Volume 107, Issue 6

Polytypism is a typical feature of layered minerals with differences only in stacking sequences. There is no obvious “phase” boundary among different polytypes, although the frequency of polytypes occurrence is related to its crystallization environment. In the past decades, X‑ray studies of molybdenite specimens from a variety of geological environments have revealed that most molybdenite crystals contain both 2 H 1 (hexagonal) and 3 R (rhombohedral) polytypes. However, the stacking sequences of these molybdenite polytypic intergrowths and their formation mechanism are not well understood. Here, we report stacking faults and domains of long-period polytypes identified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) in a molybdenite sample from a carbonatite vein in the Huanglongpu Mo-Pb ore deposit in Qinling orogenic belt, Northern China. Several layers of disordered domains intergrown with ordered 2 H 1 domain were recognized based on the contrast in HAADF image with one-dimensional lattice fringes. In addition, a 30-layer long-period polytype was unambiguously identified by a STEM image. The stacking sequences of 4-, 6-, and 8-layer disordered domains and the 30-layer long-period polytype were further examined using HRSTEM images at the atomic resolution. A 2 H 3 polytype with three repetitions was also discovered in the sample. We propose that non-equilibrium conditions related to the fluctuation of fluid composition during crystallization resulted in the oscillation of 2 H 1 and 3 R polytypes and intergrowth of various disordered domains. More broadly, our study demonstrates that HAADF-STEM imaging method may be applicable for studying other disordered layered crystals and twinned minerals.

The effect of F and Cl on the liquidus temperature of a hydrous (~3.5–4 wt% H 2 O) trachytic melt (~66 wt% SiO 2 ) at 925 to 990 °C and at 100 MPa has been experimentally investigated. We employed a novel disequilibrium approach involving diffusion couple experiments with the two-diffusion couple end-members differing solely in halogen concentrations. A shift of the liquidus temperature by ~50 °C was observed between a halogen-poor and halogen-enriched melt. Each experiment spanned the entire range of F and Cl concentrations between the two end-member compositions. We determined the halogen concentrations at the transition from crystal-bearing to crystal-free melt. These concentrations correspond to the liquidus halogen concentrations of the melt at each experimental temperature. We demonstrate that there is a limiting halogen concentration (~0.19–0.52 wt% F; ~0.07–0.24 wt% Cl), below which the melt crystallizes spherulitic clinopyroxene during heating to the run temperature. At high temperatures, upon diffusion of F and Cl into the halogen-poor melt, those crystals dissolve, leaving behind a dissolution front parallel to the diffusion interface. We propose that the dissolution is a consequence of F and Cl complexing with some of the main cationic components of clinopyroxene (Mg, Fe, Ca), thereby destabilizing this phase. Thus, the experimental dissolution of clinopyroxene is a manifestation of a liquidus depression caused by increased halogen content. Our results show that the liquidus shifts at a rate of ~1575(379) K/mol% of F and Cl in the melt, which is a minimum estimate, assuming both halogens equally drive dissolution. This liquidus depression is valid for a range of halogen concentrations (~0.06–0.87 wt% F; ~0.06–0.36 wt% Cl) and the experimental temperatures. Our findings illustrate that the degassing of halogens during or prior to an eruption can enhance crystallization in the melt and therefore influence magma physical properties that may ultimately affect eruption style.

We report on the discovery of two high-pressure minerals, tuite and ahrensite, located in two small shock-induced melt pockets (SIMP 1 and 2) in the Zagami martian meteorite, coexisting with granular and acicular stishovite and seifertite. Tuite identified in this study has two formation pathways: decomposition of apatite and transformation of merrillite under high- P-T conditions. Chlorine-bearing products, presumably derived from the decomposition of apatite, are concentrated along the grain boundaries of tuite grains. Nanocrystalline ahrensite in the pyroxene clast in SIMP 2 is likely to be a decomposition product of pigeonite under high- P-T conditions by a solid-state transformation mechanism. The pressure and temperature conditions estimated from the high-pressure minerals in the shock-induced melt pockets are ~12–22 GPa and ~1100–1500 °C, respectively, although previous estimates of peak shock pressure are higher. This discrepancy probably represents the shift of kinetic relative to thermodynamic phase boundaries, in particular the comparatively small region that we examine here, rather than a principal disagreement between the peak shock conditions.

