Search

The environmental availability of mineral nutrients plays a role in the adaptation and evolution of soil fungi and fungus-plant symbiotic systems. As nutrient availability is different from soil to soil, it is expected that the most adapted fungi can express multiple genes to promote different mechanisms of mineral attack and of metabolic routes to maximize the use of nutrient resources at the lowest energy cost. Microcosm experiments of Paxillus involutus growing on muscovite, phlogopite and K-exchanged vermiculite as K sources were carried out for 21 days. Gene expression analysis indicated that the level of K-deprivation stress was muscovite > phlogopite ≫ K-vermiculite. Genes and functions overexpressed in the experiments indicated meaningful metabolic activities including K extraction and transport. SEM-EDS and micro-XRD (∼1.15 mm resolution) analysis of mineral surfaces and chemical analysis of agar and mycelium indicated weathering processes and K uptake in good agreement with gene expression analysis. In many areas, P. involutus caused physical damage to the minerals without any apparent chemical attack, consisting of tracks of different depth, from <1 μm to ∼80 μm wide, and piles of removed material. However, all minerals were partly dissolved. Phlogopite and vermiculite presented cation-depleted areas, but not muscovite. The dissolution trends of phlogopite and vermiculite were similar, with loss of K, Al and Mg, and concentration of Si, Ti and Fe, although Si and Fe were also lost with respect to Ti. The different mobilization of Al and Fe is noteworthy. Release of K from vermiculite by cation exchange was extensive, both abiotic and driven by P. involutus. The experimental conditions closest to most environmental situations are those in K-vermiculite tests. The results provide a new and coherent insight into fungal adaptation to mineral nutrient stress where fungal activity was regulated by both K requirement and success of the actions to secure it. [ABSTRACT FROM AUTHOR]

Abstract Phyllosilicate minerals have the potential to be used as indicators of past environmental conditions and climate. However, the appropriate use of this tool requires that the origin of the phyllosilicates be ascertained to correspond to the age under investigation, as mineralogical changes postdating it would lead to wrong conclusions. The soft sandstone of the Ashdown Formation (south-central and southeaster UK) contains fine-grained Al-phyllosilicates of limited interlayer expandability that have been described by other authors as vermiculite, smectite, interstratified illite-vermiculite and interstratified illite-smectite. The origin of these mineral phases is not yet agreed, with some authors reporting them as detrital and some as authigenic, generated by the weathering of detrital mica in the sandstone. In order to investigate whether or not these fine-grained minerals are detrital, four samples of the sandstone of variable softness were studied because they most likely had different clay content and because the different clay content could be possibly due to differential weathering. Investigation of mineralogy (X-ray diffraction), chemistry (bulk and SEM-EDS microanalysis) and rock fabric (image analysis of SEM micrographs) indicated that the sandstone consists of quartz, microcline, albite, anorthite, muscovite/illite, kaolinite, interstratified illite-smectite and smectite, where smectite layers expand only partially. Kaolinite is mainly detrital. Rock hardness was controlled by quartz abundance in the original sediment, not by later weathering. Alteration of the investigated sandstone samples after deposition was limited but reaction models are compatible with partial (<10 wt%) alteration of muscovite/illite to illite-smectite and smectite, and of microcline and plagioclase to kaolinite. Analysis of mineralogical data of the fraction <2 μm of the Ashdown Formation from the literature is compatible with this interpretation and indicates that the range of muscovite/illite alteration to interstratified illite-smectite and smectite varied widely (0–45% in the <2 μm size fraction) depending on location and depth. Highlights • Hardness of Sandstone from the Ashdown Formation (UK) follows quartz/clay ratio. • Quartz/clay ratio changes are controlled first by sedimentation, then by diagenesis. • The sandstone contains expandable clays of disputed origin. • Mineralogy and reaction models suggest that the expandable clays are authigenic. [ABSTRACT FROM AUTHOR]

