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A study of the morphology and evolution of the microstructure during thermal decomposition of Y2(C2O4)3·10H2O was conducted, and the stages and factors having the greatest impact on particle size and specific surface area were identified. The effect of the yttrium oxalate hexahydrate phases on the course of decomposition was also investigated. The evolution of the morphology and microstructure of decomposition products was explained from the analysis of volume shrinkage at various stages of the reaction. The formation of oxycarbonate is accompanied by the largest shrinkage during the reaction. At this stage, there is a significant increase in the specific surface area to 60–90 m2/g. Conversely, the morphology and microstructure of the particles during the transformation of oxycarbonate into yttrium oxide change insignificantly. Yttrium oxide powders obtained from the monoclinic and triclinic hexahydrate phases have the same specific surface area, but different morphology and bulk density due to pseudomorph formation. The carbon formed during thermolysis was shown to affect the specific surface area of the decomposition product. Two methods for producing yttrium oxide with high specific surface area have been proposed, and nanocrystalline yttrium oxide with a specific surface area of 65 m2/g was obtained.

In November of 2020, an exciting occurrence of epidote pseudomorphs after orthoclase was rediscovered in the Orogrande district in southern New Mexico (fig. 1). Considering that propylitic alterati...

The thermal-assisted exfoliation phenomena of boehmite particles under moderate heating rates were examined. The exfoliation that generated flakes of 5–6 nm in thickness can be achieved because of the perfect cleavage on the boehmite particles that are stripped when thermal treatments bring about dehydration and γ-Al2O3 formation in sequential phase transformation of boehmite. Examinations of the exfoliation effects were carried out on calcined boehmite single crystal particles, which were about 500 nm in diameter, and obtained at three heating rates 0.5, 1.0, and 2.0 °C/min with the heating schedules. The TEM techniques, BET-N2 measurements, XRD-Scherrer equation, and AFM images were employed in the examination. That the BET values increased as increasing of exfoliated flakes reflected two stages of exfoliation. In the beginning stage, during which the BET values were

Permafrost is considered to be one of the important sources of additional water for the vast Late Quaternary Caspian Sea transgressions. The insufficiency of proxy evidence on landscape and climate dynamics during the long Atelian regression (MIS 4 – MIS 3) complicates the analysis of changes in the Caspian water balance. Traces of Late Pleistocene cryogenesis structures were catalogued in the alluvial and loess-soil deposits of the Srednyaya Akhtuba reference section. Four stages of permafrost development were described for the first time in the Lower Volga region. Under conditions of seasonal and longer-term freezing in the Late Pleistocene, cryogenic transformation of the deposits took place; this determined the composition, structure and properties of the loess-paleosol complex at the site. Cryolithological, micromorphological analyzes and particle size distribution of mineral matter were carried out for each horizon, and traces of cryogenic processes were described in order to evaluate the regional paleoclimatic conditions that prevailed during their formation. Optically stimulated luminescence dating was used to establish a chronological framework for the main phases of the development of cryogenic modification in the Lower Volga region. The fourth stage of formation involved deep cracking of the substrate, which occurred during freezing in MIS 4, coincident with the Atelian regression of the Caspian Sea. The third stage is expressed as large wedge-shaped structures (pseudomorphs) on the border of loess and alluvial packages, with their formation known to be associated with the degradation of ice wedges. During the second and first stages of freezing, plastic deformation of deposits occurred.

Polycrystalline calcite pseudomorphs after dolomite can exhibit mosaic or drusy fabric caused by replacement of the dolomite by calcite (one-step dedolomitization) or dolomite dissolution followed by calcite cementation (two-step dedolomitization), respectively. This study discusses the differences between the two types and interprets the characteristics of the fluid causing them. Conclusions drawn here are founded on a microscopic and petrophysical examination of an outcrop of Tuwaiq Mountain Formation, central Arabian Peninsula. Petrographic examination shows that dedolomite has distinct microfabrics in each part of the section. The lower part of the section is not affected by dolomite calcitization, the middle part is dominated by replacement fabrics, and the upper part is dominated by dissolution and/or cementation fabrics. Petrophysical examination shows a wider range of porosity and permeability values for the upper part of the section, relative to the middle and lower parts. One-step dedolomitization is characterized by preservation of the external morphology of the parent dolomite, confinement to the volume previously occupied by the dolomite, development of intracrystalline porosity and permeability that allows the fluid to maintain contact with the reaction front, and no significant effect on porosity of the original rock. Two-step dedolomitization is characterized by corrosion of the external morphology of the parent dolomite, association with other forms of calcite cementation, development without intracrystalline porosity and permeability, and positive or negative effects on the porosity. While one-step dedolomitization appears to occur within a stagnant zone of fluid circulation, the two-step dedolomitization is considered to reflect an active fluid-flow system.

