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In the work, the processes of phase reconstruction and properties of ceramics obtained from high-siliceous clay-sand, clay-cullet, and clay-sand-cullet mixtures are considered. It is possible to perform plastic molding of blanks from the given mixtures due to the presence of montmorillonite in clay and sand and sintering at 800 oC for 8 h under conditions of oxygen deficiency. Depending on the composition of the initial mixtures, specimens ranging in color from gray to black can be obtained. It has been established by the XRD, IR spectroscopy, and electron microscopy methods that the synthesized material is glass ceramics consisting of quartz, feldspars, and a glass phase. Depending on the content of the initial components in the mixtures, it is possible to obtain glass ceramics with high strength properties or coarse-pored glass ceramics whose properties are similar to those of foamed ceramics.

Uranium can be found in the Earth's crust in different reservoirs, with igneous rocks being the primary source of this element from which many types of secondary deposits are formed. Fluorspar deposits generally do not contain uranium, but in some cases, fluorite can carry both uranium in solid solutions and inclusions of uranium minerals. We studied the concentration (ICP-MS), composition (electronic microprobe), and spatial distribution (microscopy and auto-radiography) of elemental uranium and uranium minerals at different scales (microscopy and auto-radiography in fluorite from the La Azul fluorspar deposit (Taxco, Mexico) to assess the origin of uranium and its significance in this ore deposit. Auto-radiography images with the CR-39 detector were found to be impressive in their ability to elucidate uranium distribution at the millimeter scale. The limit between the solid solution of elemental uranium in natural fluorite and the appearance of uranium oxides as inclusions appeared to be between 20 μg g−1 and 40 μg g−1 bulk uranium concentration in this fluorspar ore. The maximum concentration of U in fluorite from the La Azul deposit was about 100 μg g−1. Using Raman spectroscopy and microprobe analysis, we identified the micro-inclusions of uranium minerals as uraninite (of the pitchblende variety); its composition suggested a hydrothermal origin for this fluorspar deposit. We also calculated a chemical age that can be compared with the previously published regional geology and isotopic (U-Th-Sm)/He ages in fluorite. Micro-thermometric studies of fluid inclusions were carried out in different samples of uranium-rich fluorite to identify the nature and origin of the mineralizing fluid and the precipitation mechanisms of uranium minerals. We concluded that the uranium-rich fluorite precipitated in the initial phases of mineralization from a reducing fluid, with low salinity (<8% NaCl eq.) and an intermediate temperature (110–230 °C), and that the presence of organic compounds and sulfides (mainly pyrite) favored the simultaneous precipitation of uraninite (pitchblende variety) and fluorite. [ABSTRACT FROM AUTHOR]

Laminated travertine forms in and around an active hot spring on the west coast of Tamiahua Lagoon, north of the state of Veracruz, Mexico. Fluid chemistry is characterized by discharging slightly acidic pH hot water and gas at a constant flow rate. Moreover, finely interbedded mineralogical products from discharging waters at 70 °C host scattered hydrocarbons. The mineralogy and geochemistry of the travertine formations were characterized to determine their origin. Rock samples were collected and further studied by transmitted light petrography, X-ray diffraction, and EDS-coupled scanning electron microprobe. Identified mineralogy from outcrop samples includes aragonite, gypsum, anhydrite, and elemental sulfur as essential minerals, with calcite, celestine, barite, jarosite, opal, and fluorite as accessory minerals. Isotopic analyses for C and O were determined in carbonates, S isotope ratios on both elemental sulfur and sulfates, whereas measurements for trace elements and lanthanides were performed on carbonates. A suit of brines and condensates from gas samples was collected for H and O isotopic analyses and concentration determinations of the main ions and major and trace elements. Isotopic values of δ13C and δ18O of aragonite are in the range of +1.75‰ to +2.37‰ and −1.70‰ to −0.78‰, respectively. The δ34S isotopic values of native sulfur and sulfates ranged from −4.0‰ to +1.2‰. The isotopic values of δ2H (−5.50‰) and δ18O (+7.77‰) of hot water samples collected in terraces where aragonite precipitates suggest a mixture between meteoric water and the Gulf of Mexico oil-field related waters. It was concluded that the aragonitic formations near Tamiahua Lagoon are hypogenic and were generated by CO2 and H2S emanations of deep origin and by oxidation-reduction reactions that can be linked to surficial bacterial activity. [ABSTRACT FROM AUTHOR]

