Description: Wodegongjieite occurs in the Cr-11 chromitite orebody of the Luobusa ophiolite in the Kangjinla district, Tibet, China. It is found in two inclusions in corundum: (1) as a partial overgrowth (holotype) up to 1.5 μm thick around a spheroid 20 μm across of wenjiite (Ti10(Si,P,□)7), kangjinlaite (Ti11(Si,P)10), zhiqinite (TiSi2) and badengzhuite (TiP), and (2) as pools up to 0.25 μm wide filling interstices between wenjiite, jingsuiite (TiB2), osbornite–khamrabaevite (Ti[N,C]) and corundum. Energy dispersive analyses gave Al2O3 34.09, SiO2 49.11, K2O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K0.58Sr0.26Ca2.25Al7.20Si8.80O31.20, ideally KCa3(Al7Si9)O32, for Si + Al = 16 cations. Single-crystal studies were carried out with three-dimensional electron diffraction providing data for an ab initio structure solution in the hexagonal space group P6/mcc (#192) with a = 10.2(2) Å, c = 14.9(3) Å, V = 1340(50) Å3 and Z = 2. Density (calc.) = 2.694 g⋅cm–3. The refinement, which assumes complete Si–Al disorder, gives average T1–O and T2–O bond lengths both as 1.65 Å. It was not practical to use unconstrained refinement for the occupancies of the large cation sites 6f and 2a. The ab initio model shows clearly that the two cation sites have different sizes and coordination. Consequently, we imposed the condition (1) that all the K occupies the 2a site as the average K–O bond length of 3.07 Å is close to the average K–O bond lengths reported in kokchetavite and (2) that all the Ca occupies the 6f site as the average Ca–O bond length of 2.60 Å (2.36 Å and 2.84 Å for Ca–O1 and Ca–O3, respectively) is reasonable for Ca–O. Assuming that all K and all Ca are located at the 2a site and 6f site, respectively, Sr occupancies of these sites could be refined. Thermal parameters are positive and in a reasonable range. The structure is a sheet silicate isostructural with the K-feldspar polymorph kokchetavite, with two crystallographically distinct sites for K, but not with the topologically identical anorthite polymorph dmisteinbergite (CaAl2Si2O8) with only a single site for Ca. Substitution of K by Ca at the 6f site is associated with marked rotation of the Si,Al tetrahedra and a collapse of the structure to accommodate the smaller Ca ion. The spheroid of intermetallic phases is believed to have formed from the interaction of mantle-derived CH4 + H2 fluids with basaltic magmas at depths of ~30–100 km, resulting in precipitation of corundum that entrapped intermetallic melts. Associated immiscible silicate melt of granodioritic composition crystallised metastably to wodegongjieite instead of a mixture of anorthite and K-feldspar.

Description: The replacement of magnetite by hematite is commonly observed in various geologic systems. In contrast to the formation of hematite by a solid-state oxidation, numerous experimental results have demonstrated that it can also occur by a redox-independent dissolution-reprecipitation reaction. However, the orientation relationship and the intermediate products between magnetite and hematite remain unknown during the redox-independent replacement. In this work, high-resolution transmission electron microscopy (TEM) was used to investigate porous hematite from the Baishiya skarn deposit, East Kunlun orogenic belt, aiming to build a refined growth model applicable to the replacement of magnetite by hematite in natural hydrothermal systems. Initially, hydrothermal leaching of Fe2+ from magnetite led to the formation of ferrihydrite that transformed to goethite and hematite nanocrystals successively. Oriented attachment of pseudocubic hematite nanoparticles induced by Cl along specific crystallographic directions formed hematite mesocrystals on the exposed dodecahedral facets of magnetite, leading to an orientation relationship between magnetite and adjacent hematite (i.e., (110)magnetite || (012)hematite), which is different from that of oxidation (i.e., (110)magnetite || (110)hematite). However, oriented attachment can be imperfect in some instances, and dislocations of adjacent nanoparticles result. The dislocations in the hematite mesocrystals have acted as a closed space to capture the remaining ferrihydrite. When the Si concentration in ferrihydrite was sufficient, the solid-state conversion of the remaining ferrihydrite to hematite was blocked. We propose that repeated dissolution and reprecipitation of hematite mesocrystals (i.e., defective crystals) are required to remove Si, and thereby form defect-free hematite crystals, providing a genetic link between the widespread hematitisation and related multistage fluid infiltration in some of the world's richest deposits (e.g., Olympic Dam and Bayan Obo deposits). This is the first time that Si and Cl have been verified to act as key factors to shape the hematite growth pathway during the ore-forming processes, challenging the ‘ion-by-ion’ growth model that has underpinned our knowledge of mineral solubility, nucleation, and mass transfer from nano- to micron-scales in natural hydrothermal systems.