This paper reports on an ice-wedge pseudomorph that formed and is preserved in metavolcanic host material that was later transformed to metamorphic solid bedrock. It has been dated to 1,895 ± 5 Ma by U–Pb geochronology of zircon in the bedrock, an Early Proterozoic age. Detailed observation of the deformation structures of the wedge points to an ice-wedge pseudomorph based on typical downbending around the wedge and vertical lamination in the inner part of the wedge due to slumping into the wedge after the ice melted, along with a few remains of lateral pressure structures (such as folds and upturned strata) in the adjacent host sediment. The interpretation of the wedge structure as an ice-wedge pseudomorph confirms previous work on this topic. This ice-wedge pseudomorph demonstrates for the first time the existence of permafrost at c. 1.9 Ga. It indicates that permafrost and associated conditions were present in lowlands at low latitude at discrete time intervals early in Earth’s history. Although some caution should be applied, mean annual air temperature appears to have been slightly below the freezing point at that time.

Thermokarst lakes and alases are widespread in the North-East of Russia. These structures are part of Quaternary deposit of the region and mostly of Holocene age. The alas complex of the older age is less common. Here we present the study of alas complex sections as old as Lower – Upper Neopleistocene from the Laptev Sea coast (Oyagoss yar, Shirokostan peninsula). The principal composition of these structures of different ages is quite similar. Their formation is correlated with warm intervals when the moisture content increased. Melting of the ice-rich sediment took place locally. Accumulation of the melted water increased the melting and lead to formation of separated thermokarst depressions where succession of the lake-bog-soil sediments with ice-wedge casts (pseudomorphosis) began to accumulate. Current research expands the knowledge about distribution of the alas complex in the north-eastern Russia, allows to recognize its features (the presence of tabers, lacustrine sediments with a complex of pseudomorphs along the ice wedge, the increase in grain size of lacustrine sediments from the center to the periphery of the basins, a large number of values of allochthonous peat in the middle and autochthonous in the top of alas strata), and suggests how to extract paleogeographic information from this type of deposit.

The Cheoeum vent field (CVF) is the first example of an inactive ultramafic-hosted seafloor massive sulphide (SMS) deposit identified in the middle part of the Central Indian Ridge. Here, we report on the detailed mineralogy and geochemistry of ultramafic-hosted sulphide sample atop a chimney, together with a few small fragments. Hydrothermal chimneys are characterised by high concentrations of Au (up to 17.8 ppm) and Sn (up to 1720 ppm). The sulphide mineralisation in the CVF shows (1) early precipitation of anhedral sphalerite and pyrite–marcasite aggregates under relatively low-temperature (

The study of samples of volcanic rocks, sulfide ores, and near-ore metasomatites from the bottom of the Atlantic Ocean resulted in the discovery of vital forms of organisms: foraminiferal shells, mineral pseudomorphs after the bodies of polychaetes, deposit feeders, and other fauna. This allows us to understand not only modern geological processes in the depths of the oceans, but also to reconstruct events of the distant geological past based on the principle of actualism.

The paradigm for hydrous high-strain zones that cut dry host rocks is for fluid-rock interaction to have involved aqueous fluids. However, the role of silicate melt is increasingly recognised. This contribution examines the formation of glimmerite (biotitite) bands during melt migration in the Gough Dam shear zone, a high-strain zone in central Australia that was active during the Alice Springs Orogeny (c. 450–300 Ma). The glimmerite bands cut and replace a range of quartzo-feldspathic protoliths, including granitic gneiss and quartzite. Melt that migrated through the high-strain zone is interpreted to have penetrated relict layers along a network of fractures, enhancing dissolution of the precursor rock and causing replacement by glimmerite crystallisation. Microstructures indicative of the former presence of melt in the high-strain zone include: pseudomorphs of former melt pockets of granitic composition; small dihedral angles of interstitial phases; elongate grain boundary melt pseudomorphs; neighbourhoods of grains connected in three dimensions; and localised static grain growth and recovery. Other microstructures indicative of melt-present deformation include randomly oriented neosome grains, and evidence of activation of multiple slip systems during deformation. The degree of quartzite modification to glimmerite is recorded by an increase in biotite mode, and correlated with higher Ti concentrations in biotite (higher apparent temperature) and changes to trace element and REE compositions. Melt-assisted coupled dissolution-precipitation reactions during melt flux are interpreted to partially reset Proterozoic monazite U-Pb ages inherited from the protolith (> 1630 Ma) to younger Palaeozoic ages, with a complex age pattern partially congruent with the Alice Springs Orogeny (apparent ages range from c. 606–371 Ma, with a dominant age peak at c. 451 Ma). We propose that the glimmerite formed during dynamic melt migration of an externally-derived hydrous peraluminous melt, driving reaction replacement of various felsic protoliths during this orogenic event. Refereed/Peer-reviewed