Agricultural soil erosion, both mechanical and eolic, may impact cloud processes as some aerosol particles are able to facilitate ice crystals formation. Given the large agricultural sector in Mexico, this study investigates the ice nucleating abilities of agricultural dust collected at different sites and generated in the laboratory. The immersion freezing mechanism of ice nucleation was simulated in the laboratory via the Universidad Nacional Autónoma de México (UNAM)-Micro Orifice Uniform Deposit Impactor (MOUDI)-Droplet freezing technique (DFT) (UNAM-MOUDI-DFT). The results show that agricultural dust from the Mexican territory promote ice formation in a temperature range from -11.8°C to -34.5°C, with ice nucleating particle (INP) concentrations between 0.11 L-1 and 41.8 L-1. Furthermore, aerosol samples generated in the laboratory are more efficient than those collected in the field, with T50 values (i.e., the temperature at which 50% of the droplets freeze) higher by more than 2.9°C. The mineralogical analysis indicated a high concentration of feldspars i.e., K-feldspar and plagioclase (> 40%) in most of the aerosol and soil samples, with K-feldspar significantly correlated with the T50 of particles with sizes between 1.8 µm and 3.2 µm. Similarly, the organic carbon (OC) was correlated with the efficiency of aerosol samples from 3.2 µm to 5.6 µm and 1.0 µm to 1.8 µm. Finally, a decrease in the efficiency as INPs, after heating the samples at 300°C for 2 h, evidenced that the organic matter from agricultural soils can influence the role of INPs in mixed-phase clouds. [ABSTRACT FROM AUTHOR]

Grain size, heavy minerals, mineralogical, and geochemical analyses were undertaken on the sand samples from Huatabampo (HB) and Altata (AA) beaches to infer palaeoweathering conditions and source rock characteristics. The mean grain size of HB and AA sands is fine‐grained and fine‐ to medium‐grained, respectively. Magnetite and hypersthene are the common heavy minerals identified in HB sands, whereas magnetite and ilmenite are the dominant heavy minerals in AA sands. HB sands have a high proportion of quartz and plagioclase followed by K‐feldspar and zeolite. Quartz is the dominant mineral in the AA sands, followed by plagioclase and K‐feldspar. Geochemically, the sand samples from HB are classified as wacke, whereas AA sands are classified as wacke and arkose. In chondrite‐normalised REE plot, HB and AA sands show LREE enriched and HREE depleted pattern with negative Eu anomaly. Chemical index of alteration (CIA), plagioclase index of alteration (PIA) values and Th/U ratios of HB and AA sands suggest a low intensity of chemical weathering in the source area. The tectonic discriminant function diagram suggestsan arc setting for HB sands, whereas a rift setting for AA sands. The chondrite‐normalised REE patterns, Eu anomaly, Al2O3/TiO2 ratio, trace elemental ratios, discriminant function, and ternary diagrams suggest that the HB and AA sands were dominantly derived from felsic source rocks. The comparison of REE patterns of HB and AA sands with the source rocks exposed near to the study area suggest that the central granites of Laramide plutonic rocks exposed in Sonora were contributing sediments to the HB beach, whereas AA sands received sediments mainly from the felsic Francisco gneiss in the western Sonobari Complex of Sinaloa. [ABSTRACT FROM AUTHOR]

