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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.

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

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

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.

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 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.

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 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).

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.

This contribution deals with new geology, petrography, and bulk-rock/mineral chemistry of the poorly studied ultramafics of Wadi Ibib–Wadi Shani (WI–WS) district, South Eastern Desert, Egypt. These ultramafics are dismembered ophiolitic rocks that can be subdivided into serpentinites and serpentinized peridotites. Primary minerals such as olivine and pyroxene are absent in serpentinites, but relics of them occur in serpentinized peridotites. Pseudomorph after olivine is indicated by common hourglass textures with less mesh, whilst schistose bastites reflect a pyroxene pseudomorph. Chromite can be subdivided into Cr-spinel and Al-spinel. Cr-spinel ranges from chromite to magnesochromite in composition, whereas Al-spinel belongs to the spinel field. Cr-spinel includes YCr (Cr/(Cr+Al+Fe+3), YAl (Al/(Al+Cr+Fe+3), and YFe+3 (Fe+3/(Fe+3+Al+Cr), similar to forearc peridotite, whilst Al-spinel is more similar to abyssal peridotite, and may be formed during inanition of subduction processes in proto forearc environments. The main secondary minerals are tremolite, talc, and chlorite—which is subdivided into pycnochlorite and diabantite—and their temperature ranges from 174 to 224 °C. The examined rocks had undergone high partial melting degrees (&gt, 25%), as indicated by the Cr# of their unaltered cores (Cr-spinel, &gt, 0.6), whole rocks (Al2O3, SiO2, CaO, and MgO), trace and REEs, depleted Na2O, Al2O3, and Cr2O3 of clinopyroxene, and high forsterite content ((Fo = 100 Mg/Mg + Fe), av. 95.23 mol%), consistent with forearc settings.

Glendonites are carbonate pseudomorphs after ikaite which use as indicators of near-freezing temperatures. We describe the first glendonites of Middle–Late Jurassic to Early Cretaceous age from the Barents Shelf, including the first glendonites of Late Barremian–Early Aptian age ever discovered on Earth and provide the detailed mineralogical-geochemical and isotopic studies. Mesozoic glendonites are well-known across the Arctic realm, however the ikaite-calcite transformation and implication of glendonite findings are still debatable. Pseudomorphs are mainly composed of calcite. Cathodoluminescence and scanning electron microscopy studies reveal three types (I-III) of calcite: type I - elongated, rounded and irregular calcite; type II - blocky, fibrous and needle-like calcite/siderite; type III - blocky calcite in pores. Elemental analyses distinguish geochemical differences between the calcite types: type I comprises pure CaCO3; type II calcite contains Mg and P (in all studied samples), Fe (Bajocian–Callovian), S (Middle Volgian) or Sr (Upper Barremian–Lower Aptian); type III calcite is depleted in Mg, P and Fe. Concentrations of Mn, Fe, Sr and calculated Mn/Ca, Fe/Ca and Sr/Ca ratios are suggestive of diagenetic alteration. Bulk δ18O values range from −5.39 to −1.71‰ VPDB, supporting overprinting of primary values during burial diagenesis, while bulk δ13C values range from −33.3 to −22.6‰ VPDB, providing the influence of organic matter on ikaite-glendonite formation. However, 87Sr/86Sr values range from 0.7070 to 0.7110, mainly corresponding to Jurassic–Cretaceous seawater. Rare-earth elements characterize porewater REE-patterns, with slight middle REE-enrichment and depletion in light and heavy REE. Weak positive and negative Ce anomalies indicate fluctiating from anoxic to oxic conditions during ikaite-glendonite formation. Our new biostratigraphic data refine the ages of glendonite-bearing horizons in the Barents Shelf region, allowing a more definitive correlation with coeval glendonite occurrences across the Arctic realm and with global climatic changes during the Jurassic–Cretaceous.

En el presente trabajo se estudian los procesos de carbonatación de frústulas de diatomeas y la consecuente formación de calizas/dolomías diagenéticas, en estratos de diatomitas marinas (Messiniense, sección de Níjar) y lacustres (Turoliense, sección de Tresjuncos). Se realiza un estudio mineralógico y petrológico para conocer el reemplazo de ópalo A por calcita o dolomita, considerando los factores que favorecen este reemplazo en ambos ambientes. Los datos obtenidos son relevantes para la investigación del uso de diatomitas en la captura y alma­cenamiento de dióxido de carbono a través de su carbonatación. Los procesos pseudomórficos de carbonatación de las valvas se llevaron a cabo mediante el reemplazo directo del ópalo A por calcita o dolomita y la cementación de la microporosidad. La perfecta reproducción de las valvas indica un mecanismo de reacción con acoplamiento de disolución-precipitación en la interfase de reacción. Se interpreta que la organización de las microesferas de ópalo A, dentro de las valvas, condicionaría la organización de complejos amor­fos de varios iones o nanopartículas de carbonatos amorfos, que posteriormente recristalizarían a calcita o dolomita. En las diatomitas lacustres, el proceso de carbonatación de las valvas fue favorecido por la materia orgánica proveniente de la gran cantidad de biota (anfibios, crustáceos, insectos, plantas... etc.) que incorporan. En este ambiente, los procesos de sulfato-reducción durante la degradación de la materia orgánica, generaron CO2, que junto con cambios de pH y salinidad facilitaron los reemplazos. Es posible que las carbonataciones de las valvas fueran coetáneas a la precipitación de calcita y dolomita que fosiliza la biota y por lo tanto, diagenéticas tempra­nas. En las diatomitas marinas, el proceso desencadenante del reemplazo de las valvas fue la formación de capas lenticulares y nódulos opalinos, ya que al disolverse los microfósiles calcáreos que las diatomitas incluían, se liberó CO2 y Ca. Pequeñas variaciones de pH en un entorno alrededor de 9, y la presencia de sulfatos facilitarían el reemplazado durante la diagénesis por enterramiento.