Abstract A range of phyllosilicate compositions have been detected spectroscopically on Mars, but the largest fraction by far corresponds to clay minerals rich in Fe and Mg. Given that most of our understanding of Martian clays comes from remote sensing data, it is critically important to explore the details of how compositional variation affects spectral features of phyllosilicates. The greatest efforts have focused so far on near-infrared (NIR) spectroscopy. Recently, ambiguities have been detected in the NIR spectra of 2:1 phyllosilicates with intermediate Fe Mg content that preclude mineral and chemical discrimination. Such ambiguities highlight the relevance of exploring the visible spectral range as a complementary tool to characterize Martian phyllosilicates precisely. This article reports the investigation of laboratory reflectance spectra (330–800 nm) from 34 Earth analogues of Martian phyllosilicates with a wide range of Mg Fe composition, including nontronite, celadonite and saponite end-members, as well as interstratified glauconite-nontronite, talc-nontronite, and talc-saponite. The spectra indicated the presence of Fe(III) by absorption modulations and a decrease in total reflectance, especially in samples with tetrahedral Fe(III). Absorption bands at 370 and 420 nm were diagnostic of octahedrically and tetrahedrally coordinated Fe(III), respectively. Band amplitudes in the second derivative of the Kubelka-Munk function correlated positively with Fe(III) content (R 2 > 0.8). Standard color analyses of the visible reflectance spectra under the CIE illuminant D65 indicated that the CIELAB color parameter a * 10 was positively correlated with tetrahedral Fe(III), b * 10 was positively correlated with octahedral Fe(III), and L * 10 was negatively correlated with Fe(III) in both structural sites. Because Fe(II) was in relatively low amount, it did not provide clear spectral evidence. Multiple regression models using the amplitude of the diagnostic absorption bands predicted well absolute Fe content in the phyllosilicates (R 2 = 0.89) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.84). CIELAB color parameters improved the prediction of total Fe (R 2 = 0.92) and the ratio Fe/(Fe + Mg + Al) (R 2 = 0.93). Application of these analyses to Martian data has challenges set by Fe oxide dust coating and spatial and spectral resolution. However, these results mark an avenue to develop testable tools using visible-wavelength spectral data from both satellite and lander probes to help establishing Fe content and mineral identification of Martian phyllosilicates. Highlights • Visible-range spectra of phyllosilicates shaped by Fe content and site occupancy. • Phyllosilicate Fe content is predicted well by 2nd-derivative and color analyses. • Vis spectra from Martian rocks with phyllosilicates comparable to lab spectra. • 2nd-derivative and color analyses hopeful tool to estimate Fe in Martian clays. [ABSTRACT FROM AUTHOR]

Most known clay mineral reactions are slow. Clay formation by weathering is recognized as faster (hundreds of thousands to few million years) than clay reverse weathering (few to tens My), even if the latter takes place in deep diagenetic conditions at temperatures above 100 °C. Identifying hot spots of fast clay retrograde reactions acting as effective potassium sinks would contribute to eliminating the present imbalance of the potassium budget towards release into the oceans. We have identified and modeled the reaction kaolinite + Fe + cations → Fe - illite occurring in Brazilian mangroves, which takes place through kaolinite-smectite and illite-smectite intermediates. The reaction is very fast, transforming 72–154 g m − 2 y − 1 of kaolinite into Fe-illite, or 16–38% of the newly sedimented kaolinite each year. This kaolinite-to-illite transformation is 10s–100s times faster than kaolinite formation by weathering. Extrapolation of the calculated potassium consumption to mangrove forests worldwide produces 0.6–3 × 10 12 g y − 1 , or 1–6% of the yearly potassium riverine input into the oceans. [ABSTRACT FROM AUTHOR]