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Fragipan is a compacted but non-cemented subsurface horizon, considered as a pedogenic horizon, but the mechanism of its formation is not well understood. The main hydro-consolidation hypothesis involves a collapse of soil structure when it is loaded and wet, resulting a reorganisation of pore space. Soils with fragipan never have been marked in Russian soil maps. In the South Karelia, located in Eastern Fennoscandia (34.50921 E and 61.33186 N, 110 m asl) we studied a soil profile of Albic Fragic Retisol (Cutanic), developed in the glacial till of Last Glaciation with flat subhorizontal topography under an aspen-spruce forest. The aim of this study was to demonstrate how the fragic horizon was formed in the Retisol located in South Karelia. Observations were made in each soil horizon using micromorphological method, particle size analysis and the study of mineralogical composition of clay fraction by X-ray diffraction. The analysis of the morphological description combined with the laboratory data have led us to the conclusion that the consolidation of the fragipan occurred after the textural differentiation of the profile, following the Atlantic Optimum, and does not depend on the presence of swelling clay minerals. The well-developed argic horizon was probably formed around 6000 years ago, under climatic conditions more favourable for clay illuviation than in present time. Fragipan is supposed to be developed during the Sub-Boreal cooling. [ABSTRACT FROM AUTHOR]

Mining and metallurgy generate residues that may contain thallium (Tl), a highly toxic metal, for which it is currently not feasible to determine its geochemical speciation through X-ray absorption spectroscopy due to a combination of very low contents and the interference of accompanying high arsenic contents. Therefore, fractionation studies in residues and soils are required to analyze the mobility and bioavailability of this metal, which in turn provide information to infer its speciation. For this purpose, in this work a modification of the BCR procedure was applied to residues and contaminated soils from three mining zones of Mexico and two mining zones of Spain, spanning samples with acidic to alkaline pH values. The Tl extraction procedure consisted of the following fractions: (1) water-extractable, (2) easily exchangeable and associated to carbonates, associated to (3) poorly-crystalline and (4) crystalline Fe and Mn oxyhydroxides, and (5) associated to organic matter and sulfides; and finally a residual fraction as associated to refractory primary and other secondary minerals. The extracted contents were analyzed by Inductively-Coupled Plasma with Mass Spectrometry. Surprisingly, water-soluble, in Tl(I) oxidation state, was detected in most areas, regardless of the pH, a fact that has not been reported before in these environments, and alerts to potential health risks not previously identified. Most of the samples from a metallurgy area showed high levels of Tl in non-residual fractions and a strong correlation was obtained between extracted Mn and Tl in the third fraction, suggesting its association to poorly crystalline manganese oxides. In the majority of samples from purely mining environments, most of the Tl was found in the residual fraction, most probably bound to alumino-silicate minerals. The remaining Tl fractions were extracted mainly associated to the reducible mineral fractions, and in one case also in the oxidizable fraction (presumably associated to sulfides). Capsule: Soluble Tl(I) was found in all soil samples contaminated with either mining or metallurgical wastes. Additionally, in those affected by metallurgical wastes a very strong Tl-Mn correlation was found. [ABSTRACT FROM AUTHOR]

Microbialites are modern analogs of ancient microbial consortia that date as far back as the Archaean Eon. Microbialites have contributed to the geochemical history of our planet through their diverse metabolic capacities that mediate mineral precipitation. These mineral-forming microbial assemblages accumulate major ions, trace elements and biomass from their ambient aquatic environments; their role in the resulting chemical structure of these lithifications needs clarification. We studied the biogeochemistry and microbial structure of microbialites collected from diverse locations in Mexico and in a previously undescribed microbialite in Cuba. We examined their structure, chemistry and mineralogy at different scales using an array of nested methods including 16S rRNA gene high-throughput sequencing, elemental analysis, X-Ray fluorescence (XRF), X-Ray diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS), Fourier Transformed Infrared (FTIR) spectroscopy and Synchrotron Radiation-based Fourier Transformed Infrared (SR-FTIR) spectromicroscopy. The resulting data revealed high biological and chemical diversity among microbialites and specificmicrobe to chemical correlations. Regardless of the sampling site, Proteobacteria had the most significant correlations with biogeochemical parameters such as organic carbon (Corg), nitrogen and Corg:Ca ratio. Biogeochemically relevant bacterial groups (dominant phototrophs and heterotrophs) showed significant correlations with major ion composition, mineral type and transition element content, such as cadmium, cobalt, chromium, copper and nickel. Microbial-chemical relationships were discussed in reference to microbialite formation, microbial metabolic capacities and the role of transition elements as enzyme cofactors. This paper provides an analytical baseline to drive our understanding of the links between microbial diversity with the chemistry of their lithified precipitations. [ABSTRACT FROM AUTHOR]