The studied mafic granulites are located at Xiwangshan, Xuanhua region in the north of the Trans-North China Orogen (TNCO), occurring as lens within tonalite-trondhjemite-granodiorite (TTG) gneisses in the eastern part of the Xiwangshan area. The rocks contain the representative granulite-facies minerals such as garnet, clinopyroxene, orthopyroxene, plagioclase, amphibolite, rutile and quartz, and also well-developed melt pseudomorph and antiperthite. Although the prograde metamorphic stage (M1) cannot be retrieved due to rare preservation of pre-peak-stage mineral associations, three distinct mineral assemblages that formed in different metamorphic stages can be identified, based on petrography and mineralogy characteristics. The peak stage (M2) is characterized by Grt2+Cpx2+Amp2+Pl2+Rt+melt pseudomorphs, and a post-peak decompression stage (M3) contains a mineral assemblage of Grt3+Opx3+ Cpx3+Amp3+Pl3, while a later-retrogression stage (M4) is featured by coronas of Amp4+Pl4 surrounding garnet. By calculating metamorphic P-T conditions using THERMOCALC, stage M2 was constrained to be 13.2–14.8 kbar and 1 050–1 080 °C, and M3 recorded P-T conditions of 5.7–7.3 kbar and 825–875 °C, while M4 yielded P of ~5 kbar and T of ~660 °C, consistent with amphibolite facies metamorphism. Taking into account of all these petrological data, we propose that the mafic granulite experienced a high-pressure (HP) and ultra-high temperature (UHT) granulite-facies metamorphism during the peak metamorphism, which was accompanied with a clockwise P-T path. U-Pb dating of metamorphic zircons in the granulites yields two groups of ages at 1 853±14 and 1 744±44 Ma, respectively. We suggest that the older age corresponded to the HP-UHT metamorphism, while the younger age represented an retrograde metamorphic event during cooling.

Eclogite‐facies breccias from the Monviso metaophiolite complex (N‐Italy and W‐Alps) represent a unique opportunity to study the triggering mechanisms for transient brittle deformation at eclogite‐facies conditions in ductilely deforming subducted oceanic lithosphere. Here the intact foliation of Fe‐Ti‐rich and Mg‐Al‐rich metagabbro clasts, cemented by unfoliated eclogitic matrices (paragenesis: omphacite ± garnet ± lawsonite), demonstrates pristine brecciation at eclogite‐facies conditions. Successive generations of high‐pressure veins and eclogitic matrices reveal multiple brittle rupture events. Pseudosection modeling, textural observations, and geochemical data suggest that minerals of both prograde veins and first brecciation event (M1 matrix) crystallized in presence of fluids buffered by the surrounding metagabbro minerals, while M2 matrix composition suggests an incipient infiltration of external fluids. Trace element composition of the third M3 matrix, associated to its impressive lawsonite pseudomorphs, clearly points to the massive ingression of external, serpentinite‐derived fluids. Therefore, these rocks record the progressive increase in the scale of fluid circulation along multiple, stepwise eclogitic brecciation events, from locally released fluids (closed system) to kilometer‐scale fluid infiltration. The successive brecciation steps promoted permeability creation (at least transiently), which resulted in progressive opening of the system to large‐scale sustained fluid circulation. Eclogite‐facies brecciation also controlled the initial stages of strain localization, which highlights the importance, in ductilely deforming low‐permeability eclogite‐facies rocks, of brittle deformation events for both strain localization initiation and creation of large‐scale fluid circulation pathways. Thus, Monviso breccia blocks record the complete, stepwise development of a major shear zone, via progressive strain localization, permeability creation, and increasing scale of fluid circulation.