Description: The microbial key players at methane seeps are methanotrophic archaea and sulfate-reducing bacteria. They form spherical aggregates and jointly mediate the sulfate-dependent anaerobic oxidation of methane (SD–AOM: CH4 + SO42– → HCO3– + HS– + H2O), thereby inducing the precipitation of authigenic seep carbonates. While seep carbonates constitute valuable archives for molecular fossils of SD–AOM-mediating microbes, no microfossils have been identified as AOM aggregates to date. We report clustered spherical microstructures engulfed in 13C-depleted aragonite cement (δ13C values as low as –33‰) of Pleistocene seep carbonates. The clusters comprise Mg-calcite spheres between ~5 μm (single spheres) and ~30 μm (clusters) in diameter. Scanning and transmission electron microscopy revealed a porous nanocrystalline fabric in the core area of the spheres surrounded by one or two concentric layers of Mg-calcite crystals. In situ measured sphere δ13C values as low as –42‰ indicate that methane-derived carbon is the dominant carbon source. The size and concentric layering of the spheres resembles mineralized aggregates of natural anaerobic methanotrophic archaea (ANME) of the ANME-2 group surrounded by one or two layers of sulfate-reducing bacteria. Abundant carbonate-bound 13C-depleted lipid biomarkers of archaea and bacteria indicative of the ANME-2-Desulfosarcina/Desulfococcus consortium agree with SD–AOM-mediating microbes as critical agents of carbonate precipitation. Given the morphological resemblance, in concert with negative in situ δ13C values and abundant SD–AOM-diagnostic biomarkers, the clustered spheres likely represent fossils of SD–AOM-mediating microbes.

Description: We report on Precambrian microfossils from igneous rocks of the Volyn pegmatite district, associated with the Paleoproterozoic Korosten pluton, northwestern Ukraine. The fossils were recovered from meter-sized miarolitic cavities and show a well-preserved 3D morphology, mostly filamentous but with a large variety of types and also in irregular, flaky shapes reminiscent of former biofilms, as well as rare spherical objects. Based on literature data, pyrolysis experiments, and reflected light microscopy results, the organic matter (OM) is characterized as (oxy-)kerite. Further investigations with microscopic techniques, including scanning and transmission electron microscopy, and electron microprobe analysis show that fossilization likely occurred during a hydrothermal, post-pegmatitic event by silicification dominantly in the outermost 1–2 µm of the microfossils. The hydrothermal fluid, derived from the pegmatitic environment, was enriched in SiF4, Al, Ca, Na, K, Cl, and S. The OM shows O enrichment in which N and S content is low, indicating simultaneous N and S loss during anaerobic oxidation. Mineralization with Al silicates starts at the rim of the microfossils, continuing in its outer parts into identifiable encrustations and intergrowths of clay minerals, feldspar, Ca sulfate, Ca phosphate, Fe sulfide, and fluorite. Breccias, formed during collapse of some the miarolitic cavities, contain decaying OM, which released high concentrations of dissolved NH4+, responsible for the late-stage formation of tobelite-rich muscovite and buddingtonite. The age of the fossils can be restricted to the time between the pegmatite formation, at ∼1.760 Ga, and the breccia formation at ∼1.49 Ga. As the geological environment for the growth of the microorganisms and fossilization, we assume a geyser system in which the essential biological components C, N, S, and P for growth of the organisms in the miarolitic cavities were derived from microorganisms at the surface. Fossilization was induced by magmatic SiF4-rich fluids. The Volyn occurrence is a distinct and uncommon example of Precambrian fossils, and the results underline the importance of cavities in granitic rocks as a possible habitat for microorganisms preserved in the deep biosphere.

Description: Transmission electron microscopy and 3D focused ion beam/scanning electron microscope nanotomography are applied to grain and phase boundaries between quartz, plagioclase, K-feldspar, clinopyroxene, amphibole, and calcite. The samples come from metamorphic, plutonic and volcanic rocks, and hydrothermal quartz, and experienced cooling and decompression after highly variable P–T peak conditions. Most of the boundaries are partially open in the range of up to several hundred nanometres and partly to totally filled with secondary minerals, such as actinolite, biotite, chlorite, sheet silicates, and quartz, as well as with amorphous matter. Cracking and opening of boundaries are suggested to be related to anisotropic thermoelastic response of crystals to cooling. It starts below the brittle–ductile transition of the involved minerals. The partially open grain and phase boundaries, together with dissolution-generated cavities, can form porosity of more than 2 vol.% and permeability under conditions of at least lowermost greenschist facies. Such networks of partially open or partially refilled boundaries potentially affect properties of crystalline rocks and processes in the upper crust, such as metasomatism, weathering, migration of radionuclides through bedrock of geological repositories of nuclear waste, and deformation in nature and in experiment.

