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<jats:title>Abstract</jats:title> <jats:p>This study provides a comprehensive Raman spectral characterization of nontronite and glauconitenontronite mixed-layer phases from seafloor hydrothermal fields. These 2:1 phyllosilicates, which show isomorphous cation exchange between Mg2++Fe2+ and Fe3++Al3+ in the dioctahedral sheets, exhibit three diagnostic Raman peaks in the low wavenumber region (v1 ~241–257 cm−1; v2 ~600–606 cm−1; v3 ~690 cm−1), and one peak at ~3548–3570 cm−1 (v4). With increasing (Mg2++Fe2+)oct, the presumed stretching band of octahedral OH-O bonds (v1) is displaced to a higher wavenumber, whereas the stretching band of tetrahedral Si-O-Si bonds (v2) is shifted to a lower wavenumber. Peak v4, which relates to O-H bonds of hydroxyls linked to octahedral cations, shows a downshift with increasing (Mg2++Fe2+)oct. The band v4 can be mathematically fitted by three bands, two of which strongly correlate with the cation occupancy in the octahedral sheets; i.e., vibrations of hydroxyls linked to triva-lent cations (Fe3+ and Al3+) are mainly represented by a band at ~3560–3573 cm−1, whereas divalent cations (Mg2+ and Fe2+) mainly contribute to a band at ~3538–3540 cm−1. This result is consistent with theoretical considerations for dioctahedral phyllosilicates, which predict for the incorporation of Mg2+ and Fe2+ a weakening/lengthening of O-H bonds in the OH groups, accounting for a downshift of the O-H vibrations. Hence, this is one of the first studies that trace how even subtle chemical modifications in phyllosilicates influence Raman spectral features. The reported findings have implications for mineral identification and provenance analysis, such as during surface exploration on Mars, where compositionally diverse phyllosilicates occur.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Saponite occurs in a wide range of environments from hydrothermal systems on the Earth to surface deposits on Mars. Of practical importance is that Mg-saponite forms when glasses for nuclear waste are altered in Mg-bearing aqueous solutions. In addition, saponite is favorably considered as candidate buffer material for the disposal of high-level nuclear waste and spent nuclear fuel in harsh environments. However, the thermodynamic properties, especially for Mg-saponites, are not well known. Here the author synthesized Mg-saponite (with nitrate cancrinite) following a previously reported procedure and performed solubility experiments at 80 °C to quantify the thermodynamic stability of this tri-octahedral smectite in the presence of nitrate cancrinite. Then, in combination with the equilibrium constant at 80 °C for the dissolution reaction of nitrate cancrinite from the literature, the author determined the solubility constant of saponite at 80 °C based on the solution chemistry for the equilibrium between saponite and nitrate cancrinite, approaching equilibrium from the direction of supersaturation, with an equilibrium constant of –69.24 ± 2.08 (2σ) for dissolution of saponite at 80 °C. Furthermore, the author extrapolated the equilibrium constant at 80 °C to other temperatures (i.e., 50, 60, 70, 90, and 100 °C) using the one-term isocoulombic method. These equilibrium constants are expected to have applications in numerous fields. For instance, according to the extrapolated solubility constant of saponite at 50 and 90 °C, the author calculated the saturation indexes with regard to saponite for the solution chemistry from glass corrosion experiments at 50 and 90 °C from the literature. The results are in close agreement with the experimental data. This example demonstrates that the equilibrium constants determined in this study can be used for reliable modeling of the solution chemistry of glass corrosion experiments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Troctolites are increasingly recognized as a common rock found in association with oceanic core complexes. They are similar to komatiite in composition, and hence troctolite alteration may provide insight into H2 production on Early Earth. We investigated the hydrothermal alteration of olivine-rich troctolites in two batch experiments (300 °C, and 400 °C – 40 MPa) by reacting forsteritic olivine and anorthite-rich plagioclase with salt solutions. The alteration process was evaluated based on concomitant fluid samples and solids retrieved upon the termination of the experiments. In both experiments, the initial rock powder was turned into a hard, compact mass through cementation by secondary phases. The heterogeneity of this mass was documented using µ-computed tomography and electron microscopy. Thermodynamic computations were conducted to determine the equilibrium phase assemblages and fluid compositions with increasing reaction turnover.