Fungi dissolve soil minerals by acidification and mechanical disruption. Dissolution may occur at the microscale (contact between fungus and mineral) and medium scale (entire mineral grains). Mineral weathering by fungi and other microorganisms is thought to be of significant global contribution, perhaps producing specific weathering signatures. We report fungal dissolution of phlogopite mica in experiments with three fungal strains ( Alternaria tenuissima, Cladosporium cladosporioides , Stilbella sp.) on solid medium for 30 days at 21 °C and 96–100% relative humidity. The study used variable-pressure SEM-EDS equipped with charge contrast imaging. Statistical analysis of the results discriminated between the weathering activities of the three fungal species, which increased from Stilbella sp. to C. cladosporioides to A. tenuissima , in agreement with the respective decreasing pH in the media (6.4, 5.8, 5.2 ± 0.03). Phlogopite weathering features were irregular and variable, apparently not caused by direct contact with fungal hyphae. EDS values indicated two or more dissolution mechanisms, one of them suggesting cation rearrangement in the mica towards formation of Al-rich smectite. Intimate fungus-mineral interaction was observed, and the lack of observable dissolution traces from such contact interaction is interpreted as the result of effacing by the more intense acid leaching operating at larger scale. [ABSTRACT FROM AUTHOR]

The aim of this study was to further our understanding of the pedogenic and lacustrine modification of clay minerals. Some of these modifications are of special interest because they constitute reverse weathering reactions, rare in surface environments, and because there is not yet an accurate assessment of their global relevance in mineralogical and geochemical cycles. For this study, two sections from the Central System in Spain were selected. Both are sections through the Uppper Cenomanian-Turonian mixed clastic and carbonate succession, containing both calcite and dolomite, in the Sierra de Guadarrama. Mid-Turonian sea level fall resulted in the formation of a coastal plain environment in which extensive pedogenesis occurred around saline lagoons. The mineralogical changes that have occurred as a result of sedimentation in saline lagoons and as a consequence of pedogenesis are described. Textural relationships indicate that the dolomite cement pre-dates the calcite. Silicate minerals are represented by quartz, kaolinite, illite-smectite, illite, minor plagioclase and alkali feldspar, and trace chlorite and palygorskite. There is a positive correlation between the intensity of pedogenesis and the proportion of illite in the clay assemblage in one of the sections, indicating pedogenic illitisation. In this section, the intensity of the illitisation process increases up, reaching a maximum where pedogenesis is most intense in the middle part, and then decreases as marine influence increases towards the top of the Alcorlo Formation and the overlying marine Tranquera Formation. The clay assemblages are consistent with a slow transformation process from kaolinite to illite by way of illite-smectite, taking place under surface conditions. The illitisation process has resulted in a less Fe-rich, more Mg-, and Al-rich illite than the majority of previously documented cases in the near surface. Formation of Al-rich illite is not therefore restricted to the deep subsurface. The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg 2 + substitution for Al 3 + (or Fe 3 + ) and Fe 3 + to Fe 2 + reduction. Mg 2 + enrichment may have occurred principally in saline lagoons or lakes, while Fe 3 + to Fe 2 + reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg. [ABSTRACT FROM AUTHOR]

Microbial activity following invasion of human-made structures and artifacts can have profound social and economic consequences including the permanent loss of cultural heritage. The unique frescoes in the 11th century Saint Sophia's Cathedral (Kyiv, Ukraine) have recently suffered from dark-spot biodeterioration. The aim of this work was to elucidate the microbial nature of biodeterioration and the biogeochemical processes occurring in the areas of the dark spots. Culture-independent approaches including scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), micro-X-ray diffraction and real-time quantitative polymerase chain reaction (qPCR) analysis were used in this study. SEM and qPCR data demonstrated that the main agents of fresco biodeterioration were mycelial fungi, with bacteria unlikely to play a major role in the development of the dark spots. SEM-EDS results showed that fungi colonization of the dark spotted areas resulted in mechanical and chemical weathering involving dissolution of mineral components of the plaster (mainly calcite) and displacement of mineral grains, which compromise the stability of the plaster or fresco. SEM-EDS also detected fungal biomineralization of secondary mycogenic minerals: calcium malate, hydrated aluminium and ferric phosphates. Biomineralization of calcium malate by fungi, as found in this study, is a rare biogeochemical phenomenon, possibly linked to the presence of calcite and nitrogen limitation. • 11th century frescoes in Saint Sophia's Cathedral (Kyiv, Ukraine) were affected by dark spot biodeterioration. • The main agents causing fresco defacement were filamentous fungi. • Fungi acted on fresco plaster through biomechanical and biochemical mechanisms. • Fungal activity resulted in dissolution of mineral components of the plaster, e.g. calcite. • Calcium malate, hydrated aluminum and ferric phosphate minerals were formed by fungi. [ABSTRACT FROM AUTHOR]