This paper reports detailed studies on harzburgite and serpentinite in the Hegenshan ophiolitic melange. Harzburgite consists mainly of olivine and orthopyroxene with trace amounts of clinopyroxene and chromian spinel. Clinopyroxene occurs as isolated crystals or in the intergrowth of chromian spinel–clinopyroxene–orthopyroxene. Harzburgite is moderately to highly depleted, displaying high Fo contents in olivine (90.8–92.2), moderate Al2O3 contents in orthopyroxene (1.59–2.79 wt%), low heavy REE abundances in clinopyroxene, and moderate Cr# values of spinel (0.50–0.62). The modal proportions of olivine and orthopyroxene pseudomorph grains imply that the parent of the Hegenshan serpentinite should be harzburgite. Whole-rock compositions of the harzburgite and serpentinite samples are characterized by depletions in Al2O3 and CaO and enrichments in light REE, Sr, and U. Geochemical modeling suggests that the Hegenshan harzburgite represents residues after 17–18% partial melting of the primitive mantle. The melt in equilibrium with clinopyroxene is more depleted than typical forearc basalt and boninite. Various pyroxene thermobarometers yield equilibrated temperatures of 945–1067 °C and pressures of 4.8–8.0 kbar for the Hegenshan harzburgite. The oxygen barometer yields results of +0.4 to +1.7 log units above the fayalite–magnetite–quartz buffer for the Hegenshan harzburgite. These petrological and geochemical characteristics, as well as the estimated P–T–fO2 conditions support a back-arc setting for the Hegenshan ophiolitic melange.

National audience; Unlike most "wedge" orogens, the structure and the composition of theTibetan Plateau lower crust has kept being largely inaccessible otherwisethan by geophysical studies and by the few scattered sampling ofxenoliths brought to the surface by volcanism. In the Nangqian basin (NEQiangtangterrane), Eocene syn-contractional potassic lavas carrypartially molten foliated quartzo-feldspathic xenoliths. Zircon LA-ICPMSU-Pb dating of these crustal xenoliths yields a Lower Triassic age of ca.248.5 Ma, which is interpreted as the protolith crystallization age, withno evidence for any metamorphic zircon growth or resetting. The rarerelict ferro-magnesian minerals found (clinopyroxene, amphibole,phlogopite, garnet) are in textural and chemical disequilibrium, makingconventional thermobarometry methods inoperable. Nevertheless, thexenoliths have retained unusual metamorphic features which can helpunravelling their pre-entrapment metamorphic history. Pure anorthite"patches" with magnetite inclusions are interpreted as pseudomorphsafter garnet. Since these patches are often found in contact withclinopyroxene and rutile, these minerals may together represent a formerhigh-pressure assemblage that developed at 1.2-1.5 GPa and 620-730°C.Such pressure record would indicate that 80% of the present-day crustalthickness of the Tibetan Plateau was already acquired in Eocene times.Moreover, corundum-bearing assemblages, interpreted as pseudomorphsafter phlogopite, are best explained by kinetic disequilibrium underhigh-temperature granulitic conditions, suggesting a short-livedmetamorphic event. Taken together, this set of new petrological andgeochronological data suggests that crustal granulitic metamorphism maynot b e related to the onset of a general warming of the Tibetan crustsustained until the present, but rather to the punctual injection of mantlederivedmelts in the crust before Eocene eruption.

The detailed mineralogical and microgeochemical characteristics of polymetallic nodules collected from the Interoceanmetal Joint Organization (IOM, Szczecin, Poland) claim area, Eastern Clarion-Clipperton Fracture Zone (CCFZ, Eastern Pacific) were described in this study. The obtained data were applied for the delimitation of nodule growth generations and estimation of the growth ratios (back-stripping using the Co-chronometer method). The applied methods included bulk X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA), providing information about Mn-Fe minerals and clays composing nodules, as well as the geochemical zonation of the growth generations. The analyzed nodules were mostly diagenetic (Mn/Fe &gt, 5), with less influence on the hydrogenous processes, dominated by the presence of 10-Å phyllomanganates represented by todorokite/buserite, additionally mixed with birnessite and vernadite. The specific lithotype (intranodulith), being an integral part of polymetallic nodules, developed as a result of the secondary diagenetic processes of lithification and the cementation of Fe-rich clays (potentially nontronite and Fe-rich smectite), barite, zeolites (Na-phillipsite), bioapatite, biogenic remnants, and detrital material, occurs in holes, microcaverns, and open fractures in between ore colloforms. The contents of ∑(Ni, Cu, and Co) varied from 1.54 to 3.06 wt %. Several remnants of siliceous microorganisms (radiolarians and diatoms) were found to form pseudomorphs. The applied Co-chronometer method indicated that the nodules’ age is mainly Middle Pliocene to Middle Pleistocene, and the growth rates are typical of diagenetic and mixed hydrogenetic–diagenetic (HD) processes. Additionally, few nodules showed suboxic conditions of nucleation. Growth processes in the eastern part of the CCFZ deposit might have been induced with the Plio-Pleistocene changes in the paleooceanographic conditions related to the deglaciation of the Northern Hemisphere.