Glendonites have been found worldwide in marine sediments from the Neoproterozoic Era to the Quaternary Period. The precursor of glendonite, ikaite (CaCO3 · 6H2O), is metastable and has only been observed in nature at temperatures 7 °C. Here we investigate the nucleation range for ikaite as a function of temperature and pH. We found that ikaite precipitated at temperatures of at least 35 °C at pH 9.3 −10.3 from a mixture of natural seawater and sodium carbonate rich solution. At pH 9.3, we observed pseudomorphic replacement of ikaite by porous calcite during the duration of the experiment (c. 5 hours). These results imply that ikaite can form at relatively high temperatures but will then be rapidly replaced by a calcite pseudomorph. This finding challenges the use of glendonites as paleotemperature indicators.

Eudialyte-hosted critical metal deposits potentially represent major sources of rare earth elements (REE), zirconium and niobium. Here, we study the chemical and isotopic composition of fresh and altered eudialyte in nepheline syenite from the Il&iacute, maussaq Complex, Greenland, one of the world&rsquo, s largest known eudialyte-hosted deposits. Late-magmatic hydrothermal alteration caused partial replacement of primary magmatic eudialyte by complex pseudomorph assemblages of secondary Zr-, Nb-, and REE-minerals. Three secondary assemblage types are characterised by the zirconosilicates catapleiite, gittinsite and zircon, respectively, of which the catapleiite type is most common. To investigate elemental exchange associated with alteration and to constrain the nature of the metasomatic fluids, we compare trace elements and Sm/Nd isotope compositions of unaltered eudialyte crystals and their replaced counterparts from five syenite samples (three catapleiite-type, one gittinsite-type, and one zircon-type assemblage). Trace element budgets for the catapleiite-type pseudomorphs indicate a 15&ndash, 30% loss of REE, Ta, Nb, Zr, Sr and Y relative to fresh eudialyte. Moreover, the gittinsite- and zircon-type assemblages record preferential heavy REE (HREE) depletion (&le, 50%), suggesting that the metasomatic fluids mobilised high field strength elements. Initial Nd isotope ratios of unaltered eudialyte and catapleiite- and gittinsite-type pseudomorphs are indistinguishable, confirming a magmatic fluid origin. However, a higher initial ratio and stronger HREE depletion in the zircon-type pseudomorphs suggests a different source for the zircon-forming fluid. Although alteration reduces the metal budget of the original eudialyte volume, we infer that these elements re-precipitate nearby in the same rock. Alteration, therefore, might have little effect on overall grade but preferentially separates heavy and light REE into different phases. Targeted processing of the alteration products may access individual rare earth families (heavy vs. light) and other metals (Zr, Nb, Ta) more effectively than processing the fresh rock.

Skarn alteration occurs over a wide range of temperatures and geological settings, and hydrothermal magnetite (a common mineral in skarn) represents a potentially useful temperature and tectonic indicator. Here we describe a new 2D/3D quantitative element mapping approach to evaluate the element mobility of magnetite during skarn alteration/mineralization. Magnetite grains from the giant Beiya Fe-Au skarn deposit (Yunnan, SW China) are either coarse-grained euhedral (hexagonal) with core-rim zoning (type I) or subhedral unzoned (type II) coexisting with (some enclosing) garnet. Concentrations of Na, Mg, Al, Si, Ca, Sr, Cr and Mn in the type I magnetite core and rims are drastically different, which are high in the core and outer rim and low in the inner rim. This suggests fluid geochemical fluctuations, probably led by multiple phases of ore fluid incursions that are common in many world-class skarn deposits. Concentrations of Mg, Al, Si, Ca, Mn and Sr in the garnet are higher than those in type II magnetite. This is likely because the earlier garnet crystallization (during the prograde skarn alteration) had depleted these elements in the later magnetite ore fluids. Both the garnet and the magnetite core are characterized by high Ti concentration. The high Ti concentration in the garnet is governed by the crystal structure, while that of Ti in hydrothermal magnetite is governed by Ti mobility. The 3D-mapping and simulation modeling have shown clear element variation patterns in some magnetite grains that were previously determined to be compositionally homogeneous by spot analysis. Our new 2D/3D-mapping have revealed the presence of mineral inclusions such as garnet and calcite as constrained by the trace element mobility during the replacement of garnet by magnetite, as shown in the euhedral zoned magnetite in garnet pseudomorph.

Calcite pseudomorphs after aragonite were recorded throughout the high-pressure marbles of the Basal Unit (Evia Island, Greece). The pseudomorphs form a columnar microstructure oriented normal to the foliation, which developed at peak metamorphic conditions within the stability field of aragonite. Modification of the columnar microstructure by bending of the long axis of the calcite pseudomorph and dynamic recrystallization, was mainly dependent on the intensity of subsequent, exhumation-related deformation. We used the degree of preservation or modification of the columnar microstructure observed in shear zones, to distinguish between the subduction- and exhumation-related nature of the studied shear zones. The degree of modification of the columnar microstructure was also used as a tool to investigate variations in strain intensity, recrystallization mechanism and kinematics during the exhumation of the high-pressure marbles. Combined field and microstructural data suggest that subduction-related ductile deformation preceded the growth of the columnar microstructure and was associated with internal imbrication by SE-directed thrusting. The ductile stage of the exhumation was characterized by strain localization at the middle and upper structural levels of the Basal Unit, and E-directed thrusting (i.e., northeast in pre-Neogene configuration). Final exhumation of the Basal Unit was achieved by top-to-the-E ductile to brittle thrusting, which was accompanied by syn-orogenic extension at the upper crustal levels.

A new preparation method for the synthesis of TiO2 microrods in aqueous media starting with solid hydrated titanyl sulfate crystals with defined morphology is presented. The method is based on the extraction of sulfate ions from the crystals and their replacement with hydroxyl groups in aqueous ammonia solution leaving the Ti–O framework intact. The particle size and morphology of the starting hydrated titanyl sulfate is closely preserved in the pseudomorphs of amorphous metatitanic acid including such details like the layered structure of the original hydrated titanyl sulfate crystals. When annealed up to 1200 °C, the rod-like morphology of particles is retained, while the phase composition changes to anatase/rutile. The rod-like particles of metatitanic acid possess excellent sorption properties toward radionuclides. The mechanism of pseudomorph formation is discussed based on the structures of the precursors, including the hitherto unknown structure of titanyl sulfate dihydrate determined by electron d...

