpasses, covering the wavelength range 0.4–1.0μm, for mineralogical mapping of potential chondritic material and aqueous alteration products on dwarf planet Ceres. Our parameters are inferred from laboratory spectra of well-described and clearly classified carbonaceous chondrites representative for a dark component. We additionally investigated the FC signatures of candidate bright materials including carbonates, sulfates and hydroxide (brucite), which can possibly be exposed on the cerean surface by impact craters or plume activity. Several materials mineralogically related to carbonaceous chondrites, including pure ferromagnesian phyllosilicates, and serpentinites were also investigated. We tested the potential of the derived FC parameters for distinguishing between different carbonaceous chondritic materials, and between other plausible cerean surface materials. We found that the major carbonaceous chondrite groups (CM, CO, CV, CK, and CR) are distinguishable using the FC filter ratios 0.56/0.44μm and 0.83/0.97μm. The absorption bands of Fe-bearing phyllosilicates at 0.7 and 0.9μm in terrestrial samples and CM carbonaceous chondrites can be detected by a combination of FC band parameters using the filters at 0.65, 0.75, 0.83, 0.92 and 0.97μm. This set of parameters serves as a basis to identify and distinguish different lithologies on the cerean surface by FC multispectral data.

The physical–chemical regime of argillaceous interseam sediments in the Emet borate district, Turkey: A transition from non-metallic volcano-sedimentary to metallic epithermal deposits

Among the world-class borate deposits, the borate deposits at Emet Espey and Hisarık in Western Anatolia, Turkey, stand out by their enrichment in As-bearing minerals realgar and orpiment. In terms of abundance these Turkish deposits are on a par with the world-class borate deposits in North America. In Turkey As-bearing base and precious metal deposits of epithermal low-sulfidation type developed adjacent to these non-metallic deposits under consideration. The borate deposits formed during the Miocene under arid climatic conditions within intermontane basins. The latter were filled with volcano-sedimentary series, which host a playa lake at the center. If we use the non-borate minerals, mainly phyllosilicates, as marker minerals and as a link to the equivalent shallow metal deposits, we can distinguish six paragenetic mineral associations (PAS). They reflect the following intrabasinal processes responsible for the formation of the deposits: (1) siliciclastic detrital input into the depocenter, (2) basic volcanic input into the depocenter, (3) felsic volcanic input into the depocenter, (4) evaporation in the depocenter, (5) hot brine activity in the depocenter and at its rim, and (6) oxidation. The physical–chemical regime of the various stages, including the temperature of formation, and the Eh and pH conditions are discussed in the current study indicating the interaction of intrabasinal and extrabasinal fluids:PAS-1: The alluvial–fluvial drainage systems delivered siliciclastics into the playa lake, which is a result of the weathering and denudation from the crystalline footwall rocks. The phyllosilicate association may be described as “kaolinite-in” and “pyrophyllite-out” bearing witness of a true supergene siallitic weathering, unaffected by hydrothermal activity (Eh>0, pH<7, T≈30°C).PAS-2: The phyllosilicate association which features chlorite and saponite is indicative of a reducing environment during smectitization. A moderate activity of basinal hot brines can be recognized, affecting the mineralogical input derived from basic magmatic rocks into the Neogene borate basin (Eh<0, pH6–7, T=30°–100°C).PAS-3: The alteration within the felsic tuffaceous rocks is marked by two discrete trends, the Mg trend with smectite as the diagnostic mineral (pH>7, T<100°C), and the K trend with sanidine and zeolite (clinoptilolite) being typical of this trend (pH>5.5, T<100°C). The temperature of alteration in the tuffaceous interseam pyroclastic deposits was higher than during alteration of stage (2).PAS-4: The borate minerals resulted from the combined action of evaporation and hydrothermal activity. The temperature of formation increased relative to the afore-mentioned processes. As far as the phyllosilicates are concerned, muscovite can be considered a marker mineral of this stage (pH=7–13, T≪100°C) (≪ means significantly lower than).PAS-5: Although dominated by As sulfides, the sheet silicate muscovite can be referred to as a marker for stage 5. This mineral association most strikingly shows the genetic relationship between the low-sulfidation epithermal metal and volcano-sedimentary non-metallic deposits, which developed within an ephemeral lacustrine environment of deposition (Eh<0⇒>0, pH=3–7, T≈100°C).PAS-6: A side-effect of a rising Eh is also observed among the Fe-bearing phyllosilicates. Nontronite, the diagnostic element acts like an umbrella, which covers the low-sulfidation epithermal metal and volcano-sedimentary non-metallic deposits (Eh>0, pH>5.5, T<100°C).A comparison of non-metallic and metallic shallow or epithermal systems reveals that the borate–smectite–muscovite system is the distal or non-metallic part of the low-sulfidation-type epithermal system when it comes to the accumulation of precious and base metals in a region under semi(arid) climatic conditions. Both systems show the largest overlap in stages (5) and (6). At the opposite end of this metallic–non-metallic catena, the alunite (APS mineral)–pyrophyllite–kaolinite system takes a position similar to the borate–smectite–muscovite system for the high-sulfidation metallic epithermal system.