During phase transitions the ordering of cations and/or anions along specific crystallographic directions can take place. As a result, extra reflections may occur in diffraction patterns, which can indicate cell doubling and the reduction of the crystallographic symmetry. However, similar features may also arise from twinning. Here the nanostructures of a glendonite, a calcite (CaCO3) pseudomorph after ikaite (CaCO3·6H2O), from Victoria Cave (Russia) were studied using transmission electron microscopy (TEM). This paper demonstrates the occurrence of extra reflections at positions halfway between the Bragg reflections of calcite in 0kl electron diffraction patterns and the doubling of d104 spacings (corresponding to 2∙3.03 Å) in high-resolution TEM images. Interestingly, these diffraction features match with the so-called carbonate c-type reflections, which are associated with Mg and Ca ordering, a phenomenon that cannot occur in pure calcite. TEM and crystallographic analysis suggests that, in fact, (101¯4) calcite twins and the orientation change of CO3 groups across the twin interface are responsible for the extra reflections.

We present the results of an optical microscopy, cathodoluminoscopy and isotopic study on nine glendonite concretions (calcite pseudomorphs replacing metastable hexahydrate ikaite) from Low...

The Messinian Qawasim and Abu Madi formations are the main exploration targets for hydrocarbons in the onshore Nile Delta. However, their diagenetic history and the burial attributes which impacted their reservoir characteristics are poorly studied. Among these are the preferential chlorite authigenesis and complete illitization of kaolinite and feldspars in the syn-MSC (Messinian salinity crisis) Abu Madi sediments. SEM-EDS analyses revealed two different chlorite pseudomorphs in the MSC Abu Madi sediments. The first (chlorite I) has Si/Al ratio similar to the ratio in smectite (∼2.2:1), and the other (chlorite II) has a ratio very similar to kaolinite (∼1.3:1). These pseudomorphs were formed due to progressive chloritization of both smectite and kaolinite. Chlorite I formed by chloritization of smectite, and its absence in the pre-MSC Qawasim facies is due to the absence of smectite itself. Moreover, the comparatively enhanced burial of the MSC Abu Madi sediments in Fe-rich system prompted the chloritization of kaolinite and formation of chlorite II. The incomplete illitization, and formation of intermediate illite-kaolinite phase in the shallower pre-MSC facies confirms the prominent depth control. The Messinain facies were subdivided into three rock types of descending reservoir quality and different clay distributions (RT I, II, and III). The pre-MSC facies contain abundant diagenetic kaolinite which has minimal impact on the facies reservoir quality (RT I). The MSC facies are dominated by depositional clays (RT III) and diagenetic chlorite (RT II) which have the most detrimental impact on the reservoir quality, and caused high degree of permeability anisotropy.

As reported in our prior work, we have recovered microdiamonds and other unusual minerals, including pseudomorph stishovite, moissanite, qingsongite, native elements, metallic alloys, and some crustal minerals (i.e., zircon, quartz, amphibole, and rutile) from ophiolitic peridotites and chromitites. These ophiolite-hosted microdiamonds display different features than kimberlitic, metamorphic, and meteoritic diamonds in terms of isotopic values and mineral inclusions. The characteristic of their light carbon isotopic composition implies that the material source of ophiolite-hosted diamonds is surface-derived organic matter. Coesite inclusions coexisting with kyanite rimming an FeTi alloy from the Luobusa ophiolite show a polycrystalline nature and a prismatic habit, indicating their origin as a replacement of stishovite. The occurrence in kyanite and coesite with inclusions of qingsongite, a cubic boron nitride mineral, and a high-pressure polymorph of rutile (TiO2 II) point to formation pressures of 10–15 GPa at temperatures ∼1300 °C, consistent with depths greater than 380 km, near the mantle transition zone (MTZ). Minerals such as moissanite, native elements, and metallic alloys in chromite grains indicate a highly reduced environment for ophiolitic peridotites and chromitites. Widespread occurrence of diamonds in ophiolitic peridotites and chromitites suggests that the oceanic mantle may be a more significant carbon reservoir than previously thought. These ophiolite-hosted diamonds have proved that surface carbon can be subducted into the deep mantle, and have provided us with a new window for probing deep carbon cycling. Keywords: Ophiolite-hosted diamond, Coesite, Stishovite, Podiform chromitites, Mantle transition zone