This is the fourth paper presenting new mosaic-like mineralogical data from the Carpathian–Pannonian region. Data are arranged by countries and localities. Every section gives a description (including XRPD, EMPA and SEM-EDX results) of the minerals and a concise description of their parageneses. Every discussed mineral is first described from the given locality and in many times even from the whole region.From Hungary the following minerals are reported: freieslebenite and beaverite-(Cu) from the Rudabánya ore deposit, planerite–aheylite–faustite–turquoise solid-solution members from the Parádfürdő ore deposit.From Romania the following minerals are identified: conichalcite (with high Pb content, conichalcite–duftite solid solution), duftite and mottramite–duftite solid-solutions from the Băiţa Bihor ore deposit, pseudomalachite pseudomorph after azurite as well as vauquelinite from the Ocna de Fier ore deposit.From Slovakia the following minerals are described: axinite-(Fe) from the Maglovec diorite-porphyrite quarry, bultfonteinite from the Vechec andesite quarry and botallackite from the Dobšiná ore deposit.

Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.

Wilhelmgümbelite, ideally [ZnFe2+Fe33+(PO4)3(OH)4(H2O)5]·2H2O, is a new secondary phosphate mineral related closely to schoonerite, [ZnMnFe22+Fe3+(PO4)3(OH)2(H2O)7]·2H2O, from oxidized zones of the Hagendorf-Süd pegmatite, Hagendorf, Oberpfalz, Bavaria, Germany. Wilhelmgümbelite occurs as radiating sprays of needle-like rectangular laths, up to 0.2 mm long and with colour varying from light yellow brown to orange red. Cleavage is perfect parallel to {010}. The mineral is associated closely with an oxidized pseudomorph of phosphophyllite, recently named steinmetzite. Other associated minerals are albite, apatite, chalcophanite, jahnsite, mitridatite, muscovite and quartz. The calculated density of wilhelmgümbelite is 2.82 g cm–3. It is optically biaxial (+) with α = 1.560(2), β = 1.669(2), γ = 1.718(2), 2V(meas) = 63(1)° and 2V(calc.) = 65°. Dispersion is weak with r > v, orientation X = b, Y = c, Z = a. Pleochroism is weak, with coloursZ = orange brown, Y = yellow brown, X = light yellow brown, Z >> Y > X. Electron microprobe analyses (average of seven analyses, seven crystals) with H2O and FeO/Fe2O3 calculated on structural grounds, gave FeO 5.8, Fe2O3 25.0, MnO 2.6, ZnO 16.4, P2O5 28.7, H2O 23.4, total 101.9 wt.%. The empirical formula, scaled to 3 P and OH– adjusted for charge balance is Zn1.50Mn0.272+Fe0.602+Fe2.333+(PO4)3·(OH)2.73(H2O)8.27. The structural formula is [Zn(Mn0.27Fe0.733+)∑1.0(Zn0.25Fe0.152+Fe0.603+)∑1.0(Zn0.25Fe0.452+)∑0.7Fe3+(PO4)3(OH,H2O)9]·2H2O.Wilhelmgümbelite has orthorhombic symmetry, Pmab, Z = 4, with the unit-cell parameters of a = 10.987(7) Å, b = 25.378(13) Å, c = 6.387(6) Å and V = 1781(2) Å3. The strongest lines in the powder X-ray diffraction pattern are [dobs in Å(Iobs) (hkl)] 12.65 (100) (020); 8.339 (5) (120); 6.421 (14) (001); 6.228 (8) (011); 4.223 (30) (120) and 2.111 (7) (0 12 0). Wilhelmgümbelite is an oxidized form of schoonerite, with the Mn2+ replaced principally by Fe3+. Its structure differs from that of schoonerite in having the Zn partitioned between two different sites, one five-coordinated as in schoonerite and the other tetrahedrally coordinated. Wilhelmgümbelite also differs structurally from schoonerite in having partial occupation of one of the Fe sites, which appears to be correlated with the Zn partitioning.

Cerium oxalate decahydrate was subjected to a thermal decomposition to obtain nanocrystalline ceria. It was shown that conditions during dehydration strongly influence morphology of the final product. If one performs dehydration of the precursor under a vacuum or in air, CeO 2 can be obtained in pseudomorph, i.e. the precursor’s crystal shape is retained. An increase of water vapor pressure leads to the formation of different intermediates and fragmentation of the precursor’s crystals. Investigation by various physical methods (TEM, XRD, and BET) provided the information that the final product consisted of CeO 2 particles 5–6 nm in size.

We report a facile preparation approach of MAPbBr(3), MAPbCl(3), or FAPbBr(3) (where MA = CH3NH3+ and FA = CH(NH2)(2)(+)) perovskite nanowires via sequential synthesis of MAPbI(3) and FAPbI(3) nanowires with chemically controlled composition and morphologies followed by an exchange of halide anions. The nanowires formation sequence includes intermediate phases such as MAI-PbI2-DMF and FAIPbIrDMF (DMF = dimethylformamide) acting as structure directing agent. The 1D shape of the adduct is preserved during the conversion to perovskite. The adducts play the role of key precursors controlling the final product morphology. Systematic investigations of the observed phase transformations and morphology features on multiple length scales revealed the effectiveness of the suggested synthetic route utilizing an original pseudomorph formation mechanism of the 1D structures to produce partly oriented films and textured layers of the nanowires via only a few experimental steps.