Tuffite has been studied for Cu(II) adsorption from aqueous and ammoniacal solutions. Tuffite was characterized by mercury porosimetry, Brunnauer–Emmet–Teller (BET), scanning electron microscopy–energy X-ray dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectra (FTIR), X-ray powder diffraction (XRD), inductively coupled plasma-atomic emission spectrometry (ICP-OES) and petrographic microscopy. The equilibrium time was 50 min. The second-order model is the best model to describe the process. It was determined that the intraparticle diffusion was not the only limited step. Process variables were studied to improve the adsorption process. The material washed contributes to increase the Cu(II) adsorption from 213.05 to 276 mg/g. The flow countercurrent system requires at least 6 g of tuffite to achieve 90% of removal. [ABSTRACT FROM PUBLISHER]

New outcrops at the Cerro Matoso open-pit nickel mine permitted to sample sediments found on top of the peridotites to study the textural relationships and composition of phyllosilicates that make up what we propose is a deep marine succession. The appearance and color of mounds and tabular layers divided them into two lithofacies formed by green claystone and black mudstone. The chemical, mineralogical, and petrographic analysis allowed the identification of berthierine and greenalite as the main components of the clay fraction of these rocks. Both phyllosilicates show sharp peaks in the X-ray diffractometry (XRD) diagrams, which show a high degree of "crystallinity". These peaks are produced when the phyllosilicates are formed under ideal conditions of temperature, alkaline pH, and reducing environments, which differ from the supergene and oxidizing environments in which they are now found. The fossiliferous content, in which bivalves, gastropods, and polychaetes predominate, together with the fissured and brecciated textures in the sediments, accompanied by a network of microcracks and open ducts are indicative of marine environments with fluid exhalation that can be related to hydrothermal systems and serpentinization of ultramafic rocks. This work allows us to redefine some units previously described as part of the lateritic profile of Cerro Matoso, i.e., ¨black saprolite and ¨canga mona¨ and to report what would be the first discovery of fauna associated with hydrothermal vents hosted in mantle rocks that were posteriorly accreted during the Cretaceous to the continent. These were recently exposed to humid tropical environmental conditions, where they developed weathering lateritic profiles. • Reducing conditions and high temperatures instead of oxidizing environments are identified during the formation of clays at the top of the Cerro Matoso peridotites. • Fossiliferous evidence helps to support ocean-deep environments in the Cerro Matoso sediments. • Units previously considered as alteration products of ultramafic rocks are redefined as deep-marine sediments (black mudstone and green claystone). • The findings coincide with the serpentinization processes that mantle rocks undergo in ocean ridges and that can host hydrothermal systems and associated fauna. • Evidence of fauna associated with hydrothermal vents is reported, and it would be the first reported in Colombia. [ABSTRACT FROM AUTHOR]