Melt must transfer through the lower crust, yet the field signatures and mechanisms involved in such transfer zones (excluding dykes) are still poorly understood. We report field and microstructural evidence of a deformation‐assisted melt transfer zone that developed in the lower crustal magmatic arc environment of Fiordland, New Zealand. A 30–40 m wide hornblende‐rich body comprising hornblende ± clinozoisite and/or garnet exhibits 'igneous‐like' features and is hosted within a metamorphic, two‐pyroxene–pargasite gabbroic gneiss (GG). Previous studies have interpreted the hornblende‐rich body as an igneous cumulate or a mass transfer zone. We present field and microstructural characteristics supporting the later and indicating the body has formed by deformation‐assisted, channelized, reactive porous melt flow. The host granulite facies GG contains distinctive rectilinear dykes and garnet reaction zones (GRZ) from earlier in the geological history; these form important reaction and strain markers. Field observations show that the mineral assemblages and microstructures of the GG and GRZ are progressively modified with proximity to the hornblende‐rich body. At the same time, GRZ bend systematically into the hornblende‐rich body on each side of the unit, showing apparent sinistral shearing. Within the hornblende‐rich body itself, microstructures and electron back‐scatter diffraction mapping show evidence of the former presence of melt including observations consistent with melt crystallization within pore spaces, elongate pseudomorphs of melt films along grain boundaries, minerals with low dihedral angles as small as

In this work, cerium dioxide nanoparticles (CeNPs) were synthesized by thermal decomposition of cerium oxalate decahydrate Ce2(C2O4)3·10H2O. Although a large amount of water was removed along with CO2, the product of decomposition retained the shape of the precursor crystal, i.e. a pseudomorph formed. The Ce2(C2O4)3·10H2O precursor crystals were of plate-like shape; this shape was inherited by CeO2 formed upon decomposition of the precursor. CeO2 plates with a thickness of less than 15 µm were transparent to visible light. X-ray diffraction analysis, transmission electron microscopy and specific surface area measurements indicated that the plates consisted of CeNPs 4–6 nm in size. The influence of the annealing temperature on the growth of crystallites of the pseudomorph was studied. Fast coarsening of the crystallites started at a temperature of 500 °C.

Carbonate apatite (CO3Ap) is an inorganic component of mature bone and is used as bone substitute instead of autografts. Recently, 100% pure CO3Ap granules formed from calcium carbonate via a dissolution-precipitation reaction have been used as a clinical precursor for bone. In this study, the feasibility of fabricating blocks of CO3Ap from blocks of octacalcium phosphate (OCP), which is an analog of the bone composition and serves as a precursor for bone, was investigated by immersion in NaHCO3. The macroscopic structure of the OCP block was retained upon conversion to CO3Ap. In addition, as the NaHCO3 concentration increased from 0.0 to 2.0 mol/L, the carbonate content increased in the resulting CO3Ap, reaching ≈ 14 wt% at maximum NaHCO3 concentration. Moreover, the morphology of the CO3Ap crystals varied as the NaHCO3 concentration increased up to 0.5 mol/L: they were pseudomorphic plate-like when the NaHCO3 concentration was low but formed particles ranging ~ 10 nm in size at high NaHCO3 concentrations. The different phase transformation modes depended on NaHCO3 concentrations closely related to CO3Ap crystal morphology and were associated with a decrease in the mechanical strength of the block.

The high-grade Th-U-Y zone is one of the numerous mineralized zones found in the El Garra El Hamra hydrothermally altered syenites in southwestern Egypt. It is characterized by the thorium minerals thorite and thorogummite, which are closely associated with xenotime-(Y). This zone of mineralization is localized in the eastern part of the El Garra El Hamra hydrothermally altered syenites. This zone does not relate to the first phase of syenites that appeared on the surface but is linked to the other, hidden phase of the syenitic body. Additionally, the fluids responsible for the mineralization of this zone belong to the late-stage hydrothermal event as well as the fault action due to the fact that it had continued reactivations for a long period of time. The studied mineralized zone also contains ceriopyrochlore-(Ce),(Ce,Ca,)2 (Nb,Ta)2O6 (OH), fluorite (CaF2) and barite (BaSO4), which are found in considerable amounts, whereas apatite (Ca5(PO4)3F) and chromite (FeCr2O4) are rarely found together with Fe-oxi-hydroxides (hematite and goethite) with a minor amount of jarosite (KFe33+(OH)6(SO4)2). Finally, the main constituent mineral in the zone is the pseudomorph perthite in addition to quartz, which is found as deformed crystals. A technological sample was collected from this zone and was found to assay 4.62 weight percent thorium (Th), 0.37 weight percent uranium (U) and 3.12 weight percent rare earth elements (REEs). From the chemical and mineralogical composition of the studied technological sample, the leachabilities of Th, U and REEs were investigated using sulfuric acid (H2SO4) agitation leaching and pug leaching methods. The latter was found to be preferred under the conditions of 0.54 ton H2SO4 per ton of altered syenite at 110 °C for two hours.

