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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]

Mineral weathering processes in soils are important controls on soil characteristics and on bio- and geochemical cycling. Elucidation of these processes and their mechanisms is crucial for understanding soil environments and their influence globally. An Umbric Podzol from the Falkland Islands was studied while investigating possible ways to counteract soil degradation and loss. The soil had lost the O, E, and Bs horizons through erosion, thus revealing the transitional B/C horizon, which grades into the underlying parent material. Samples were taken from the B/C surface and 5 cm below the surface, then analyzed with X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy, organic C and N analysis, analysis of extractable Fe and Al with the dithionitecitrate- bicarbonate and ammonium oxalate methods, and Fourier-transform infrared analysis. The soil fabric and mineralogy were compatible with derivation from sandstone rock. Clasts of heterogeneous mineral composition as well as loose material from disaggregated clasts were present. The soil had large proportions of quartz and albite, and minor amounts of muscovite, chlorite, plagioclase, feldspar, kaolinite, and non-diffracting Fe oxide (goethite and/or ferrihydrite). The most peculiar characteristic was a large component (~7 wt% of the bulk soil) of an amorphous (non-X-ray diffracting) silicate phase of small particle size (<1 µm), non-extractable, with heterogeneous composition. The average composition of this phase is similar to that of the bulk soil and approaches that of Al-Fe-rich smectite. The amorphous phase is not allophane or imogolite by any of the analyses carried out. The amorphous silicate phase is formed partly by the translocation of metals from O, E, and Bs horizons and partly by dissolution of the primary minerals of the B/C horizon, both of which precipitated in combination with low water mobility causing rapid saturation of the interstitial water. There are no reports of amorphous silicate phases with these characteristics or abundances from soils or other weathering environments. Thus, our observations indicate the existence of complex, successive weathering steps not yet identified that could be investigated in materials subjected to slow weathering such as the soil described here. [ABSTRACT FROM AUTHOR]

The efficient use of REE as tracers of weathering processes requires further data acquisition to establish robust connections between mechanisms, mineral assemblages and REE concentration patterns. Here, results are provided corresponding to rocks from four locations in the Iberian Pyrite Belt that were altered with different intensity by sulfuric fluids. The acidic alteration was mild (pH 5–7) in Calañas and El Villar, while strong (pH below 3 to 5) in Quebrantahuesos and Tharsis. The main REE reservoir before alteration were phyllosilicates (and plagioclase/feldspar in El Villar only). Mild alteration generated mainly kaolinite while illite and some plagioclase/feldspar were preserved. Mildly altered rocks approximately preserved the relative REE concentrations of their precursors, interpreted as indicating a low fluid/rock ratio environment where binding sites in minerals outnumbered those in fluids. Strong acid alteration produced alunite, jarosite, beudantite and goethite, with partial illite preservation. Where illite and/or goethite were major minerals, this alteration generated REE patterns enriched in light REE over heavy REE or did not modify the relative REE concentrations of the original rock. However, samples where REE concentrations were controlled by alunite or jarosite had LREE > > HREE. In alunite the maximum concentration corresponded to Nd and Sm. In jarosite concentrations decreased from La to Lu, except that a positive Eu anomaly was preserved from the precursor. These REE concentration patterns are interpreted as due to crystallographic factors, controlled by the best fit of REE with charge 3+ in the 12-fold coordination site of K in jarosite and alunite. This site is larger in jarosite and fits La best, whereas the smaller site in alunite best fits Nd and Sm. Goethite REE signature is variable, perhaps due to two formation routes, (1) silicates ➔ goethite and (2) silicates ➔ jarosite ➔ goethite. Our interpretations of REE concentration in alunite, jarosite and goethite are novel and provide new routes to further develop REE as tracers of acid alteration processes. [Display omitted] • Eu anomaly was negative in altered volcanic rocks, positive in altered shales. • Kaolinite, formed at pH 5–7, inherited the REE pattern of precursor phyllosilicates. • Alunite (pH 3–5) and jarosite (pH < 3) had a modified REE pattern with LREE > > HREE. • REE signature in alunite and jarosite is controlled by the size of the K crystallographic site. • Goethite variable REE signature is caused by two formation routes, from silicates or from jarosite. [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]

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]