</jats:p> <jats:p>Mineral zonation developed between the fast-reacting, fluid-dominated top portion of the solids and the more isolated portions at the bottom of the reaction cell. At 300 °C, the total reaction turnover after 1800 h was 77.5%. Serpentinization of olivine controlled the fluid composition after plagioclase had reacted away in the top layers. In contrast, a Ca- and Al-enriched assemblage of xonotlite and chlorite developed alongside unreacted plagioclase at the bottom. The porosity is very low in the top layers but high (around 15%) in the bottom part of the cemented mass. At 400 °C, the reaction turnover was only 51% as olivine was stable after plagioclase had reacted away. Clinopyroxene and andradite ± chlorite had formed in the top layers, whereas xonotlite, grossular, and chlorite had formed at the bottom. The permeability is more uniform and the mineral zonation less pronounced at 400 °C. These mineral zonations developed as a consequence of increased mobility of Ca, Al, Mg, and to a lesser extent of Fe in the experiment, which may be facilitated in the highly permeable granular materials when compared to a compact rock. Steady-state hydrogen concentrations were at least 20 mmol L−1 at 300 °C and &amp;lt;1 mmol L−1 at 400 °C. A lack of magnetite formation at the higher temperature is responsible for the low-H2 yields.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gold deposits are often the result of complex mineralization and remobilization processes. Interpretation of bulk geochemical and sulfur isotope data of the gold deposits is frequently hampered by complex zoning in pyrite, which calls for in situ determination of geochemical and sulfur isotope composition of sulfide minerals. The Qiucun deposit is a good representative of epithermal gold deposits in the Mesozoic Coastal Volcanic Belt of southeastern China. It represents a complex mineralization history, comprising three hydrothermal stages: (I) early stage of pyrite-quartz-chalcedony; (II) main ore stage of quartz-polymetallic sulfide; and (III) post-ore stage of quartz-carbonate. Detailed backscattered electron imaging (BSE) and in situ trace element and sulfur isotope analyses using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and LA-multicollector (MC)-ICP-MS were applied to reveal the gold mineralization and remobilization history of this deposit. Four texturally distinct generations of pyrite were identified, all of which host invisible gold: Py1a and Py1b in Stage I and Py2a and Py2b in Stage II. A detailed study of the texture, chemistry, and sulfur isotopic composition as well as hydrothermal evolution of auriferous pyrite from the Qiucun deposit revealed the behavior of gold in the course of pyrite evolution. Pyrite of Stages I and II contains invisible gold, whereas later-stage visible native gold and re-enrichment in invisible gold is associated with alteration rims around the primary pyrite grains. Py1a is rich in silicate inclusions, enriched in Co and Ni, and depleted in As and Au relative to later pyrite generations. This redistribution is attributed to the alteration of biotite in the sub-volcanic host rocks that effectively destabilized gold in the ore fluid during Py1a deposition. Py1b and Py2a show oscillatory zoning with bright bands having elevated As and Au contents. The oscillatory zoning is interpreted to reflect pressure fluctuations and repeated local fluid boiling around the pyrite crystals. These three pyrite generations (Py1a, Py1b, Py2a) record a narrow range of δ34SV-CDT values between –3.6 and 4.6‰, consistent with a magmatic sulfur source. Gold and some trace elements (As, Ag, Sb, Pb, Tl, and Cu) that were initially incorporated into Py2a became partially exsolved and remobilized during the replacement of porous and invisible gold-rich Py2b. This replacement was likely due to coupled dissolution and re-precipitation reactions triggered by oxidation of the mineralizing fluids. Fluid oxidation is further supported by a general decrease trend of δ34SV-CDT from Py2a (–3.2 to 4.6‰) to Py2b (–15.2 to –2.3‰). Last, previously formed auriferous pyrite underwent post-mineralization fracturing, causing local pulverization of pyrite. Thus, newly created porosity facilitated fluid circulation, hydrothermal alteration of the pyrite, and remobilization of invisible gold, which re-precipitated with pyrite in the form of electrum as small inclusions or as larger grains within fractures. Our study emphasizes that pressure-driven hydrothermal processes play a vital role in the initial enrichment and re-concentration of gold and some other trace metals during episodic deposition, replacement, and hydrothermal alteration of gold-bearing pyrite in epithermal gold deposits, ultimately forming visible gold and high-grade ore shoots as exemplified by the Qiucun deposit.