Keywords: clays clay minerals Mars infrared astrobiology Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal-OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27-2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal-chemical substitution and mixed layering affect the position of the diagnostic metal-OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified ditrioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates.

This study aims to contribute to a better understanding of the nature and evolution mechanism of interstratifi ed clay minerals. We examined the <2 μm or <0.2 μm size fraction of interstratifi ed kaolinite-smectite (K-S) formed by hydrothermal and hydrogenic alteration of volcanogenic material from a Tortonian clay deposit (Almería, Spain), a weathered Eocene volcanic ash (Yucatan, Mexico), and a weathered Jurassic bentonite (Northamptonshire, England). The methods used were X-ray diffraction analysis (XRD) of random and oriented preparations, thermogravimetry, chemical analysis, and 29 Si MAS nuclear magnetic resonance. The proportions of kaolinite and smectite in K-S (%K) were determined by fi tting the XRD patterns of ethylene-glycol-saturated samples with patterns calculated with the NEWMOD computer program. The obtained range of compositions is 0-85%K. A comparison of the results from the various techniques showed non-linear relationships, indicating that the layers in K-S are complex and hybrid in nature. The smectite-to-kaolinite reaction is a solid-state transformation proceeding through formation of kaolinite-like patches within the smectite layers. The process consists of several non-simultaneous stages: (1) removal of parts of the tetrahedral sheet, resulting in formation of kaolinite-like patches; (2) layer collapse to ~7 Å where the kaolinite-like patches are suffi ciently large; (3) Al for Mg substitution in the octahedral sheet, simultaneous or slightly delayed with respect to layer collapse, causing a layer-charge decrease and loss of interlayer cations; (4) Si for Al replacement in the tetrahedral sheet and further loss of interlayer cations. Iron remains in the kaolinite or is lost at the latest stages of the process.

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The synthesis of zeolites from three samples of fly ash was carried out through a low-temperature (25-60°C) hydrothermal process with a NaOH pre-fusion treatment preceded by sonication. The results were compared with those of conventional hydrothermal syntheses. XRD and SEM investigations demonstrate that the application of ultrasonic treatment facilitates the formation of zeolites at a lower-temperature (25°C) than syntheses not preceded by sonication. No significant difference in type, temperature of crystallization, or amount of zeolites synthesized was noted between the three different samples of fly ash, implying that the chemical composition of fly ash had little influence on the zeolite product within the compositional range of these fly ash precursors. Although there appears to be a correlation between the SiO(2)/Al(2)O(3) ratio of the fly ash and the temperature of zeolite formation by conventional synthesis, no correlation was apparent when ultrasonic pre-treatment was used at low-temperatures.

The major geochemical forms of Cr, Ni, Cu, Zn, Pb, and V in a soil from an industrial polluted site in the south of Italy were determined by means of synchrotron X-ray microanalytical techniques such as coupled micro-X-ray fluorescence/micro-X-ray diffraction and micro-X-ray absorption near edge structure spectroscopy in combination with bulk extraction methods (sequential extraction procedures, EDTA extractions, and toxicity leaching characteristic procedure tests). Cr, Ni, Zn, and Cu were found in spinel-type geochemical forms (chromite, trevorite, franklinite, zincochromite, and cuprospinel) and often in association with magnetite and hematite. Vwas mainly present as V(V) associated with iron-oxides or in the form of volborthite [Cu3(OH)2V2O7.2H2O]. Pb was speciated as minium (Pb3O4), lanarkite [Pb2O(SO4)], and, in association with Cr(VI), as crocoite (PbCrO4). In general, despite a high total concentration, metals appear to be speciated for the most part as rather insoluble geochemical forms. However, particular attention should be paid to Zn, Cu, V, and Pb that show non-negligible mobilizable fractions. On the basis of the geochemical forms identified, among others, two major former industrial activities were tentatively ascribed as being responsible for the observed major pollution: polyvinyl chloride and cement-asbestos productions.

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Please note that this is an author-produced PDF of an article accepted for publication following peer review. Abstract:Seafloor hydrothermal sites generate abundant Mg-and Fe-rich clays. These clays are structurally and compositionally interesting because these environments are characterized by large, dynamic temperature and chemical gradients in their deposition environment, which promote the formation of chemically and structurally complex clays, including interstratified phases. The system is also interesting as a proxy for the study of the large Mg-and Fe-rich phyllosilicate deposits on Mars, which are broadly characterized as smectitic clay of hydrothermal, volcanic or sedimentary origin. Thirty submarine samples and four terrestrial ones, for comparison, were studied by means of X-ray diffraction (XRD), thermogravimetry (TG), mid-IR and Mössbauer spectroscopies and chemical analysis. The samples include nontronite and the mixed-layer phases glauconite-nontronite, talcnontronite and talc-saponite. Some of the talc-saponite samples have Fe contents well above those typical for these Mg-rich, trioctahedral phases (up to 1.69 Fe per O 10 [OH] 2 , in the tetrahedral and octahedral sheets). Tetrahedral Fe ranges from 0 to 0.66 atoms per O 10 [OH] 2 across the samples. As found in previous studies of similar specimens, Fe promotes the retention of molecular water that is released upon heating above 200 C, and is mainly emplaced in non-expandable layers (talc and glauconite layers). In talc-nontronite and talc-saponite octahedral Fe (both di-and trivalent) appears to be bound to this trapped molecular water, whereas in glauconite-nontronite the bond appears to be with tetrahedral Fe. Samples typically show more than one dehydroxylation event in the TG analysis. The weight loss at each dehydroxylation event is broadly consistent with the proportion of individual layers as determined by means of XRD, but there is no good correlation between both. By contrast, the weight loss at each dehydroxylation event correlates with the chemistry of the layers, where certain cations promote chemical domains in the octahedral sheet (e.g., trioctahedral, nontronite-like, montmorillonite-like, etc.) that dehydroxylate at the several temperatures. The correlations found for talc-nontronite and glauconite-nontronite samples suggest that the distribution of cations in the octahedral sheets of most, but not all, samples is intermediate between total dispersion and total segregation, perhaps random. The talc-nontronite samples have talc layers with cation-deficient octahedral sheets. The above results are best explained by layers of polar character, where the Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site Highlights ► Seafloor hydrothermal clays are mainly mixed-layer Mg-and Fe-rich phases ► Non-expandable clay layers trap molecular water near to structural Fe ► Water in Fe-, Mg-rich clay on...