Valencianite has been described as a variety of K-feldspar (adularia) from La Valenciana mine, Guanajuato, Mexico, from which three samples were used for this study. We present new major and trace element analysis, X-ray powder diffraction with Rietveld refinement, and single-crystal 40Ar/39Ar ages of this classical mineral. A detailed review of major works on feldspars and relevant papers from adularia shows that the structure of this mineral has monoclinic and triclinic domains with variable degrees of Al/Si order that we have been only able to model by powder X-ray diffraction assuming the presence of monoclinic (~50%) plus triclinic (~50%) K-feldspar. The literature data show some extreme structural states for adularia obtained in the pre-Rietveld refinement era; these data are dubious and need to be reanalyzed. A triangular diagram using the relative development of {110}, {010} and {001} faces is proposed. The temperature of formation, the Na/K ratio, and the growth kinetics seem to be the main factors controlling the morphological changes in K-feldspar crystals. The geochemistry of valencianite shows an almost pure orthoclase composition, as is common in most adularia crystals, although compositions up to Or90Ab10 have been found. Measurement of thallium in adularia can be an exploration guide for ore deposits. The weighted mean of 15 40Ar/39Ar analyses of one valencianite from La Valenciana mine gave an age of 30.43 ± 0.27 Ma (2 standard deviations). It is discussed whether valencianite can be considered a new mineral. [ABSTRACT FROM AUTHOR]

Maya blue is a well-known pre-Hispanic pigment, composed of palygorskite or sepiolite and indigo blue, which was used by various Mesoamerican cultures for centuries. There has been limited research about its continued use during the Viceroyalty period; therefore, the sixteenth century is the perfect period through which to study the continuity of pre-Hispanic traditions. The fact that the indigenous people were active participants in the construction and decoration of convents makes their wall paintings a good sampling material. X-ray fluorescence (XRF), scanning electron microscopy (SEM) and X-ray diffraction (XRD) were performed in samples of blue found in convents across Puebla, Tlaxcala and Morelos in order to identify whether the numerous hues of blue were achieved with Maya blue or with other pigments. We found no copper (Cu) or cobalt (Co) with the XRF, so several pigments, such as azurite, smalt or verdigris, were discarded. With SEM, we discovered that the micromorphology of certain blues was clearly needle-shaped, suggesting the presence of palygorskite or sepiolite. In addition, we found silicon (Si), magnesium (Mg) and aluminum (Al) by using energy-dispersive X-ray spectroscopy (EDS) in all blue samples, which also suggests the presence of these magnesium-rich clay minerals. With the XRD samples, we verified that the blues were produced with these two clay minerals, thus confirming that several wall paintings were manufactured with Maya blue. These findings confirm that this particular manmade pre-Hispanic pigment, Maya blue, was an important pigment prior to the Viceroyal period. [ABSTRACT FROM AUTHOR]

The mineralogy of surface crusts and efflorescent salts of two old abandoned tailings (Xochula and Remedios) of the mining district of Taxco, Guerrero, was studied by short-wave infrared spectroscopy (SWIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The chemistry of the selected samples was studied with induced coupled plasma mass spectrometry (ICP–MS). Principal mineralogy is composed of hydrated sulfates of iron (di-trivalent), other less-soluble sulfates such as gypsum and jarosite, and oxides–hydroxides of iron, all of which are the result of the weathering of sulfides (mainly pyrite) accumulated in mining waste. Using quantitative X-ray diffraction (Rietveld method) and the spatial distribution of secondary minerals, two main zones (lateral and central) have been established in both tailings. The genetic model for their formation shows that the secondary iron minerals (sulfates, oxides, and hydroxides) in these deposits have been formed at least in three different stages, mainly by oxidation, dissolution, and precipitation processes in combination with dehydration and primary sulfides neutralization reactions. The chemical analyses of some salts confirm the presence of potentially toxic elements (PTE) in their structure and therefore indicate that the dissolution of the efflorescences in these deposits is a potential source of contamination during the wet season. [ABSTRACT FROM AUTHOR]