Atom exchange between aqueous Fe(II) and structural Fe in clay minerals.

Due to their stability toward reductive dissolution, Fe-bearing clay minerals are viewed as a renewable source of Fe redox activity in diverse environments. Recent findings of interfacial electron transfer between aqueous Fe(II) and structural Fe in clay minerals and electron conduction in octahedral sheets of nontronite, however, raise the question whether Fe interaction with clay minerals is more dynamic than previously thought. Here, we use an enriched isotope tracer approach to simultaneously trace Fe atom movement from the aqueous phase to the solid ((57)Fe) and from the solid into the aqueous phase ((56)Fe). Over 6 months, we observed a significant decrease in aqueous (57)Fe isotope fraction, with a fast initial decrease which slowed after 3 days and stabilized after about 50 days. For the aqueous (56)Fe isotope fraction, we observed a similar but opposite trend, indicating that Fe atom movement had occurred in both directions: from the aqueous phase into the solid and from the solid into aqueous phase. We calculated that 5-20% of structural Fe in clay minerals NAu-1, NAu-2, and SWa-1 exchanged with aqueous Fe(II), which significantly exceeds the Fe atom layer exposed directly to solution. Calculations based on electron-hopping rates in nontronite suggest that the bulk conduction mechanism previously demonstrated for hematite1 and suggested as an explanation for the significant Fe atom exchange observed in goethite2 may be a plausible mechanism for Fe atom exchange in Fe-bearing clay minerals. Our finding of 5-20% Fe atom exchange in clay minerals indicates that we need to rethink how Fe mobility affects the macroscopic properties of Fe-bearing phyllosilicates and its role in Fe biogeochemical cycling, as well as its use in a variety of engineered applications, such as landfill liners and nuclear repositories.

Iron–clay interactions: Detailed study of the mineralogical transformation of claystone with emphasis on the formation of iron-rich T–O phyllosilicates in a step-by-step cooling experiment from 90 °C to 40 °C

Iron–minerals–water interactions are of primary importance in the contexts of underground structure engineering (e.g. reactive barriers or deep geological storage) and for the understanding of secondary alteration processes in primitive meteorites. To improve our understanding of these systems, we determine the mineralogical transformations induced by the association of iron and silicates during a cooling through an experimental simulation of iron–clay interactions with a step-by-step procedure in the range of 90°C to 40°C. The run products and solutions are well characterised, by means of different techniques (X-ray diffraction, scanning and transmission electron microscopy, manocalcimetry, inductively coupled plasma optical emission spectrometry and ion chromatography), and the thermodynamic data concerning Fe-bearing phyllosilicates are well-tested comparing the modelling and experimental results. Therefore, the main mineralogical modifications observed include the remarkable formation of cronstedtite and greenalite, as well as the formation of magnetite at all temperatures, along with a significant dissolution of quartz, mixed-layer illite–smectite clays, illite (affecting more than 70% of each mineralogical phase) and a partial alteration of chlorite, kaolinite and dolomite. The experimental results confirm the reaction path predicted by thermodynamic modelling, i.e. the formation of iron-rich T–O phyllosilicates (cronstedtite and greenalite) and magnetite at the expense of metal iron and silicates. Both the experimental and thermodynamic results presented in this study provide important constraints to well predict the impact of nuclear waste canister corrosion in a claystone media and to better understand secondary alteration processes, which could also affect the mineralogical and chemical composition of primitive meteorites.

Iron oxidation state in phyllosilicate single crystals using Fe-K pre-edge and XANES spectroscopy: Effects of the linear polarization of the synchrotron X-ray beam