On Earth, trioctahedral (Mg-rich) vermiculite occurs as a transient alteration product of Mg/Fe2+-mica and chlorite, pyroxene, olivine, amphibole and even plagioclase where there is a nearby Mg source, indicating mild/short-lived weathering or hydrothermal conditions. On Mars, the abundant mafic rocks and low water availability suggest the likelihood of widespread vermiculite formation from the Noachian to the early Hesperian. Although it can be challenging to identify vermiculite by remote sensing or in situ infrared spectroscopy alone, we show that numerous previous remote sensing results suggest that vermiculite is widespread on the martian surface and perhaps a more common mineral on that planet than on Earth. Investigation of rocks from Riotinto (SW Spain) provides an example of how chlorite alters to vermiculite in an acidic environment. Similar incomplete vermiculite replacements of Mg/Fe2+-mica, chlorite and mafic minerals could have resulted on Mars from acidic to neutral weathering at low water regimes. While possible bias exists towards remote sensing detection of vermiculite on Mars in sites (1) with advanced alteration and (2) where vermiculite is not mixed (at pixel level) with other phyllosilicates, we find that vermiculite occurs in Noachian terrains, in sites (wider than pixel level) where other phyllosilicates are detected, and that vermiculite has more Fe3+ than is typical on Earth. [Display omitted] • Vermiculite is a transient alteration product of mafic minerals, mica and chlorite. • Vermiculite is widespread on Mars and found in Noachian altered terrains. • The mineral might be more commonplace on Mars than on Earth. • Most identified vermiculite on Mars so far is Fe3+-rich. • Existing biases suggest more occurrences and wider composition range. [ABSTRACT FROM AUTHOR]

Saharan dust can travel long distances in different directions across the Atlantic and Europe, sometimes in episodes of high dust concentration. In recent years it has been discovered that Saharan dust aerosols can aggregate into large, approximately spherical particles of up to 100 μm generated within raindrops that then evaporate, so that the aggregate deposition takes place most times in dry conditions. These aerosol aggregates are an interesting phenomenon resulting from the interaction of mineral aerosols and atmospheric conditions. They have been termed “iberulites” due to their discovery and description from aerosol deposits in the Iberian Peninsula. Here, these aggregates are further investigated, in particular the role of the clay minerals in the aggregation process of aerosol particles. Iberulites, and common aerosol particles for reference, were studied from the following periods or single dust events and locations: June 1998 in Tenerife, Canary Islands; June 2001 to August 2002, Granada, Spain; 13–20 August 2012, Granada; and 1–6 June 2014, Granada. Their mineralogy, chemistry and texture were analysed using X-ray diffraction, electron microprobe analysis, SEM and TEM. The mineral composition and structure of the iberulites consists of quartz, carbonate and feldspar grains surrounded by a matrix of clay minerals (illite, smectite and kaolinite) that also surrounds the entire aggregate. Minor phases, also distributed homogenously within the iberulites, are sulfates and Fe oxides. Clays are apparently more abundant in the iberulites than in the total aerosol deposit, suggesting that iberulite formation concentrates clays. Details of the structure and composition of iberulites differ from descriptions of previous samples, which indicates dependence on dust sources and atmospheric conditions, possibly including anthropic activity. Iberulites are formed by coalescence of aerosol mineral particles captured by precursor water droplets. The concentration of clays in the iberulites is suggested to be the result of higher efficiency for clay capture than for the capture of larger mineral grains. The high hygroscopicity of clay minerals probably causes retention of water in the evaporation stage and some secondary minerals (mainly gypsum) are associated with clays. [ABSTRACT FROM AUTHOR]