The detailed mineralogical, physical and chemical characterization of nine samples of imported cosmetic talc and of two samples of talc from currently non-productive Mexican ore deposits (Oaxaca and Puebla States) is presented. The imported cosmetic talc was classified into two groups, considering whether they are packed in the country of origin or in Mexico and considering their price. X-ray diffraction, infrared short wave, thermogravimetric analysis and scanning electron microscopy were used for mineralogical characterization. For the physical characterization, colorimetry and laser granulometry were used. The chemical composition (major, trace elements) was studied by ICP-MS. It was concluded that only the highest priced and imported in packaged form talcs meet the specific purity requirements for a talc of cosmetic type. The talcs that are packed in Mexico and the talc of the studied Mexican deposits present mineralogical and chemical impurities that make their use difficult in the manufacture of high-quality cosmetic talc. The low-price talc should not be sold as cosmetic talc, and the regulations in Mexico on this subject should be reviewed and updated. [ABSTRACT FROM AUTHOR]

The Chicomuselo fold-thrust belt (CFTB) is perhaps the less known cover orogen in Mexico. It is exposed in southernmost Mexico, Guatemala and Belize, along a fringe of about 500 km long and 50–100 km wide, on the southern edge of the Maya tectonostratigraphic terrane. The CFTB is constituted by a succession of sedimentary late Paleozoic marine rocks that include the Santa Rosa Inferior, the Santa Rosa Superior, the Grupera and the Paso Hondo formations. Based on the fossil content of the Santa Rosa Inferior and Paso Hondo formations, the age of these units was constrained from the Mississippian to the late Wolfcampian (early Permian). These successions are folded and thrusted, affecting the two more competent upper formations, whereas the two finer-grained lower units are also highly foliated. Folds, thrusts, and axial-plane foliation (S 1) are more consistent with a NE-SW shortening direction and the asymmetry indicates a vergence towards the NE. Such structures are unconformably covered by red-beds of the Middle Jurassic Todos Santos Formation. No angular unconformity was found among the Paleozoic formations, as reported in previous works, which indicates that shortening deformation may have occurred in an orogenic cycle that took place between the latest Permian and Triassic. Structures related to such deformation are cut by kink bands and lateral faults (both dextral and sinistral), spaced across the belt. In order to constrain the thermal conditions of deformation in the CFTB, about 53 samples of phyllite, slate and shale were collected, and their illite crystallinity index (ICI) determined along the belt axis. Calibrated ICI's indicate that deformation occurred between low epizone to the upper diagenetic zone (Kübler index between 0.2 and 1 Δ°2θ angle), with a slow increase in crystallite size from the lower to the upper units, along and across the belt. Deformation-related illite from the Santa Rosa Inferior and Paso Hondo formations was dated with the 40Ar-39Ar method. In the first sample, mica fish of tens of microns dominate the fabric, though a finer-grained illite parallel to the axial plane of late kink bands overgrows such fabric. In the second sample, only one generation of micrometric size illite is visible under the SEM. However, the characterization and dating of several clay size fractions (0.2–0.5, 0.5–1 and 1–2 μm, in addition to 2–4 μm in the first sample) indicate that both samples contain the same two end-member generations of illite, one formed during a first event of intense shortening at 252.3 ± 8.1 Ma, and another associated to spaced faulting formed at 175.1 ± 6.5 Ma. All these data indicate that all sedimentary units within the CFTB were shortened simultaneously during an orogeny that started in the Permian-Triassic threshold and were presumably affected by Middle Jurassic faulting during the opening of the Gulf of Mexico. • The Chicomuselo fold-thrust belt (ChFTB) is a regional tectonic feature exposed in southern Mexico, central Guatemala, and Belize. • Ar-Ar age of deformation-related illite indicate this slate belt formed in the Late Permian-Triassic, during Pangea assemblage. • The southern edge of the ChFTB is the reverse-dextral Motozintla shear zone (MSZ), similar but later to the Caltepec fault. • The MSZ superimposes the Chiapas massif crystalline basement rocks over shortened rocks of the CFTB. [ABSTRACT FROM AUTHOR]