Kaolinite (Kt) is the most studied mineral, being widely used in the ceramic, pharmaceutical and cellulose industries, in addition to being the main soil mineral in the world. Found in different parts of the planet, it differs in genesis and may be formed as a result of local weathering of the rocks, occurring in the silt fraction; and also due to the mineral’s neogenesis with a predominance of clay-sized particles. The plain of upper Iguaçu river has the largest kaolinitic deposit in the south of Brazil and it’s formation raised doubts if this kaolin was transported or formed in situ due the high organic matter in the alluvial plain. To elucidate the origin of kaolin deposits, we sampled a possible font of the mineral, in the mountains of Serra do Mar and sampled two tubes that reach 4 m depth in the upper Iguaçu plain. We performed textural analysis, organic carbon, X-ray diffraction, Kt crystallography in silt and clay fractions, thermal analysis (TDA/TG) to quantify Kt and Gb in the clay fraction and scanning electron microscopy (SEM) with dispersive energy spectroscopy (EDS). The TDA/TG analysis demonstrated that saprolite has 66% of the kaolinite found in the plain. The XRD analysis shows a significant presence of mica (Mc) in all samples of the silt fraction, both in Serra do Mar and in the plain. In SEM/EDS, crystals with planar growth are observed, and the presence of pseudomorphic Mc-Kt in the silt fraction of all analyzed samples, with emphasis on the tubes sample with the crystal having almost twice the size of that observed in the saprolite sample from Serra do Mar, allowing to infer that the Kt of the silt fraction of the wetland soils were formed on site by the diagenesis of mica particles. The results obtained in this work indicate that the kaolinitic material found in the wetland of the upper Iguaçu plain is the result of weathering processes in the wetland itself, evidenced by the large pseudomorphs found, even greater than those observed in Serra do Mar.

The Messinian lagoonal carbonate–evaporite sequence of Cyrenaica, north‐east Libya, hosts: (i) opaline nodules in gypsiferous microbial‐rich mudstone; (ii) nodules replacing poikilotopic gypsum cementing bioclastic carbonates; and (iii) bedded porcelanite with large lenticular gypsum pseudomorphs intercalated with recrystallized microbial mats. Optical microscopy, scanning electron microscopy, backscattered electron images – secondary electron images, X‐ray fluorescence, X‐ray diffraction and cathodoluminescence techniques were employed to elucidate on the early stages of chert diagenesis and demonstrate the role of moganite in the formation of varieties of chalcedony during silicification. The opaline nodules composed of opal‐A, exhibit shrinkage cracks lined by isopachous fibrous quartzine underlain by a thin birefringent zone. This suggests that quartzine may develop by crystallization and conversion of opal‐A through a transitional moganite. The quartzine is followed by amorphous silica gel that converts into length‐fast chalcedony. By contrast, lutecite evolved by silica replacement of gypsum through a transitional moganite phase, which forms an amorphous zone in apparent optical continuity with the fibrous lutecite; as moganite advances by pseudo‐crystalline terminations controlled by the gypsum cleavage. In gypsified oyster shells, intercrystalline organic matter enveloping pseudomorphed shell microstructures enabled moganite nucleation that merges syntaxially around the gypsum crystal rim. The associated lutecite occurs as radial‐fibrous beekite aggregates that are consistent with early shell silicification. The bedded porcelanite consists of opal‐CT groundmass undergoing recrystallization and enclosing micro‐cavities lined by quartzine. The associated pseudomorphs display phases of gypsum replacement by moganite and lutecite at the outer margin, followed by dissolution and cavity filling by quartz varieties in response to fluid flushing. In all cases, organic matter and microbes played a vital role in the silicification process.

The Curiosity rover investigated a topographic structure known as Vera Rubin ridge, associated with a hematite signature in orbital spectra. There, Curiosity encountered mudstones interpreted as lacustrine deposits, conformably overlying the 300 m‐thick underlying sedimentary rocks of the Murray formation at the base of Mount Sharp. While the presence of hematite (α‐Fe2O3) was confirmed in‐situ by both Mastcam and ChemCam spectral observations and by the CheMin instrument, neither ChemCam nor APXS observed any significant increase in FeOT (total iron oxide) abundances compared to the rest of the Murray formation. Instead, Curiosity discovered dark‐toned diagenetic features displaying anomalously high FeOT abundances, commonly observed in association with light‐toned Ca‐sulfate veins but also as crystal pseudomorphs in the host rock. These iron‐rich diagenetic features are predominantly observed in “grey” outcrops on the upper part of the ridge, which lack the telltale ferric signature of other Vera Rubin ridge outcrops. Their composition is consistent with anhydrous Fe‐oxide, as the enrichment in iron is not associated with enrichment in any other elements, nor with detections of volatiles. The lack of ferric absorption features in the ChemCam reflectance spectra and the hexagonal crystalline structure associated with dark‐toned crystals points toward coarse “grey” hematite. In addition, the host rock adjacent to these features appears bleached and show low‐FeOT content as well as depletion in Mn, indicating mobilization of these redox‐sensitive elements during diagenesis. Thus, groundwater fluid circulations could account for the remobilization of iron and recrystallization as crystalline hematite during diagenesis on Vera Rubin ridge.