We investigated the influence of linear polarization of the synchrotron X-ray beam on the determination of iron oxidation state in phyllosilicates. Fe K-edge XANES spectra and pre-edge peaks have been recorded for various orientations of single crystals of biotite (Bt), clinochlore (chlorite group; Cli), talc (Tc), and antigorite (serpentine group; Ant). Ab initio XANES calculations, performed for 6 orientations of the biotite structure, support the experimental results. Depending on crystal orientation, the experimental results show, (1) important changes both for XANES and pre-edge peaks, (2) characteristic changes of spectral signatures regardless of the mineral species, (3) uncorrelated changes between XANES and pre-edge peaks, and (4) important changes of the energy position of pre-edge peaks, but with no significant change for their integrated areas. Regarding the crystal orientation, the pre-edge peak centroid varies in energy by ~0.4 eV in the case of Bt and Tc, and by ~0.2 eV for Cli and Ant. Such variations correspond to XFe 3+ (i.e., Fe 3+ /Fe total ) of 0.22 and 0.15, respectively. Comparison with the analysis of powdered samples show that Fe-redox can be generally framed as follows: -(2/3) XFe 3+ &lt; XFe 3+ (powder) &lt; +(1/3)XFe 3+ . In good agreement with the so-called “magic angle” theorem for powdered samples, we propose an ideal orientation of the single crystals that provide similar pre-edge peaks as for powdered samples. With the wavevector (i.e., beam direction) perpendicular to (100) or (010), measurements should be done with an angle of 35° between the electric-field vector and the [001] crystal direction. Moreover, measurements performed with the wavevector perpendicular to (001) systematically result in an overestimation of XFe 3+ up to 0.07. Finally, we show that the appropriate positioning of single crystals reduces the XFe 3+ uncertainty to the intrinsic error of pre-peak measurements. This approach opens possibilities for in situ analyses of Fe-bearing phyllosilicates in thin sections that are potentially relevant for scientific fields such as hydrothermal/metallogenic, metamorphic, meteoritic or environmental mineralogy

Isolation and Microbial Reduction of Fe(III) Phyllosilicates from Subsurface Sediments

Fe(III)-bearing phyllosilicates can be important sources of Fe(III) for dissimilatory microbial iron reduction in clay-rich anoxic soils and sediments. The goal of this research was to isolate Fe(III) phyllosilicate phases, and if possible, Fe(III) oxide phases, from a weathered shale saprolite sediment in order to permit experimentation with each phase in isolation. Physical partitioning by density gradient centrifugation did not separate phyllosilicate and Fe(III) oxide phases (primarily nanoparticulate goethite). Hence we examined the ability of chemical extraction methods to remove Fe(III) oxides without significantly altering the properties of the phyllosilicates. XRD analysis showed that extraction with acid ammonium oxalate (AAO) or AAO in the presence of added Fe(II) altered the structure of Fe-bearing phyllosilicates in the saprolite. In contrast, citrate-dithionite-bicarbonate (CDB) extraction at room temperature or 80 °C led to minimal alteration of phyllosilicate structures. Reoxidation of CDB-extracted sediment with H(2)O(2) restored phyllosilicate mineral d-spacing and Fe redox speciation to conditions similar to that in the pristine sediment. The extent of microbial (Geobacter sulfurreducens) reduction of Fe(III) phyllosilicates isolated by CDB extraction and H(2)O(2) reoxidation (16 ± 3% reduction) was comparable to what took place in pristine sediments as determined by Mossbauer spectroscopy (20 ± 11% reduction). These results suggest that materials isolated by CDB extraction and H(2)O(2) reoxidation are appropriate targets for detailed studies of natural soil/sediment Fe(III) phyllosilicate reduction.

A comparative analysis of potential biosignatures in basalt glass by FIB-TEM

Bulk analyses of fresh and altered subseafloor basalts have elucidated the large-scale geochemical fluxes that result from water–rock interactions, which ultimately affect both seawater and mantle compositions, but these analyses do not provide insights into the physical and chemical processes occurring on the scale of microorganisms. Micron-sized tubules that may represent microbial boring into basalt glass are ubiquitous throughout the seafloor and can potentially be preserved over geological time. These putative biosignatures give clues into the history of life on Earth, and may be significant for the search for life on other planets; however, their formation and mineralization are still poorly understood. We have used a combination of focused ion beam (FIB) milling and transmission electron microscopy (TEM) to analyze the textural and geochemical characteristics of tubular alteration features from four basalt glass samples representing different sample locations, collection depths, and ages, including one sample from the Cretaceous Troodos ophiolite.High-resolution TEM imaging consistently identified platy layers of partially crystalline material infilling the tubules. The lattice spacings and diffraction patterns showed that this material comprises a mix of two-layer and three-layer silicates. Closer inspection of the contact point between the tubules and the surrounding basalt glass commonly revealed a thin leached rim around the tubules. Electron energy loss spectroscopy mapping demonstrated that these rims are depleted in everything but Si and O, which implies that they are formed by a leaching process, likely the incongruent dissolution of the glass during formation of the tubule. The infilling material is depleted in Ca and enriched in K, and in most cases also enriched in Fe, which is consistent with their identification as Fe-bearing phyllosilicates. These observations are consistent across the samples, which implies that the processes that form and mineralize tubular glass alteration features are similar throughout the oceanic subsurface and through time.