Coralline algae are worldwide carbonate builders, considered to be foundational species and biodiversity hotspots. Coralline habitats face increasing pressure from human activities and effects related to Global Change, yet their ecological properties and adaptive responses remain poorly understood. The relationships of the algal microbiota with the mineral bioconstructions, as well as plasticity and resilience of coralline holobionts in a changing environment, are of particular interest. In the Gulf of California, Neogoniolithon trichotomum (Rhodophyta) is the main carbonate builder in tidal pools. We performed a multi-disciplinary assessment of the N. trichotomum microstructure using XRD, SEM microscopy and SR-FTIR spectromicroscopy. In the algal perithallus, magnesium-calcite and aragonite were spatially segregated and embedded in a polysaccharide matrix (rich in sulfated polysaccharides). Mg-calcites (18–19 mol% Mg) were the main mineral components of the thallus overall, followed by iron carbonates related to dolomite (ankerite) and siderite. Minerals of late evaporitic sequences (sylvite and bischofite) were also present, suggesting potential halophilic microenvironments within the algal thalli. The diverse set of abundant halophilic, halotolerant and oligotrophic taxa, whose abundance increase in the summer, further suggests this condition. We created an integrated model, based on environmental parameters and the microbiota distribution, that identified temperature and nutrient availability (particularly nitrate and silicate) as the main parameters related to specific taxa patterns. Among these, Hahella, Granulossicoccus, Ferrimonas , Spongiibacteraceae and cyanobacterial Xenococcaceae and Nostocaceae change significantly between seasons. These bacterial components might play relevant roles in algal plasticity and adaptive responses to a changing environment. This study contributes to the understanding of the interplay of the prokaryotic microbiota with the mineral microenvironments of coralline algae. Because of their carbonates with potential resistance to dissolution in a higher pCO 2 world and their seasonally dynamic bacteria, coralline algae are relevant targets to study coastal resilience and carbonated systems responses to changing environments. Unlabelled Image • The interplay of microorganisms and algal mineral bioconstructions remains poorly understood. • Carbonates rich in Fe and Mg found make CA relevant targets to study coastal resilience. • Halophiles and evaporite minerals concurrently suggest halophilic microenvironments in the thallus. • Bacterial microbiota correlated significantly with temperature and nutrients. • Key bacteria might play relevant roles in adaptive responses of coralline algae. [ABSTRACT FROM AUTHOR]

Desert varnish is a dark microlayer that forms on rocky surfaces that is usually associated with arid and desert environments. It consists mainly of clay minerals (60%), while the rest are Fe and Mn oxides. Growth rates are very slow and vary from <1 to 40 μm/ky. Although different proposals exist to explain their formation mechanisms, these processes are still unknown. Around the world, various groups and human communities have created petroglyphs with different meanings in desert varnish. In the Sonoran Desert, the archaeological site La Proveedora is known for having many petroglyphs made in granite rock varnishes. It is believed that people groups occupied the area during the mid-Holocene (ca 5000-3000 BC). The research was carried out by analyzing and looking for possible signals and contributions from past environments and current processes that are preserved in the varnish layers. The X-ray diffraction, laser breakdown spectra and scanning electron microscopy with energy dispersive spectroscopy analyses in varnish samples, showed that the distribution and concentrations of Fe and Mn exhibit an alternating behavior on a depth scale from the surface until contact with granite. The DNA analysis showed a major presence of the Proteobacteria and Actinobacteria groups. Notably, some of these microorganisms can incorporate Fe and Mn into their metabolic processes and mobilize them through the varnish for its formation. The 14C radiocarbon dating (370 ± 54 cal BP) indicates a very young age associated primarily with recent microorganisms. However, a first approximation was obtained for the minimum age (5000-1000 yr BP). The comparison between the areas of varnish and the surfaces within the petroglyphs suggests that the varnish formation occurred under conditions in the past when the humidity was higher, and that it is probably a very slow, intermittent or uncoated formation mechanism today. [ABSTRACT FROM AUTHOR]