Description: New petrographic and microanalytical studies of mineral inclusions in the Purang ophiolitic chromitites (SW Tibet) are used to scrutinize the evolution of the associated Cretaceous sub-oceanic lithospheric mantle section. Silicate inclusions in the chromite grains include composite and single-phase orthopyroxene, clinopyroxene, amphibole, and uvarovite. Most inclusions are sub-rounded or globular, whereas a few inclusions exhibit cubic/octahedral crystal morphologies. The latter are randomly distributed in the large chromite grains, though discrete aggregates are consistently confined to the grain centers. Abundant micrometer-scale, clinopyroxene inclusions are topotaxially aligned along crystallographic planes. Less-abundant sulfide, wüstite, apatite, and uvarovite inclusions are observed in some samples. The trace element geochemistry of the Purang chromitite evoke parental MORB- and boninite-like melts, consistent with the supra-subduction zone setting. The δ26Mg values of the high-Cr and high-Al chromitites range from –0.25 to –0.29‰ and –0.05 to –0.32‰, respectively. The associated harzburgite has nearly overlapping δ26Mg values of –0.13 to –0.37‰, but pyroxenite sills show distinct δ26Mg values (–0.61 to –0.67‰). The variable Mg isotope signatures, combined with abundant exotic, ultrahigh-pressure and super reduced (UHP-SuR) mineral inclusions in the chromite grains, suggest that recycling and recrystallization under different mantle conditions played an important role in the genesis and evolution of these rocks. Furthermore, discrete silicate, sulfide, and metal alloy inclusions in the Purang chromitites are comparable to those reported in other Tethyan ophiolites, and collectively suggest a common geodynamic evolution.

Description: Five crystals of pegmatitic xenotime from Ås II feldspar quarry (Evje, S Norway) were studied with comprehensive analytical methods from microscale to nanoscale with respect to fluid-mediated alterations and their geochronological implications. Xenotime crystals are strongly altered during dissolution-reprecipitation processes that resulted in the formation of (Th, U)-depleted xenotime subdomains with numerous microinclusions of (Th, U)-silicate, uraninite and minor galena. Remains of a primary xenotime (Xtm1) yielded LA-ICPMS Usingle bondPb data characterized by a reverse discordance (from −10.5 to −2.6%) with a 208Pb/232Th mean age 988.4 ± 5.9 Ma (95% conf., MSWD = 0.75, n = 71), which is within uncertainty with a 207Pb/235U mean age 979.1 ± 5.0 Ma (95% conf., MSWD = 1.4, n = 22) yielded by data filtered to below ±5% discordance (i.e., from −5.0 to −2.6%). The altered xenotime domains (Xtm2) provided highly scattered dates, including 208Pb/232Th dates from 200 ± 14 to 2135 ± 120 Ma (n = 135). Discordant Usingle bondPb data yielded upper intercept age 909 ± 16 Ma (MSWD = 7.9). EPMA Th-U-total Pb measurements and compositional characteristics of uraninite inclusions indicate three age populations of ca. 852–983, 594–687 and 37–101 Ma. TEM investigations revealed initial alterations within domains Xtm1, representing a primary xenotime, which progressed along parallel submicron- to nanoscale fractures. These fractures represent partially open grain boundaries, which are empty or are filled with secondary inclusions of (Th, U)-silicates, uraninite or coffinite. The submicron-sized inclusions are accompanied by subdomains of (Th, U)-depleted xenotime. Altered xenotime (Xtm2) is well crystalline in TEM imaging and electron diffraction patterns in contrast to primary unaltered xenotime domains (Xtm1) that demonstrate, in Raman spectra, moderate degree of radiation damage caused by U and Th decay. Secondary inclusions of Th and U phases are nanocrystalline or amorphous, which increases their potential for Pb-loss or accumulation of Pb in excess. Some of them contain nanoinclusions of Pb3O4 or metallic Pb, whereas 〈100 nm-sized inclusions of (Pb, Sb)-oxide formed in the altered xenotime. To summarize, this study provides important insights for our understanding of coupled dissolution-reprecipitation processes that affect xenotime and mobilization of released U, Th and Pb. Removal of highly mobile U in a fluid, and the presence of nanoinclusions of Pb3O4, (Pb, Sb)-oxide and metallic Pb have particular importance for xenotime dating and explain disturbance towards older ages. Nevertheless, both removal of U, Th and Pb from altered and recrystallized xenotime as well as presence of submicron- to nanoinclusions can result in age disturbance resulting in spread of dates along a concordia curve.