Orthopyroxene-oxide symplectites after olivine are among the most enigmatic features of corona assemblages in metagabbros. Two coronitic metagabbro bodies from the Algonquin suite in the Grenville Orogen, Ontario, contain exceptionally well preserved orthopyroxene + Fe–Ti oxide symplectite formed during prograde Ottawan (ca. 1060 Ma) granulite-facies metamorphism. Based on textural evidence, we propose a new hypothesis for the formation of these symplectites. Under oxidising conditions associated with fluid infiltration, magmatic olivine and ilmenite underwent a coupled reaction whereby magnetite produced by oxidation of olivine replaced adjacent igneous ilmenite. Ilmenite was re-precipitated as a fine-grained intergrowth with orthopyroxene and some magnetite in the former olivine sites. This hypothesis is supported by textural evidence showing partial replacement of magmatic ilmenite by magnetite and a close spatial association between magmatic oxides and orthopyroxene + Fe–Ti oxide symplectite, which locally radiates from ilmenite into olivine. Measured orthopyroxene/oxide ratios in the symplectite (20–35% oxides) agree with the ratio predicted from the proposed reaction (ca. 30%). Coronas and pseudomorphs formed during high-grade metamorphism, with increasing fO2 interpreted to result from fluid infiltration at near-peak conditions of ca. 13 kbar, 800 °C. The same samples contain red-brown fine-grained aggregates interpreted as iddingsite pseudomorphs after olivine. Raman spectroscopy suggests that the iddingsite consists largely of amorphous silica and Fe-hydroxide; textural evidence indicates that it formed by late-stage oxidation and hydration of olivine that survived earlier metamorphism. The unusual co-occurrence of granulite-facies pseudomorphs after olivine with an alteration product formed at near-surface conditions indicates that some olivine may survive protracted high-grade metamorphism in environments where fluid access is limited.

The extraordinary lateral continuity of isopachous stromatolite laminae in the ∼87 m-thick Mesoproterozoic Lakshanhatti Dolomite (India) evinces chemical precipitation. Fan-shaped crystals grown on lamina surfaces further corroborate this contention; growth of fan-shaped crystals under the overhanging stromatolite column-margin indicates direct carbonate precipitation from ambient waters. The fan-shaped crystals are stacked up, separated only by thin dark micritic laminae. In a relatively upper stratigraphic interval of the formation, lighter laminae characterized by a clotted texture and traversed by numerous winding tubular voids change gradually upwards into dark micritic laminae. Some sporadically distributed lenticular intraclastic beds also have the similarly dark micritic coatings. Clear carbonate cement crusts also occur between laminae and between successive dark micritic coats around intraclasts. Dull cathodoluminescence (CL) characterizes this cement as well as the cement lining within early diagenetic voids. In contrast, the laminae with clotted textures show dirty orange luminescence, while the dark micritic laminae and the dark micritic grain-coats display clear bright orange luminescence. Pyrite and its pseudomorphs are preferably concentrated along the dark micritic laminae. Carbon content in these dark micritic components, whether laminae or coats, is much higher than in the lighter components, exceeding what can be accounted for their CaMg(CO3)2 composition. A large part of this carbon is kerogen, plausibly biogenic. The dark components are, therefore, reasonably, though not unequivocally, assumed to be microbial mats. Degradation of the mats might have given rise to the light laminae with clotted textures. The fuzzy-margin tubes within the light laminae probably manifest the escape of gases generated during organic matter decomposition. Si–Al-rich terrigenous fines thinly draping the dark carbonaceous laminae was possibly the result of baffling and trapping of terrigenous fines by filamentous microbiota. Dark carbonaceous laminae encasing intraclasts was considered to be the result of binding and stabilization by microbiota. Spike-like growth of discrete laminae strongly suggests an occasional breakdown of colonial homeostasis of phototrophic microbiota. The microbial community thus appears to have played an active role in stromatolite-building in the Lakshanhatti Dolomite Member, even though the simultaneous existence of direct carbonate precipitates from sea water indicates a hybrid origin of these stromatolites. Resting on shelf sandstone and being capped by dark offshore shale, the Lakshanhatti Dolomite had been deposited in distal offshore, but not at the great depth, perhaps in an epeiric sea. Progressive deepening inhibited direct carbonate precipitation. δ13C and δ18O values suggest normal open marine salinity during deposition.

Titanite textures and chemistry have been investigated from the Roxby Downs Granite, host to the Olympic Dam Cu–U–Au–Ag deposit, South Australia. Three textural subtypes of titanite are documented: primary magmatic (cores and rims); deuteric; and hydrothermal (low T recrystallisation). Magmatic cores are defined by enrichment in LREE (~ 3 wt%), Nb (up to 1 wt%) and Zr relative to rims, which typically contain

The Tremadocian (Early Ordovician) is currently considered a time span of greenhouse conditions with tropical water surface temperature estimates, interpolated from oxygen isotopes, approaching 40 °C. In the mid-latitude Baltoscandian Basin, conodonts displaying low δ O values, which suggest high temperatures (>40 °C) in the water column, are in contrast with the discovery of contemporaneous glendonite clusters, a pseudomorph of ikaite (CaCO ·6H O) traditionally considered as indicator of near-freezing bottom-water conditions. The massive precipitation of this temperature sensitive mineral is associated with transgressive conditions and high organic productivity. As a result, the lower Tremadocian sediments of Baltoscandia apparently contain both “greenhouse” pelagic signals and near-freezing substrate indicators. This paradox points to other primary controlling mechanisms for ikaite precipitation in kerogenous substrates, such as carbonate alkalinity, pH and Mg/Ca ratios, as recently constrained by laboratory experiments. Preservation of “hot” conodonts embedded in kerogenous shales rich in δ O-depleted glendonites suggests both the onset of sharp thermal stratification patterns in a semi-closed basin and the assumed influence of isotopically depleted freshwater yielded by fluvial systems.

Ikaite, found as a constituent of tufa chimneys and mounds in Ikka Fjord, Greenland, is only formed in waters close to freezing point. At higher temperatures it inverts to calcite, forming impressive pseudomorphs which have been found at a large number of locations world-wide of varying ages, sometimes in association with glacial deposits. The Ikka Fjord deposits, first described by the Danish geologist Hans Pauly, were key to understanding the nature of these widely reported pseudomorphs.