Zolenskyite (IMA 2020-070), FeCr 2 S 4 , is a new sulfide mineral that occurs within troilite, with clinoenstatite and tridymite, in the matrix of the Indarch meteorite, an EH4 enstatite chondrite. The mean chemical composition of zolenskyite determined by electron probe microanalysis, is (wt%) S 43.85, Cr 35.53, Fe 18.94, Mn 0.68, Ca 0.13, total 99.13, yielding an empirical formula of Fe 0.99 Mn 0.04 Ca 0.01 Cr 1.99 S 3.98 . The ideal formula is FeCr 2 S 4 . Electron backscatter diffraction shows that zolenskyite has the C 2/ m CrNb 2 Se 4 -Cr 3 S 4 -type structure of synthetic FeCr 2 S 4 , which has a = 12.84(1) Å, b = 3.44(1) Å, c = 5.94(1) Å, β = 117(1)°, V = 234(6) Å 3 , and Z = 2. The calculated density using the measured composition is 4.09 g/cm 3 . Zolenskyite is a monoclinic polymorph of daubréelite. It may be a high-pressure phase, formed from daubréelite at high pressures (several gigapascals) and moderate temperatures in highly shocked regions of the EH parent asteroid before becoming incorporated into Indarch via impact mixing. Zolenskyite survived moderate annealing of the Indarch whole-rock. The new mineral is named in honor of Michael E. Zolensky, an esteemed cosmochemist and mineralogist at NASA’s Johnson Space Center, for his contributions to research on extraterrestrial materials, including enstatite chondrites.

Li-rich micas are crucial in the exploration for and exploitation of Li resources. The determination of Li in mica using classical bulk chemical methods or in situ microanalytical techniques is expensive and time-consuming and has stringent requirements for the quality of micas and reference materials. Although simple linear and nonlinear empirical equations have been proposed, they are inconsistent with the complex physicochemical mechanisms of Li incorporation and commonly lead to large errors. In this study, we introduce a refined method of multivariate polynomial regression using a machine learning algorithm to estimate Li from multiple major oxide abundances. The performance of our regression model is evaluated using the coefficient of determination ( R 2 ) and the root-mean-square error (RMSE) of the independent test sets. The best-performed models show R 2 of 0.95 and a RMSE of 0.35 wt% for the test set of data set 1 (all compiled data, n = 2124) and R 2 of 0.96 and a RMSE of 0.22 wt% for the test set of data set 2 (only data obtained using in situ techniques, n = 1386). Our results indicate that integration of electron probe microanalysis and multivariate polynomial regression (based on data set 1) presents a robust and convenient approach to quantify Li contents in micas. The application of the proposed approach to micas from central Inner Mongolia, NE China, suggests that in addition to the Weilasituo ore bodies, the Jiabusi granite and greisen and the Shihuiyao metamorphic sediment formation have good potential for Li exploration. Our study also provides preliminary constraints on the genesis of Li deposits.

Picrites, dominantly composed of highly forsteritic olivine, can serve as important constraints on primary magma composition and eruption dynamic processes in global continental flood basalt (CFB) provinces. Picrites are commonly divided into high-Ti and low-Ti groups based on whole-rock TiO 2 content or Ti/Y ratio. Here, we use an artificial neural network (ANN) to classify the individual olivine in picrites from global CFB provinces according to whether their parental magma is high-Ti or low-Ti to better understand the primary origin and magmatic processes. After training the ANN on 1000 olivine major element compositions data points, the network was able to differentiate chemical patterns for high-Ti and low-Ti olivine and classify olivine into correct types with an accuracy of >95%. Moreover, we find that two types of olivine mix in some single samples from Etendeka, Emeishan, and Karoo CFB provinces. Combining the results with chemical markers of source lithology, we suggest that the two types of olivine originate from two different sources and their olivine populations mixed during the ascent. This mixing then makes the spatial and temporal variation of picrites types in some CFB provinces unclear.