Description: The distribution of Th and U in a crystal of pegmatitic xenotime-(Y) (henceforth named xenotime) from Ås II feldspar quarry (Evje, S Norway) has been studied with transmission electron microscopy (TEM) to determine submicron- to nanoscale processes that might have affected U-(Th-)Pb age record. Xenotime contains high Th and U unaltered domains, and Th and U depleted altered domains, which contain numerous inclusions of (Th, U)-silicate and uraninite. A focused ion beam foil (FIB-foil) prepared from the unaltered xenotime, examined with transmission electron microscopy (TEM), revealed a network of ca. 15–20 nm-thick and up to several hundred nanometers-long dislocations. The dislocation cores contain U, Th and Si enriched material, and are surrounded on both sides by radiation-damaged xenotime. Rarely present “empty” dislocations are indicative of nanoscale-sized porosity in xenotime. The presence of a network of line defects in xenotime indicates plastic deformation of the xenotime crystal, which resulted in an initial stage of the formation of partially open phase boundaries. The dislocations network has been infiltrated by fluid, which mediated coupled dissolution-reprecipitation reactions. Segregation of U and Th into the dislocation cores, and selective transport of highly fluid-mobile U outside of the xenotime is suggested to be a source of Pb-excess and reverse discordance in the 207Pb/235U vs. 206Pb/238U concordia diagrams. The new finding of migration and finally segregation of U and Th into dislocation cores at the atomic-scale demonstrates the importance of nanoscale investigations for better understanding of the obtained age data and provides crucial implications for applications of xenotime geochronology.

Description: Palladium and Pt are present in magmatic sulfide deposits mainly as discrete platinum-group minerals (PGM) closely associated with base metal sulfides (BMS). It is always debated whether these PGM phases are of magmatic or subsolidus origin. The mechanism by which Pt- and Pd-mineral phases form depends on how Pd and Pt are accommodated in magmatic sulfide phases: as cation substitutions or as stable nano Pd- and Pt-ligand particles. To know how Pd and Pt are hosted in magmatic sulfides and how they behave during cooling, we have investigated magmatic monosulfide solid solution (MSS) (quenched from 950 °C) and low-temperature (slowly cooled from 950 to 25 °C) decomposed MSS, both synthesized from PdSb, PdTe2, PdBi2, PtSb2, PtTe2-or PtBi2-saturated CuNiFe-sulfide mixture. Transmission Electron Microscopy (TEM) revealed that at 950 °C, Pd is hosted in MSS as nano Pd-telluride and antimonide melt droplets. Platinum is hosted in MSS as PtTe2 (moncheite), PtS (cooperite) and PtSb2 (geversite) nanocrystals. Moncheite and cooperite nanoparticles are aligned along the (0001) plane, and share one crystallographic plane (hkl) with the host, hexagonal MSS. At 25 °C, the Pd-telluride and antimonide melt droplets crystallized to merenskyite (PdTe2) and Ni-rich sudburyite (Pd(Ni)Sb). The Pt nano phases at 25 °C keep their composition and fabric and show no preference to pyrrhotite and pentlandite. Results imply that Pt and Pd minerals nucleate at magmatic temperature and grow by assembling PGE-ligand nanoparticles, not by exsolution of cationic and anionic metal species from BMS. Results also prove a weak Pd-S chemical affinity at the magmatic stage; Pd atoms are incorporated in MSS and the intermediate solid solution (ISS) when semimetals are not available. During subsolidus transformations of MSS and ISS, Pd preferentially concentrates in pentlandite.

Description: There are seven meteorite craters and several Neogene placers in the Ukrainian Shield containing impact apographitic diamond. In this work impact diamonds from the Bilylivka meteorite crater and from the Samotkan’ Neogene titanium-zirconium placer were studied in detail. The results of a comprehensive study of impact diamond crystals — morphology, microtopography, microstructure, carbon isotope composition, photoluminescence, optical, infrared, and Raman spectroscopy — are presented. The size of the impact diamonds is up to 0.5 mm. Impact diamond crystals are mostly two- or three-phase polycrystalline aggregates (diamond, lonsdaleite, graphite). They show external morphological and internal microstructural features of solid-state phase transition of graphite to diamond during impact shock metamorphism – they are paramorphoses on graphite crystals. Microstructural features of the graphite-diamond transition in the studied crystals of impact diamonds are their polysynthetic (11 2¯¯¯ 1) twinning and the polycrystalline structure of the twins themselves. The carbon isotopic composition of impact diamonds ranges: for Bilylivka diamonds – from –14.80 to –21.84 ‰ δ13C VPDB, with an average value of –17.21 ‰ δ13C and for Samotkan’ diamonds – from –10.35 to –23.06 ‰ δ13C VPDB, with an average value of –17.64 ‰ δ13C. The photo luminescent and spectroscopic features of the studied diamonds indicate the absence of nitrogen defects in crystals that are characteristic for mantle diamond. The location of the source rocks and potential routes how diamond have been incorporated into the Samotkan’ placer are discussed.