Ikaite (CaCO3·6H2O) is a metastable phase that crystallizes in nature from alkaline waters with high phosphate concentrations at temperatures close to 0 °C. This mineral transforms into anhydrous calcium carbonate polymorphs when temperatures rise or when exposed to atmospheric conditions. During the transformation in some cases the shape of the original ikaite crystal is preserved as a pseudomorph. Pseudomorphs after ikaite are considered as a valuable paleoclimatic indicator. In this work we conducted ikaite crystal growth experiments at near-freezing temperatures using the single diffusion silica gel technique, prepared with a natural aqueous solution from the polymictic lake Laguna Potrok Aike (51°57′S, 70°23′W) in Patagonia, Argentina. The ikaite crystals were recovered from the gels and the transformation reactions were monitored by in situ Raman spectroscopy at two different temperatures. The first spectra collected showed the characteristic features of ikaite. In successive spectra new bands at 1072, 1081 and 1086 cm−1 and changes in the intensity of bands corresponding to the OH modes were observed. These changes in the Raman spectra were interpreted as corresponding to intermediate stages of the transformation of ikaite into calcite and/or vaterite. After a few hours, the characteristics of the Raman spectrum were consistent with those of calcite. While ikaite directly transforms into calcite at 10 °C in contact with air, at 20 °C this transformation involves the formation of intermediate, metastable vaterite. During the whole process the external shape of ikaite crystals was preserved. Therefore, this transformation showed the typical characteristics of a pseudomorphic mineral replacement, involving the generation of a large amount of porosity to account for the large difference in molar volumes between ikaite and calcite. A mechanism involving the coupled dissolution of ikaite and crystallization of calcite/vaterite is proposed for this replacement., Deutsche Forschungsgemeinschaft (DFG MA 4235/4-1 to C.M.), Ministerio de Economía y Competitividad, Spain (Project CGL2013-47988-C2-1-P)

The new mineral zincomenite, β-ZnSeO3, is found in active fumaroles belonging to the Northern fumarole field at the First scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with sofiite, sellaite, fluorite, halite, anhydrite, cotunnite, chubarovite, flinteite, etc . Zincomenite occurs as tabular, equant or prismatic crystals up to 0.2 mm sometimes combined in clusters up to 0.3 mm across or interrupted incrustations up to 0.7 × 1 cm overgrowing basalt scoria. Partial to complete pseudomorphs of zincomenite after sofiite are typical. The crystal forms are {101}, {010}, {100} and {013}. T-shaped twins with (012) as twin plane were found. Zincomenite is transparent, colourless, white or pale beige with adamantine lustre. Cleavage and parting were not observed and the fracture is uneven. D calc is 4.760 g cm−3. Zincomenite is optically biaxial (−), α = 1.744(5), β = 1.860(5), γ = 1.875(5). The IR spectrum is reported. The chemical composition (wt%, electron microprobe data) is: ZnO 42.53, SeO2 56.67, total 99.20. The empirical formula calculated on the basis of 3 O apfu is: Zn1.02Se0.99O3. Zincomenite is orthorhombic, Pbca, a = 7.1971(2), b = 6.2320(2), с = 11.9914(3) A, V = 537.84(2) A3 and Z = 8. The strongest reflections of the powder X-ray diffraction pattern [ d ,A( I )( hkl )] are: 4.612(26)(102), 3.601 (77)(200), 3.119(48)(210), 3.048(38)(113), 3.014(100)(211, 021), 2.996(56)(004), 2.771(19)(123, 104, 212), 2.459(23)(213, 023), 2.311(20)(123, 221, 204), and 2.162(19)(214, 024). Zincomenite is a representative of the CuSeO3 structure type. Its crystal structure, solved from single-crystal X-ray diffraction data ( R = 0.0188), contains layers formed by Zn2O8 dimers (consisting of edge-sharing ZnO5 trigonal bipyramids; each dimer shares four vertices with the neighbouring ones) linked via (SeO3)2− groups (with Se4+ in trigonal pyramidal coordination) to form an open framework. The mineral is named in allusion to its chemical composition: zinc selenite (the Greek μηναζ means moon , indicating selenium).

The mineral name “maufite” was proposed in 1930 to describe a “bright emerald green” nickeliferous vein material occurring within a serpentinite of the Great Dyke in the Umvukwe Range of Zimbabwe, then Southern Rhodesia. Study of this material shows that it is not a distinct species but rather an interstratified nickel-bearing lizardite-clinochlore, with lizardite dominant over clinochlore. The name “maufite” is discredited. The lizardite is a Group-A polytype, and the clinochlore is a Ia polytype in the nomenclature developed by S.W. Bailey. The formula calculated on the basis of 14 negative charges is (Mg1.74Al0.88Ni0.13□0.25)Σ3.00(Si1.60Al0.40)Σ2.00O5(OH)4; on the basis of nine oxygen atoms including 14.6 wt.% H2O determined thermogravimetrically, the formula is (Mg1.68Al0.78 Ni0.13□0.41)Σ3.00(Si1.55Al0.45)Σ2.00O4.51(OH)4.49. Analytical electron microscopy shows that the composition is variable on a fine scale. The randomly interstratified lizardite-clinochlore occurs as a pseudomorph after amphibole and plagioclase and is a product of serpentinization. The material is poorly crystalline and very fine grained, producing broad reflections on powder diffraction and microbeam X-ray diffraction patterns. Unlike most of the interstratified serpentine-chlorite described in the literature, lizardite is here dominant over clinochlore.