Abstract: The effect of oil on As(V) adsorption on clay minerals has been investigated using batch experiments at low and high pH, NaCl concentration and oil contents. Four clay minerals were chosen because of their abundance in sediments and their different crystal chemistry: illite, kaolinite, montmorillonite, and chlorite. The values for pH were 4 and 8 and salt concentrations were 0.001 and 0.7M NaCl to appreciate the effects of changing salinity, e.g. from fresh water to seawater conditions. For the coating experiments, a well-characterised oil was used to survey the main effects of complex organic mixtures on adsorption and oil to clay mineral (w/w) ratios were 0.0325 and 0.3250. As(V) adsorption increased with increasing NaCl concentration, suggesting that the mechanisms of As(V) adsorption are related to the formation of surface complexes in which Na+ ions act as bridges between the clay surface and the As(V) anions. Cation bridging is also indicated by zeta potential measurements which show that higher NaCl concentrations along with the presence of As(V) can cause the clay particles and adsorbed ions to have a more negative overall charge. Adsorption is lower at higher pH due to the reduced number of positively charged sites on the edge of clay mineral layers. Oil coating reduces As(V) adsorption by decreasing the available surface area of clay minerals, except in the case of oil-coated montmorillonite, where surface area following dispersion in water is increased. The main variables controlling As(V) adsorption are surface area and surface charge density, as confirmed by a simplified quantitative model. These findings advance our ability to predict the effects of complex pollution events in various freshwater and marine settings. [Copyright &y& Elsevier]

• Kaolinite transforms into Fe-smectite and Fe-illite in mangrove soils. • REE were not influenced by anthropic activity. • There is a trend of depletion of Heavy REE (HREE)in the normalized patterns. • Terrigenous kaolinite control the fractionation of light REE (LREE) over HREE. • Adsorption of HREE by 2:1 clays reduced the fractionation of LREE over HREE. Mangroves are intertidal tropical ecosystems influenced by marine and terrigenous input. They are important sinks of trace elements, like Rare Earth Elements (REE), which are recognized as excellent indicators of geochemical processes in estuaries. REE data of bulk soil samples from eight mangroves located along the Brazilian coast were correlated with quantitative clay mineral data based on XRD full-profile modelling. Kaolinite has been found to transform to Fe-illite in these mangroves. The clay assemblage was dominated by interstratified phases, including kaolinite-smectite (kaolinite and smectite-rich), illite–smectite and other illitic phases. REE concentrations in bulk soils were LREE > MREE > HREE (where L, M and H indicate light, medium and heavy). Multivariate statistical approach (PCA analysis) using clay mineral concentrations and proxies for relative REE concentrations ([La/Gd] N , [La/Yb] N and [Gd/Yb] N ratios) showed (r > 0.75) that fractionation of LREE over HREE was highest in mangroves where kaolinite, fed with the highly weathered sediments form the Neogene Barreiras Group, was most abundant (67–85% of all clay minerals), while REE fractionation decreased as the relative proportion of Fe-rich smectite increased (26–46 % of all clay minerals). Identified processes of REE control are: 1) Level of sediment weathering within the continent, where higher weathering implies higher kaolinite content and higher REE fractionation (LREE > MREE > HREE); 2) selective leaching of LREE from mangrove soils by saline water; 3) preferential adsorption of HREE by Fe-rich smectite and illite neoformed in the mangrove soils. It is possible to conclude that clay minerals exert an important control on REE fractionation in tropical mangroves. [ABSTRACT FROM AUTHOR]