We report the first calorimetric observation of the glass transition for a carbonate melt. A carbonate glass [55K 2 CO 3 –45MgCO 3 (molar)] was quenched from 780 °C at 0.1 GPa. The activation energy of structural relaxation close to the glass transition was derived through a series of thermal treatments comprising excursions across the glass transition at different heating rates. Viscosities just above the glass transition temperature were obtained by applying a shift factor to the calorimetric results. These viscosity measurements (in the range of 10 9 Pa·s) at supercooled temperatures (ca. 230 °C) dramatically extend the temperature range of data for carbonates, which were previously restricted to superliquidus viscosities well below 1 Pa·s. Combining our calorimetrically derived results with published alkaline-earth carbonate melt viscosities at high temperatures yields a highly non-Arrhenian viscosity-temperature relationship and confirms that carbonate liquids are “fragile.” Based on simulations, fragile behavior is also exhibited by Na 2 CO 3 melt. In both cases, the fragility presumably relates to the formation of temperature-dependent low-dimensional structures and Vogel-Fulcher-Tammann (VFT) curves adequately describe the viscosity-temperature relationships of carbonate melts below 1000 °C.

A series of experiments on monazites from Victoria, Australia, is presented to further understand their fission track etching properties. Using a 6 M HCl etchant at 90 °C, SEM images on crystal (100) pinacoid faces reveal well-etched rhombic spontaneous fission track openings. Average rhombic etch pit diameters Dpc and Dpb, parallel to the crystallographic c - and b -axes are 0.81 ± 0.20 μm and 0.73 ± 0.26 μm, respectively. An angular distribution experiment on (100) faces found that spontaneous fission tracks initially etch anisotropically, being preferentially revealed at an azimuth of 90° to the crystallographic c -axis up to ~60 min of etching. As etching continues, however, the distribution becomes progressively more uniform and is essentially isotropic by 90 min. Two experimental methods determined the rate at which the etchant penetrated along the lengths of implanted 252 Cf fission tracks. This involved the application of a focused ion beam scanning electron microscope (FIB-SEM) to mill progressively into slightly etched monazite crystals followed by an etch-anneal-etch approach. Results indicate that at least the greater part of the etchable ranges of the latent fission tracks were penetrated by the 6 M HCl etchant within the first few minutes. Continued etching to 5 min indicates that track etching slows down toward the ends of the tracks, but the maximum ranges are estimated to be reached after 5–15 min, which represents the longest time the latent segments of the tracks are exposed to potential annealing at the etchant temperature. Taking into account that implanted 252 Cf fission tracks in monazite anneal on average ~4% of their length at 90 °C after 1 h (Jones et al. 2019), suggests that a much shorter duration for exposure to this temperature causes less than ~1% of fission track length reduction during etching.

The dehydration of serpentine mineral antigorite, Mg 2.8 Si 2 O 5 (OH) 3.6 , is regarded as the key step in metamorphic transformation of ultramafic hydrated rocks in subduction zones, which affects seismicity and feeds volcanic activity. The abundance of alkali-chloride brines derived from deep subduction/upper mantle sources implies the possibility of a large control of the H 2 O activity by the dissolved salts. The present study examines the effect of alkali chlorides, lowering the H 2 O activity in fluid, on antigorite stability at high pressure. The decomposition of natural antigorite (Ural) in the presence of a halite-saturated NaCl-H 2 O fluid was studied up to 3 GPa and 700 °C by in situ X‑ray diffraction combined with resistively heated diamond-anvil cell. Reference experiments were also performed on salt-free sample. At 1.5–3 GPa in the presence of halite-saturated fluid ( X NaCl ≈ 0.15), antigorite decomposes to an intermediate product assemblage of talc+forsterite at about 550 °C, which is ≈150 °C lower compared to salt-free H 2 O-unsaturated system. Such a low-temperature shift supports the previous models of a broadened P-T area of serpentinite dehydration in the subducting slab. In addition, the present experiments reveal active dissolution of the product Mg silicates, first of all forsterite, in the NaCl-H 2 O fluid at 600–700 °C/1.5–3 GPa. This implies that dehydrated serpentinites are a potential source of fluids enriched in MgO and SiO 2 , which play an important role in deep metasomatic processes.