The Sierra de Cacheuta vein-type Se mineralization in the Mendoza Province predominantly consists of clausthalite, klockmannite, eskebornite, eucairite, and naumannite. These primary selenides formed in a fault zone, cutting through fine-grained trachytic host rock. Cross-sections perpendicular to the veinlets, polarized light microscopy, and scanning-electron microscopy, combined with electron-microprobe analysis, provide a record of the relationship between different crystallization and deformation events. Mineralization encompasses four episodes of fault formation (d1–d4): early zonal selenide crystallization (stage (I)); ductile deformation of the selenides (stage (II)); fault re-opening, fluid-mediated metal mobilization, metalliferous-fluid infiltration, and mineral precipitation (stage (III)); and subsequent alteration (stage (IV)). The Se vein originated from multiple injections of highly oxidized, metal-rich fluids. These low-T solutions (estimated max. temperature 100 °C, max. pressure 1 bar) possessed high to exceptionally high Se fugacities (log fSe2 between −14.5 and −11.2) that prevailed for most of the evolution of the deposit. The source of the Se and the accompanying metals (Cu, Ag, Pb, and Fe) is probably the neighboring bituminous shale. The deposition of Se minerals occurred when the oxidized metal-bearing solutions came in contact with a reductant, which caused the reduction of mobile selenate to immobile selenide or elemental Se. We identified several features that permit us to safely distinguish samples from Cacheuta from Argentinian Se deposits in the Province of La Rioja: (I) trachytic host rock fragments containing bitumen and TiO2 pseudomorphs after titanomagnetite; (II) early Co-rich and Ni-poor krut’aite (Co < 6.7 wt %, Ni < 1.2 wt %) partly replaced by clausthalite, umangite, klockmannite, eskebornite, Ni-poor tyrrellite (Ni < 2.7 wt %), Ni-poor trogtalite (Ni < 1.2 wt %), and end-member krut’aite and petříčekite; (III) lack of calcite gangue; and (VI) Se-bearing alteration minerals comprising chalcomenite, molybdomenite, cobaltomenite, an unnamed Cu selenide (for which the ideal formula may be either Cu2Se3 or Cu5Se8), and possibly mandarinoite, mereheadite, orlandiite, and scotlandite as new species for this occurrence.

The study of the mechanisms by which mineral fibres promote adverse effects in both animals and humans is a hot topic of multidisciplinary research with many aspects that still need to be elucidated. Besides length and diameter, a key parameter that determines the toxicity/pathogenicity of a fibre is biopersistence, one component of which is biodurability. In this paper, biodurability of mineral fibres of social and economic importance (chrysotile, amphibole asbestos and fibrous erionite) has been determined for the first time in a systematic comparative way from in vitro acellular dissolution experiments. Dissolution was possible using the Gamble solution as simulated lung fluid (pH = 4 and at body temperature) so to reproduce the macrophage phagolysosome environment. The investigated mineral fibres display very different dissolution rates. For a 0.25 μm thick fibre, the calculated dissolution time of chrysotile is in the range 94–177 days, very short if compared to that of amphibole fibres (49–245 years), and fibrous erionite (181 years). Diffraction and SEM data on the dissolution products evidence that chrysotile rapidly undergoes amorphization with the formation of a nanophasic silica-rich fibrous metastable pseudomorph as first dissolution step whereas amphibole asbestos and fibrous erionite show minor signs of dissolution even after 9–12 months.

Crystallographic methods are used to identify the primary mineral phase of pseudomorphs of crystals embedded in 3.48 Ga bedded carbonate-chert rocks from the Dresser Formation, Pilbara Craton, Australia. This identification provides valuable information on the chemical environments at the onset of life on Earth., Crystallography has a long history of providing knowledge and methods for applications in other disciplines. The identification of minerals using X-ray diffraction is one of the most important contributions of crystallography to earth sciences. However, when the crystal itself has been dissolved, replaced or deeply modified during the geological history of the rocks, diffraction information is not available. Instead, the morphology of the crystal cast provides the only crystallographic information on the original mineral phase and the environment of crystal growth. This article reports an investigation of crystal pseudomorphs and crystal casts found in a carbonate-chert facies from the 3.48 Ga-old Dresser Formation (Pilbara Craton, Australia), considered to host some of the oldest remnants of life. A combination of X-ray microtomography, energy-dispersive X-ray spectroscopy and crystallographic methods has been used to reveal the original phases of these Archean pseudomorphs. It is found with a high degree of confidence that the original crystals forming in Archean times were hollow aragonite, the high-temperature polymorphs of calcium carbonate, rather than other possible alternatives such as gypsum (CaSO4·2H20) and nahcolite (NaHCO3). The methodology used is described in detail.

The Triassic Saima alkaline complex in the Liaodong Peninsula of northeastern China covers an area of about 20 km 2 and is dominated by nepheline syenite, with phonolite at its center, and a concealed body of eudialyte-bearing nepheline syenite in the northwest of the complex. The phonolite has similar features to miaskite, while the nepheline syenites are classified in the agpaitic group according to their mineral assemblage, and the alkalinity and aluminum saturation indexes. Zircon is the dominant Zr-bearing mineral in the phonolite, whereas wadeite occurs as the only primary Zr-bearing mineral in the nepheline syenites. The transitional crystallization from zircon to wadeite reveals an increase in alkalis and a high K/Na ratio as the magmas evolved from the volcanic to the intrusive stage. The primary wadeite grains underwent varying degrees of hydrothermal alteration. Overall, the areas of weak, medium, and strong alteration are characterized by the following respective associations: (1) wadeite + secondary catapleiite/gaidonnayite, (2) wadeite + secondary catapleiite/gaidonnayite + zircon, and (3) pseudomorphs after wadeite. The pseudomorphs are widespread and mainly consist of residual wadeite, secondary zircon, catapleiite/gaidonnayite, K-feldspar, calcite, and some Zr-bearing titanite and vesuvianite. All of the secondary zircon grains in the three associations are typically enriched in Ca and Al compared with the primary Ca-free zircons of the phonolite. The progressive alteration of wadeite suggests that the Saima complex underwent multiple episodes of fluid activity during a hydrothermal stage, including an initial Na-metasomatism via alkaline fluids, then stages most likely involving progressively CO 2 -rich fluids, and an intensive episode involving a mixture of these fluids with externally derived Ca-rich fluids.