Soil formation occurs through numerous physical and chemical weathering processes acting to alter the parent rock on the Earth's surface. Samples of surface soils were collected over a range of elevations (2000-3600 m) from profiles directly overlying basaltic to more felsic parent rocks, over a region in NW Ethiopia. The soils were investigated to determine their chemical composition and X-ray diffraction was used to identify and quantify individual mineral phases. The data set was analyzed using non-parametric statistics (Spearman's Rank and Mann-Whitney U tests) to compare the soils forming over the two parent rocks. Principal component analysis (PCA) was used to identify the mineral alteration assemblage and formation during pedogenesis. The extent of alteration was quantified using several chemical weathering indices (Chemical Index of Alteration = CIA; Chemical Index of Weathering = CIW), including an index calculated by multivariate analyses of the soil chemical composition data (weathering "W" index). Further to this we devised and tested a new weathering index (Wmin) using multivariate analysis of the soil mineralogy, to estimate the extent of weathering and physico-chemical proprieties of the parent rock from which the soil formed. The soils present a fair to advanced stage of alteration, with abundant iron (Fe) oxides (up to 40 wt%) and phyllosilicates (up to 57 wt%), including kaolinite-smectite (K-S) mixed-layer phases. The K-S was composed of either 30-50% kaolinite or 94-98% kaolinite layers. Discrete kaolinite was also present. The bimodal K-S mineralogical composition is likely due to two precursor phases: feldspar for the kaolinite-rich K-S and volcanic glass for the smectite-rich K-S. K-S with intermediate composition (50-94% kaolinite) was rare, due to its instability. Statistical analysis showed significant differences between the chemical compositions of the soils developed on the two different parent volcanic compositions. The soils overlying the more felsic parent rocks were less altered than those overlying the flood basalt. When comparing the weathering indices calculated in this study, we conclude that while the CIA and CIW may be more readily determined, the W and Wmin indices can elucidate information on the composition of the original rock from which they formed. The W index is more sensitive to certain variables when compared with the newly derived mineralogical Wmin index; however the Wmin index takes into account mineral phases within the sample, which provides a more detailed interpretation of weathering rates than chemistry alone. In addition the Wmin index correlated with meteorological variables, such as elevation (and consequently temperature and precipitation), known to influence the degree of pedogenesis. The Wmin index can be used to enhance our understanding of the processes that occur during weathering processes to supplement information gained from traditional chemical weathering indices. [ABSTRACT FROM AUTHOR]

Abstract: Six-year experiments of volcanic glass reacting with waters of different chemistry (two freshwaters, seawater and brine) at ∼22°C have produced the thorough transformation of mm-size glass chips into quartz, with minor alunite and calcite. These results contradict the current thinking about glass weathering and quartz formation. The reaction from glass to quartz took place at an estimated velocity ranging from 300–2000 times to 106 times faster than the accepted values for quartz precipitation, depending on how rates were assessed. The most likely process taking place is the rapid transformation of cation-depleted glass into quartz. In addition, alunite formed very efficiently from low-S glass (40±15ppm). Such effective reaction in the absence of the accepted conditions for alunite formation is attributed to the high Al and K content combined with the generation of low pH conditions at the microscale. Local, low-pH conditions may arise due to proton-for-Na substitution at the earliest stage followed by liberation of the protons as glass later corroded. The surprising results show a new pathway of glass weathering and point towards reactions controlled by microscale conditions producing high activities locally. Such conditions may be common in the mineral–fluid interface with saline waters, immobile waters or systems with low water:rock ratios. The rapid precipitation of quartz at low temperature is relevant to the origin of quartz and the silica budget in a variety of sedimentary environments and prompts their reconsideration. [Copyright &y& Elsevier]

The extent of natural weathering of phiogopite crystals in a karstic environment and the composition of its weathering products have been examined through a multi-method investigation carried out on a volcanoclastic deposit discovered in the Grotta del Cervo cave, Pietrasecca, central Italy. Phlogopite crystals are a mixture of 1M and 2M1 polytypes and show mixed- layering with a low amount (1-4%) of smectite in the glass groundmass. Phiogopite crystals vary in composition from phlogopite to ferroan phiogopite. Their average structural formula is (K0.92Na0.03Ba0.03Ca0.02) (Mg2.05Al0.01Fe2+0.19Fe3+0.04Mn0.01Ti0.15 0.19 (Si2.63Al1.37)O10.16(OH1.57F0.27). Micromorphological, X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), scanning electron microscopy (SEM), and microprobe analyses (EMPA) showed that the mineralogical evolution of phiogopite crystals is consistent with in-situ pseudomor- phic weathering of mica to Al-rich dlioctahedral (rather than Mg-rich) clay minerals (beideffite-like) via phlogopite-smectite mixed-layer phases. The main mechanism of transformation is a layer-by-layer modification in which water composition, enriched in Si and Al from glass dissolution, is a major control. Mg loss, Ca and Na exchange for K, enrichment in Si and Al are the main processes required for beidellitization of phiogopite. Glass alteration produced sequentially smectite, chabazite, phillipsite and analcime, as leaching progressed and the interstitial fluids became increasingly alkaline. The type of zeolite found in the several samples (in the chabazite, phillipsite, analcime series) indicates that the alteration extent increases from wall to the ceiling and to the floor of the cave. [ABSTRACT FROM AUTHOR]

Abstract: Using a series of experiments, we show that the formation of clay minerals can occur biologically via the processes of sediment ingestion and excretion by worms. Specimens of lugworm, Arenicola marina, were fed a mixture of coarse grained quartz sand and unweathered, finely crushed Icelandic basalt in experimental tanks that simulated an intertidal, shallow-marine, sedimentary environment. Faecal casts as well as the starting material and samples from a control tank, collected and separated into < 2 μm fractions, were analysed using X-ray diffraction and Fourier Transform Infrared spectroscopy. The faecal samples were found to have lost plagioclase feldspar due to dissolution and to contain new clay minerals (kaolinite, illite and a 14 Å clay) not present in the original or control samples. The experiments show that macrobiotic ingestion processes induce rapid clay growth. These clays coat sand grains and may be the precursors to porosity-preserving chlorite coats found in sandstones. [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]