Abstract: The legendary cruise of H.M.S. Challenger (1872–1876) around the globe must always occupy an eminent place in the annals of oceanography, as being the first systematic attempt made on a global scale to explore the ocean. This expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise. Sediment with high content of metals (later called “metalliferous”) was among the enigmatic findings taken onboard. Although the nature of metalliferous sediments is well known today, the very first sampled sediments of this type have not been studied to date. Motivated by the historical value of Challenger’s metalliferous sediment collection we undertook an investigation addressing two questions: (1) the composition of sediments from seafloor for which we have very limited data; (2) Sr–Nd–Pb–Fe–Zn-isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century. The SE Pacific metalliferous sediments sampled by the Challenger’s explorers are of 2 types: (1) metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth (CCD); and (2) stripped of CaCO3 metalliferous sediments located beneath the CCD in the deeps near the mid-ocean ridges. The abiogenic part of these sediments is composed mainly of poorly-crystalline to X-ray amorphous Fe–Mn-oxyhydroxides, and an amorphous silicate phase. These sediments have geochemical features similar to those of all the other metalliferous sediments: very high Fe and Mn content (on abiogenic basis), very low Al/(Al+Fe+Mn), and high content (on abiogenic basis) of As, Ba, Be, Bi, Cd, Co, Cu, Mo, Ni, Pb, Sb, Th, Tl, U, V, W, Y, Zn and Zr. Their REE distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint (weak positive or no Eu anomaly). Seawater and/or terrigenous input from South America control the Sr–Nd–Pb-isotope signature of the Challenger metalliferous sediments and have almost completely obliterated any original MORB-derived hydrothermal signal. Zn isotopes are mainly contributed from seawater although other Zn sources (hydrothermal fluid and detrital aluminosilicates, barite and volcanic glass) are necessary to fully explain Zn-isotope ratios. Fe isotopes indicate relatively slow Fe2+ to Fe3+ oxidation in the non-buoyant plume, thus producing relatively lighter Fe-isotope signature of the FeOOH particles that formed the studied metalliferous sediments. [Copyright &y& Elsevier]

Abstract: The kinetics of smectite illitization in shale samples from eight wells have been modeled using the equation −dS/dt = kK 0.25 S 5 [Cuadros, J., Linares J., 1996. Experimental kinetic study of the smectite-to-illite transformation. Geochim. Cosmochim. Acta 60, 439–453], where S is the proportion of smectite layers in illite-smectite (I–S), t is time, k is the rate constant and K is the concentration of potassium. The I–S compositions and the physical parameters were obtained from the literature. Potassium concentration is the only unknown parameter and it was chosen to fit the experimental data. I propose that this variable is termed “effective K concentration,” as it is affected not by the actual K concentration alone but also by those of other cations that compete with K for the smectite interlayer sites, especially Ca, by ion mobility in the sediments and possibly by other variables. The kinetic equation allows an accurate fit to the experimental data in all the wells, reproducing both the illitization onset and the patterns of the plots of % smectite vs. depth. The relative changes of the values of “effective K concentration” compare well with K/Ca equivalent ratios in the whole rock. Three of the modeled wells, located in near proximity, produced similar “effective K concentration” patterns. These results support the validity of the equation used in the models. They also suggest that this equation can be used to assess K concentrations in sediments and K mobility within basins. [Copyright &y& Elsevier]