Glendonites are pseudomorphs of the mineral ikaite (CaCO3·6H2O) after loss of hydration water and occur in distinctive euhedral crystalline forms, sometimes clustered as rosettes of up to tens of centimeters in diameter. While it is generally accepted that organic-rich environments, methane seeps, and high phosphate levels are important for ikaite formation, glendonite occurrences in ancient sedimentary sequences are widely considered to reflect near-freezing temperatures, even at high latitudes during periods of greenhouse climates. To fully understand the paleoenvironmental significance of glendonites, a comprehensive examination of the modern ikaite setting is necessary. Temperature is the most important parameter that has been quantitatively constrained for the presence of ikaite. Low bottom-water temperature, while a required condition for formation of the mineral, is not adequate for its growth; other controls are necessary to explain the absence of ikaite in many cold environments. In this ...

Research has shown that mullite in brick from the Kazan kremlin crystallizes in the form of primary (‘flaky’) mullite, pseudomorphs of glass with needle-shaped mullite in terms of feldspar, and pseudomorphs of glass with short-prismatic mullite. Mossbauer absorption spectra showed that in the brick studied mainly hematite was found on the surface and magnetite in the interior volume.

Iron-rich carbonates and the oxidized remains of former carbonates (iron-oxide concretions) underlie bleached Navajo Sandstone over large portions of southern Utah. Iron in the carbonates came from hematite rims on sand grains in the upper Navajo that were dissolved when small quantities of methane accumulated beneath the sealing Carmel Formation. As a second buoyant gas (CO 2 derived from Oligocene–Miocene magmas ) reached the seal and migrated up dip, it dissolved in the underlying water, enhancing the solution9s density. This water carried the released ferrous iron and the methane downward. Carbonates precipitated when the descending, reducing water degassed along fractures. The distribution of a broad array of iron-rich features made recognition of the extent of the ancient flow systems possible. Although siderite is not preserved, dense, rhombic, mm-scale, iron-oxide pseudomorphs after ferrous carbonates are common. Distinctive patterns of iron oxide were also produced when large (cm-scale), poikilotopic carbonate crystals with multiple iron-rich zones dissolved in oxidizing waters. Rhombic pseudomorphs are found in the central cores of small spheroids and large (meter-scale), irregular concretions that are defined by thick, tightly cemented rinds of iron-oxide–cemented sandstone. The internal structure and distribution of these features reveal their origins as iron-carbonate concretions that formed within a large-scale flow system that was altered dramatically during Neogene uplift of the Colorado Plateau. With rise of the Plateau, the iron-carbonate concretions passed upward from reducing formation water to shallow, oxidizing groundwater flowing parallel to modern drainages. Finally they passed into the vadose zone. Absolute dating of different portions of these widespread concretions could thus reveal uplift rates for a large portion of the Plateau. Iron-rich masses in other sedimentary rocks may reveal flow systems with similar histories.

International audience; Sediment-hosted stratiform ore deposits that underwent metamorphism and orogenesis are interpreted either to have a multiphase origin that might in part predate these events or to be exclusively related to fluid migration during the orogeny. This controversy concerns the formation of many world-class sediment-hosted ore deposits such as in the Central African Copperbelt. Here we present Re-Os dating results for disseminated and stratiform Cu-Co sulfide pseudomorphs after anhydrite in nodules and layers from the Copperbelt. Results demonstrate that at least some of the ores formed at around 800 Ma during rifting and basin development, and thus predate the Pan-African Lufilian orogeny. Younger Re-Os ages, i.e., between 682 +/- 28 and 230 +/- 36 Ma, may be due to the replacement of the early Cu-Co sulfides by younger sulfide phases. Alternatively, these younger ages may result from the disturbance or resetting of the Re-Os system during recrystallization/metamorphism or the transition of chalcocite from its high- to its low-temperature polymorph.

Neoproterozoic cap carbonates are characterized by “anomalous” sedimentary features, including sea-floor precipitates, represented by aragonite pseudomorph cement crusts and crystal fans. These features are found in restricted periods of geological history, and their generation in the context of the Neoproterozoic has been attributed to very specific postglacial conditions leading to alkalinity oversaturation. New sedimentary and geochemical results on well-preserved Neoproterozoic deposits of aragonite pseudomorph crystal fans at the base of the Sete Lagoas Formation (Bambui Group, central Brazil) are reported. The micrite-settling facies shows the highest abundance of crystal fans, arranged in centimeter-scale layers interfingered with the micrite matrix; this facies is interpreted as a result of postglacial calcium carbonate oversaturation in restricted areas characterized by marine dysoxic waters. A numerical model constrains the kinetics of formation of aragonite crystals or micrite by calculating the induction times and precipitation rates of both carbonate species for postglacial conditions of elevated temperature and high atmospheric pCO2. In these conditions, the Mg/Ca ratio of the Neoproterozoic seawater is calculated at ca. 1.2. Alkalinity oversaturation is monitored by a variable evaporation degree. Aragonite formation is kinetically favored, and the discrepancy between calcite and aragonite precipitation rate is greater as atmospheric pCO2, hence alkalinity, is higher. These modeling conditions do not preclude the contribution of incomplete organic-matter degradation to alkalinity as suggested by negative carbon isotope signatures. In addition, the large number of aragonite–micrite pairs suggests a seasonal or paleoclimatic forcing. Other cases of abiotic aragonite precipitation through time are also briefly examined by comparison.

Amorphous calcium carbonate (ACC) is a key precursor of crystalline CaCO3 biominerals and biomimetic materials. Despite recent extensive research, its formation and amorphous-to-crystalline transformation are not, however, fully understood. Here we show that hydrated ACC nanoparticles form after spinodal liquid–liquid phase separation and transform via dissolution/(re)precipitation into poorly hydrated and anhydrous ACC nanoparticles that aggregate, forming a range of 1D, 2D and 3D structures. The formation of these structures appears to be achieved by oriented attachment (OA), facilitated by the calcite medium-range order of ACC nanoparticles. Both electron irradiation processes in the TEM and under humid air exposure at room temperature of the latter ACC structures result in pseudomorphs of single crystalline mesostructured calcite. While the high-vacuum/e-beam heating leads to solid-state transformation, the transformation in air occurs via an interface-coupled dissolution/precipitation mechanism. Our results differ significantly from the currently accepted model, which considers that the low T ACC-to-calcite transformation in air and during biomineralization is a solid-state process. These results may help to better understand how calcite biominerals form after ACC and offer the possibility of biomimetically preparing single crystalline calcite structures after ACC by tuning pH2O at room temperature.