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The origin of celadonite still remains enigmatic and fragmentary. Exceptional celadonite mineralization was discovered in the Miocene lacustrine Janggi Basin in the southeastern Korean Peninsula. This Janggi celadonite is a greenish, earthy/vitreous material filling east-west trending fault zones in basaltic flows. The scale of the celadonite body is up to a meter thick and laterally extends ~10 m. These occurrences are markedly in contrast with celadonite as vesicle-filling or mineral-replacing types in the literature. The Janggi celadonite allows exploring the puzzling genesis of celadonite and comparing its characteristics with global cases for a better understanding of celadonite formation.</jats:p> <jats:p>X-ray diffraction and microprobe analyses demonstrate that the Janggi celadonite ranges from ferroceladonite through celadonite to ferroaluminoceladonite and is mixed with opal at a ratio of up to ~3:7. Detailed fieldwork and whole-rock major, trace, and oxygen isotope analyses indicate that celadonite is formed in an open system at ~120 °C by the interaction of hybridized fluid (a mixture of &amp;lt;55% magmatic and &amp;gt;45% other origins) and basalts during the physicochemical fault brecciation of the host rock. The cations needed for celadonite formation were supplied from the smectitization/zeolitization of rhyolitic mesostasis (for Al and part of K) and pyroxene microlites (for Fe and Mg) in the basaltic breccias during the associated oxidation of micro-nanoparticles by circulating fluids (for most of K).</jats:p> <jats:p>A comparison of the Janggi celadonite with global cases highlights that celadonite genesis is neither limited to the seawater alteration of basalt nor do hosts and reactive fluids control celadonite compositions. A contextualized perspective on celadonite genesis alludes that a potassic alteration of rock that is rich in ferromagnesian components in a shallow crustal environment (&amp;lt;~200 MPa at &amp;lt;~450 °C) produces celadonite. Because of the relative availability of the necessary components for celadonite precipitation, our model predicts celadonite mineralization in many volcanic environments, where magmatic fluid and particle size reduction could contribute. These insights emphasize celadonite’s potential applications for tracing geothermal history.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gungerite, TlAs5Sb4S13, is a new mineral from the Vorontsovskoye gold deposit in Northern Urals. It occurs in limestone breccias composed of calcite and dolomite and cemented by orpiment, pyrite, realgar, stibnite, and minor baryte and quartz. It belongs to the latest phases among sulfosalts (chiefly Tl-As-Sb ones) present in the ore. The empirical formula (based on the sum of all atoms = 23 pfu) is Tl0.99As5.29Sb3.77S12.95. The Raman spectrum exhibits bands corresponding to As-S and Sb-S stretching vibrations, and a band at 263 cm−1 that is assigned to As–As stretching vibrations. Gungerite is bright orange with an orange streak, greasy luster, and perfect cleavage on {010}. It is translucent in thin fragments. The calculated density is 4.173 g/cm3. In reflected light, the mineral is yellowish-white with very weak bireflectance. In crossed polars, it is distinctly anisotropic but anisotropy effects are masked by strong internal reflections of bright orange color. Gungerite is orthorhombic, with the space group Pbcn. Unit-cell parameters determined from the single-crystal X-ray diffraction data are as follows: a = 20.1958(3) Å, b = 11.5258(2) Å, c = 20.1430(2) Å, and V = 4688.74(12) Å3 (Z = 8). The crystal structure consists of doughnut-shaped (As,Sb)-S clusters, which have van der Waals contacts to most of the surroundings, and are connected to them only by sparse cation-sulfur bonds. These clusters are formed by a chelating mirror-symmetrical group, which is “stacked” on, around, and along rods of the TlS9 coordination polyhedra; these rods are oriented parallel to [010]. An individual doughnut-shaped cluster with a central TlS9 polyhedron half-inserted into it contains one As–As bond 2.449 Å long. The polar Tl rods form a chessboard arrangement with occasional stacking errors leading to twinning on (101). The large and complex structure of gungerite shows remote similarities to that of gillulyite and the rod-like structure of lorándite.