This database of Phanerozoic occurrences and isotopic characteristics of metastable cold-water calcium carbonate hexahydrate (ikaite; CaCO3⚫6H2O) and their associated carbonate pseudomorphs (glendonites) has been compiled from academic publications, explanatory notes, and reports. Our database including more than 700 occurrences reveals that glendonites characterize cold-water environments, although their distribution is highly irregular in space and time. A significant body of evidence suggests that glendonite occurrences are restricted mainly to cold-water settings; however they do not occur during every glaciation or cooling event of the Phanerozoic. While Quaternary glendonites and ikaites have been described from all major ocean basins, older occurrences have a patchy distribution, which may suggest poor preservation potential of both carbonate concretions and older sediments. The data file described in this paper is available on Zenodo at https://doi.org/10.5281/zenodo.4386335 (Rogov et al., 2020).

The differentiation process of ultrapotassic magmas is enigmatic and poorly understood. The Yaojiazhuang ultrapotassic complex is concentrically zoned by late-intruded syenite in the core and early emplaced clinopyroxenite in the periphery, combining a “bi-modal” lithology. Spatially, apatite and iron oxide-apatite (IOA) ores, glimmerite and pseudoleucite occur in the upper part of clinopyroxenite. The syenite and clinopyroxenite are composed of variable amounts of clinopyroxenite, biotite, K-feldspar, magnetite, apatite with minor analcite, titanite, and primary calcite. The pseudoleucite clinopyroxenite contains mainly clinopyroxene, biotite and garnet in the matrix, and nepheline–K-feldspar intergrowth with muscovite and minor celestine in the leucite pseudomorph. Geochemically, rocks of the Yaojiazhuang complex are significantly enriched in potassium (K), light rare earth elements (LREE), and large ion lithophile elements (LILE). Crustal contamination by Archean tonalite–trondhjemite–granodiorite (TTG) gneisses basement may play an important role to convert the syenitic melts from silica-undersaturation to saturation. Fractionation crystallization is supported by the mineral crystallization sequence to explain the bimodal lithologies instead of silicate liquid immiscibility. During the magmatic evolution, decompression, fractionation of volatile-poor clinopyroxene and the enhancement by CO2 may result in the exsolution of an aqueous fluid. The late-stage interactions between existing minerals and magmatic fluids in the crystal mush could be a key process in the generation of both leucite pseudomorphs and apatite/IOA ores.

Whether ooids are formed through biotic or abiotic progress seems to become increasingly controversial in recent decades. New studies indicate that the microbial effect could make important contributions to the construction, destruction and modification of ooids. Previous studies mainly focused on the organomineralization of carbonate minerals in ooids. This paper investigates the formation process of iron minerals in red oolitic grainstones of the Lower Jurassic Nieniexiongla Formation in the Tethyan Himalaya of southern Tibet. Petrographic studies and electron probe microanalysis revealed that the colorful minerals present in red ooids are hematites. Sulfur was detected in the hematites, which indicates that they were transformed from precursor pyrites. These hematites retain euhedral or framboid shapes of the pyrites. Hematite pseudomorphs distribute randomly in ooids and could not form a whole concentric layer of the ooids. Euhedral crystals usually have larger diameters than amorphous spheres and are scattered mostly in nuclei. Based on the distribution and morphology of hematites, we speculate that precursor pyrites were formed through metabolism of sulfate-reducing bacteria. Moreover, the timing of the pyrite formation was after the deposition but before the lithification. The decomposition of the microorganisms within the ooids by bacterial sulfate-reducing provides the HS-, the degradation of organic matters would produce spaces for the intrusion of pore water with sufficient Fe2+ to facilitate the pyrite deposition. In nuclei, the framboid aggregates of pyrites were transferred to euhedral crystals through continuous growth of the constituent microcrystals. Our study demonstrated organomineralization of sulfate-reducing bacteria mediated pyrites in carbonate ooids.