Iron electronic transitions produce intense colors in minerals with moderate to high Fe content, which phenomenon allows using color and visible spectroscopy to identify and quantify Fe-bearing minerals. This approach is especially useful for Mars investigation due to the limited analytical instruments available at Mars. The challenges are: (1) overlap between Fe electronic absorption bands of Fe-bearing minerals, (2) band wavelengths are modified by chemical and structural variables, and (3) particle size effects can modify mineral color. The magnitude of these difficulties, however, depends on the mineral mixture investigated. In this study, we further probe the potential of visible-range data for investigation of Fe-minerals. The samples were mineral mixtures from the Riotinto area (SW Spain) that resulted from acidic alteration and contained Fe in oxy-hydroxides, hydroxysulfates and phyllosilicates. Samples were studied using visible/near-infrared spectroscopy, X-ray diffraction and thermogravimetry. The wavelength range 320–1000 nm was investigated using spectral second derivatives and CIELAB color parameters. Hematite was readily detected, below X-ray diffraction detection limits, from the band at 520–560 nm. Jarosite was readily identified by the combined presence of bands at ~390 and ~ 440 nm. Thermogravimetric and CIELAB color analyses indicated goethite as the most common Fe-rich mineral in the samples. Correlations between CIELAB color parameters were controlled by goethite, where jarosite-rich specimens aligned with the correlations but some samples containing hematite plotted outside them. Quantification of goethite was attempted as calibrated against thermogravimetric data. Calibration was not possible below ~10 wt% goethite. Above this value, results from spectral second derivatives allowed quantification up to the maximum goethite content of ~35 wt% (linear correlation with R2 = 0.89). Goethite quantification with CIELAB color parameters was hindered by hematite. In samples with very low or no hematite, goethite quantification was possible with parameters L * 10 (R2 = 0.83) and h ab,10 (R2 = 0.65), also up to ~35 wt% goethite. Consequently, if accurate calibration is possible and depending on mineral mixture components, spectroscopic and CIELAB color parameters should allow goethite quantification in a wide concentration range. • Hematite detectability in mineral mixtures is high due to absorption at 520–560 nm. • Jarosite is also readily detectable due to the combined bands at 390 and 440 nm. • Goethite and jarosite share color characteristics determined with CIELAB parameters. • Hematite modifies CIELAB color characteristics of goethite-jarosite mixtures. • Goethite quantification is possible by spectral and color methods to at least 35 wt%. [ABSTRACT FROM AUTHOR]

Abstract: It is envisaged that high-level nuclear waste (HLW) will be disposed of in underground repositories. Many proposed repository designs include steel waste canisters and bentonite backfill. Natural analogues and experimental data indicate that the montmorillonite component of the backfill could react with steel corrosion products to produce non-swelling Fe-rich phyllosilicates such as chamosite, berthierine, or Fe-rich smectite. In K-bearing systems, the alteration of montmorillonite to illite/glauconite could also be envisaged. If montmorillonite were altered to non-swelling minerals, the swelling capacity and self-healing properties of the bentonite backfill could be reduced, thereby diminishing backfill performance. The main aim of this paper was to investigate Fe-rich phyllosilicate mineral stability at the canister–backfill interface using thermodynamic modelling. Estimates of thermodynamic properties were made for Fe-rich clay minerals in order to construct approximate phase-relations for end-member/simplified mineral compositions in logarithmic activity space. Logarithmic activity diagrams (for the system Al2O3–FeO–Fe2O3–MgO–Na2O–SiO2–H2O) suggest that if pore waters are supersaturated with respect to magnetite in HLW repositories, Fe(II)-rich saponite is the most likely montmorillonite alteration product (if values are significantly lower than magnetite–hematite equilibrium). Therefore, the alteration of montmorillonite may not be detrimental to nuclear waste repositories that include Fe, as long as the swelling behaviour of the Fe-rich smectite produced is maintained. If exceeds magnetite–hematite equilibrium, and solutions are saturated with respect to magnetite in HLW repositories, berthierine is likely to be more stable than smectite minerals. The alteration of montmorillonite to berthierine could be detrimental to the performance of HLW repositories. [Copyright &y& Elsevier]