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nitscheite (IMA2020-078), (NH4)2[(UO2)2(SO4)3(H2O)2]·3H2O, is a new mineral species from the Green Lizard mine, Red Canyon, San Juan County, Utah, U.S.A. It is a secondary phase found in association with chinleite-(Y), gypsum, pyrite, and Co-rich rietveldite. Nitscheite occurs in subparallel and divergent intergrowths of yellow prisms, up to about 0.3 mm in length. Crystals are elongated on [101] and exhibit the forms {100}, {010}, {001}, and {111}. The mineral is transparent with vitreous luster and very pale-yellow streak. It exhibits bright green fluorescence under a 405 nm laser. The Mohs hardness is ~2. The mineral has brittle tenacity, curved fracture, and one good cleavage on {010}. The measured density is 3.30(2) g·cm−3. The mineral is easily soluble in H2O at room temperature. The mineral is optically biaxial (–), α = 1.560(2), β = 1.582(2), γ = 1.583(2) (white light); 2Vmeas = 17(1)°; no dispersion; orientation X = b, Z ≈ [101]; pleochroism X colorless, Y and Z yellow; X &amp;lt; Y ≈ Z. Electron microprobe analysis provided the empirical formula (NH4)1.99U2.00S3.00O21H10.01. Nitscheite is monoclinic, P21/n, a = 17.3982(4), b = 12.8552(3), c = 17.4054(12) Å, β = 96.649(7)°, V = 3866.7(3) Å3, and Z = 8. The structure (R1 = 0.0329 for 4547 I &amp;gt; 3σI reflections) contains [(UO2)2(SO4)3(H2O)2]2− uranyl-sulfate sheets, which are unique among minerals, with NH4 and H2O groups between the sheets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Protocaseyite, [Al4(OH)6(H2O)12][V10O28]·8H2O, is a new mineral (IMA2020-090) occurring in low-temperature, post-mining, secondary mineral assemblages at the Burro mine, Slick Rock district, San Miguel County, Colorado, U.S.A. Crystals of protocaseyite are saffron-yellow, thick blades, with pale orange-yellow streak, vitreous luster, brittle tenacity, curved fracture, two very good cleavages, a Mohs hardness of 2, and a density of 2.45(2) g/cm3. The optical properties of protocaseyite could be only partly determined: biaxial with α = 1.755(5), β &amp;lt; 1.80, γ &amp;gt; 1.80 (white light); pleochroic with X and Y yellow, Z orange (X ≈ Y &amp;lt; Z). Electron-probe microanalysis and crystal-structure solution and refinement provided the empirical formula [(Al3.89Mg0.11Ca0.02)Σ4.02(OH)6(H2O)12][H0.06V10O28]·8H2O. Protocaseyite is triclinic, P1, a = 9.435(2), b = 10.742(3), c = 11.205(3) Å, α = 75.395(7), β = 71.057(10), γ = 81.286(6)°, V = 1036.4(5) Å3, and Z = 1. The crystal structure (R1 = 0.026 for 4032 Io&amp;gt;2 σI reflections) contains both the [V10O28]6− decavanadate polyoxoanion and a novel [Al4(OH)6(H2O)12]6+ polyoxocation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>From their formation, fission tracks are complex structures, onto which their thermal histories come to be imprinted. Track etching leaves elongated voids whose lengths and orientations are used for reconstructing these histories. It is thus important to understand etching for interpreting track data. We revive an existing dissolution model that explains the geometries and dimensions of etched fission tracks in apatite. It implies that on continued etching, the track contours come to reflect the minimum and maximum apatite etch rates, at the same time that all trace of the track structure is erased. We cannot derive valid etch rates from the dimensions of the track openings or from the length increase of step-etched confined tracks. The roundedness of the track tips is not a measure of etching progress. Understanding the contours of confined tracks does permit, in most cases, to calculate their true etch times. We propose to exploit this fact to set an etch-time window and to model the confined-track data in this interval. The excluded measurements will be those of the least-etched and most-etched tracks. This numerical loss is offset by the fact that an etch-time window relaxes the requirement of a fixed immersion time, and a longer immersion multiplies the measurable confined tracks. This calls for no changes to existing procedures if the etch-time windows for different protocols give consistent results. The length data for apatites with different compositions could become comparable if their etch-time windows were linked to a compositional parameter.</jats:p>