In the Jabal Samran area (western Saudi Arabia), secondary copper mineralization in a NE-trending shear zone in which the arc metavolcanic host rocks (dacite–rhyodacite) show conjugate fractures and extensive hydrothermal alteration and bleaching. The zones contain frequent Fe–Mn(III) oxyhydroxides (FeOH–MnOH) that resulted from oxidation of pyrite and Mn-bearing silicates. In the bleached part, the groundmass is represented by Fe-bearing interstratified illite–smectite with up to 4.02 wt% FeOt. FeOH–MnOH are pre-weathering phases formed by hydrothermal alteration in a submarine environment prior to uplifting. Five varieties of FeOH are distinguished, four of them are exclusively hydrothermal with ∼20 wt% H2O whereas the fifth contains ∼31–33 wt% H2O and might represent reworking of earlier hydrothermal FeOH phases by weathering. FeOH fills thin fractures in the form of veinlets and crenulated laminae or as a pseudomorph for pyrite, goethite and finally ferrihydrite, and this oxyhydroxide is characterized by positive correlation of Fe2O3 with SiO2 and Al2O3. On the other hand, MOH shows positive correlation between MnO2 and Al2O3 whereas it is negative between Fe2O3 and SiO2. Paratacamite is the most common secondary copper mineral that fills fractures and post-dates FeOH and MnOH. It is believed that Cl− in the structure of paratacamite represents inherited marine storage rather than from surfacial evaporates or meteoric water. The mineralogy of slags suggests a complicated mineral assemblage that includes native Cu prills, synthetic spinifixed Mn-rich amphiboles with 16.73 wt% MnO, brown glass and Ca–Mn–Fe phase close to the olivine structure. EMPA indicate that the some Cu prills have either grey discontinuous boarder zone of S-rich Mn–Cu alloy (with up to 21.95 wt% S and 19.45 wt% Mn) or grey Cu–Mn–Fe alloy (with up to 15.9 wt% Cu, 39. 12 wt% Mn and 61.64 wt% Fe). Mn in the Cu prills is expelled inward as Cu–Mn–Fe alloy inclusions whereas S is expelled outward as S-rich Mn–Cu alloy crust. Remains in the Samran smelter sites suggest the use of charcoal as a source of energy, quartzite as a flux and an air-cooling technique was used.

Pseudomorphs are formed along the envelopes of whole and destroyed cells as a result of a high rate of fossilization in microbial ecosystems. The destroyed cells may be mistaken for chemogenic formations of colloidal compounds, in particular, aqueous silicon dioxide. This paper focuses on microfacies, which are either mineralized biofilms or fragments of fossilized bacterial envelopes, as shown by electron microscopy and energy-dispersive spectrometry. In particular, we establish the biomineral origin of a number of mineral aggregates of specific appearance found in the samples studied: stromatolites and segregations.

In this paper, we present a comparative study of the pseudomorphic carbonatation of gypsum and anhydrite single crystals in Mg-bearing aqueous solutions at room temperature. We have found that carbonatation of gypsum an anhydrite occurs via a similar coupled dissolution–crystallization mechanism. However, whereas pseudomorphization of anhydrite precisely preserves the external form and dimension of the initial crystals, pseudomorphs after gypsum are less perfect and the shape and volume of the original crystals are partially lost. Furthermore, the mineralogical compositions of the pseudomorphs after anhydrite and gypsum are different, because gypsum is replaced by relatively large calcite crystals, while anhydrite pseudomorphs consist of aggregates of calcite and aragonite. Such textural and compositional differences can be clearly related to the length scale of the coupling between dissolution and crystallization at the replacement fronts. In situ atomic force microscopy observations of the early stages of carbonatation have shown that dissolution and crystallization are very tightly coupled on anhydrite rather than on gypsum surfaces. This clearly indicates that during the replacement of anhydrite and gypsum, dissolution and precipitation, respectively, are the rate-limiting steps. On the other hand, the higher supersaturation levels reached during the replacement of anhydrite lead to a competition between calcite and aragonite crystallization. This explains the fact that calcite and aragonite coexist within the replacement layers after anhydrite. Finally, we have found that dissolved magnesium in the reacting solutions is almost equally incorporated into the pseudomorphs, regardless of both the precursor phase and the mineralogical composition of the replacements. Our calculations and chemical analyses have shown that the compositions of the MgxCa1–xCO3 solid solution for which supersaturations reach maximum values correspond quite well with maximum Mg/Ca ratios in the replacement layers., This work was supported by the Spanish Government (MAT2012-38810 and CGL2013-48247-P). AFM and SEM images, and EDX analyses were obtained at the ICTS Centro Nacional de Microscopia Electrónica-UCM. We thank Ana Vicente for assistance with the SEM. GIXRD patterns were recorded at the X-ray Diffraction Central Service-UCM. We also thank Ignacio Carabias for technical support and valuable help in GIXRD interpretation. C.P. is grateful to the Spanish Government for a FPU Fellowship.

Unusual speleothems, associated with hyperalkaline (pH > 12) groundwaters have formed within a shallow, abandoned railway tunnel at Peak Dale, Derbyshire, UK. The hyperalkaline groundwaters are produced by the leaching of a thin layer (3-type groundwaters and degassing of CO2. Stalagmites within the Peak Daletunnel have grown rapidly (averaging 33 mm y–1), following the closure of the tunnel 70 years ago. They have an unusual morphology comprising a central sub-horizontally-laminated column of micro- to nano-crystalline calcium carbonate encompassed by an outer sub-vertical assymetricripple-laminated layer. The stalagmites are composed largely of secondary calcite forming pseudomorphs (3·6H2O), withminor volumes of small (