Abstract: Several designs proposed for high-level nuclear waste (HLW) repositories include steel waste canisters surrounded by montmorillonite clay. This work investigates montmorillonite stability in the presence of native Fe, magnetite and aqueous solutions under hydrothermal conditions. Two series of experiments were conducted. In the first, mixtures of Na-montmorillonite, magnetite, native Fe, calcite, and NaCl solutions were reacted at 250°C, P sat for between 93 and 114 days. In the second series, the starting mixtures included Na-montmorillonite, native Fe and solutions of FeCl2 which were reacted at temperatures of 80, 150, and 250°C, P sat, for 90–92 days. Experiments were analysed using XRD, FT-IR, TEM, ICP-AES, and ICP-MS. In the first series of experiments, native Fe oxidised to produce magnetite and the starting montmorillonite material was transformed to Fe-rich smectite only when the Fe was added predominantly as Fe metal rather than Fe oxide (magnetite). The Fe-rich smectite was initially Fe(II)-rich, which oxidised to produce an Fe(III)-rich form on exposure to air. The expansion of this material on ethylene glycol solvation was much reduced compared to the montmorillonite starting material. TEM imaging shows that partial loss of tetrahedral sheets occurred during transformation of the montmorillonite, resulting in adjacent layers becoming H-bonded with a 7Å repeat. The reduced swelling property of the Fe-smectite product may be due predominantly to the structural disruption of smectite layers and the formation of H-bonds. Solute activities corresponded to the approximate stability field calculated for hypothetical Fe(II)-saponite. In the second series of experiments, significant smectite alteration was only observed at 250°C and the product contained a small proportion of a 7Å repeat structure, observable by XRD. In these experiments, solute activities coincide with berthierine. The experiments indicate that although bentonite is still a desirable choice of backfill material for HLW repositories, some loss of expandability may result if montmorillonite is altered to Fe-rich smectite at the interface between steel canisters and bentonite. [Copyright &y& Elsevier]

Visible/Near Infrared Reflectance spectroscopy (350–2500 nm) is an efficient and effective tool to identify and semi-quantify clay minerals, Fe3+ (oxyhydr)oxides, and their crystal-chemistry/structure. Even so, there have been few studies systematically describing how spectroscopy can be applied to mineralogical evolution during weathering. This study investigated reflectance spectra of a long core of a thick basaltic weathering sequence in Hainan Island and a series of transformation products from montmorillonite to kaolinite to present spectral features associated with weathering trends. In combination with X-ray diffraction and geochemical results, mineralogical transitions and weathering intensity of the core samples were described from a spectroscopic perspective. Several different protoliths were spectroscopically recognized in the Hainan weathering sequence, consistent with geological observation and geochemical analysis. Overprinted on these multiple protoliths was a discernable weathering sequence observable by infrared spectroscopic methods. Clay minerals changed upwards in section from TOT type (nontronite) to TO type (kaolinite), and finally to O type (gibbsite) hydroxide clay-size particles as the result of gradual hydrolysis of silicates and leaching of mobile elements with weathering. The relative abundance of hydroxyl to interlayer water, observable by the band depth ratio of spectral absorptions around 1400 nm and 1900 nm (BD1400/BD1900), increased progressively and correlated positively with the chemical index of alteration and Al 2 O 3 concentration. The increasing BD1400/BD1900 index value with weathering was the result of gradual leaching of interlayer cations (e. g. Na+, K+), and their water complexes within interlayers, and gradual formation of M-OH layers by hydrolysis. Owing to leaching of Mg and release of Fe, the octahedral M-OH formed by hydrolysis was gradually dominated by Al-OH. Reflectance spectroscopy is a powerful tool to identify protolith changes in weathering sequences and for studying clay mineralogical transitions and weathering intensity. This spectroscopic approach could have great value for understanding weathering related to paleoclimate on Earth or other planets using field spectroscopy and remote sensing. • • Infrared reflectance spectroscopy was applied to a drill core through weathered rocks. • • Spectral indices of clay minerals were developed to characterize weathering trends. • • Ratios of spectral band depths measure interlayer water and hydroxyl content. • • These indices can be applied to remote sensing data of Mars. [ABSTRACT FROM AUTHOR]