This study provides a comprehensive Raman spectral characterization of nontronite and glauconite-nontronite mixed-layer phases from seafloor hydrothermal fields. These 2:1 phyllosilicates, which show isomorphous cation exchange between Mg 2+ +Fe 2+ and Fe 3+ +Al 3+ 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 (Mg 2+ +Fe 2+ ) 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 (Mg 2+ +Fe 2+ ) 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 trivalent cations (Fe 3+ and Al 3+ ) are mainly represented by a band at ~3560–3573 cm −1 , whereas divalent cations (Mg 2+ and Fe 2+ ) 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 Mg 2+ and Fe 2+ 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.

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.

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 H 2 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. 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 <1 mmol L −1 at 400 °C. A lack of magnetite formation at the higher temperature is responsible for the low-H 2 yields.

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.

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: Py 1a and Py 1b in Stage I and Py 2a and Py 2b 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. Py 1a 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 Py 1a deposition. Py 1b and Py 2a 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 (Py 1a , Py 1b , Py 2a ) record a narrow range of δ 34 S V-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 Py 2a became partially exsolved and remobilized during the replacement of porous and invisible gold-rich Py 2b . 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 δ 34 S V-CDT from Py 2a (–3.2 to 4.6‰) to Py 2b (–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.

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. X‑ray difraction 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 <55% magmatic and >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). 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 (<~200 MPa at <~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.

Gungerite, TlAs 5 Sb 4 S 13 , 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 Tl 0.99 As 5.29 Sb 3.77 S 12.95 . 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/cm 3 . In reflected light, the mineral is yellowish-white with very weak bireflectance. In crossed polars, it is distinctly anisotropic but anisotropy efects 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 difraction data are as follows: a = 20.1958(3) Å, b = 11.5258(2) Å, c = 20.1430(2) Å, and V = 4688.74(12) Å 3 ( 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 TlS 9 coordination polyhedra; these rods are oriented parallel to [010]. An individual doughnut-shaped cluster with a central TlS 9 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.

Nitscheite (IMA2020-078), (NH 4 ) 2 [(UO 2 ) 2 (SO 4 ) 3 (H 2 O) 2 ]·3H 2 O, 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 {111̅}. 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 H 2 O at room temperature. The mineral is optically biaxial (–), α = 1.560(2), β = 1.582(2), γ = 1.583(2) (white light); 2 V meas = 17(1)°; no dispersion; orientation X = b , Z ≈ [101]; pleochroism X colorless, Y and Z yellow; X < Y ≈ Z . Electron microprobe analysis provided the empirical formula (NH 4 ) 1.99 U 2.00 S 3.00 O 21 H 10.01 . Nitscheite is monoclinic, P 21/ n , a = 17.3982(4), b = 12.8552(3), c = 17.4054(12) Å, β = 96.649(7)°, V = 3866.7(3) Å 3 , and Z = 8. The structure ( R 1 = 0.0329 for 4547 I > 3σ I reflections) contains [(UO 2 ) 2 (SO 4 ) 3 (H 2 O) 2 ] 2− uranyl-sulfate sheets, which are unique among minerals, with NH 4 and H 2 O groups between the sheets.

Protocaseyite, [Al 4 (OH) 6 (H 2 O) 12 ][V 10 O 28 ]·8H 2 O, 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 safron-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/cm 3 . The optical properties of protocaseyite could be only partly determined: biaxial with α = 1.755(5), β < 1.80, γ > 1.80 (white light); pleochroic with X and Y yellow, Z orange ( X ≈ Y < Z ). Electron-probe microanalysis and crystal-structure solution and refinement provided the empirical formula [(Al 3.89 Mg 0.11 Ca 0.02 ) Σ4.02 (OH) 6 (H 2 O) 12 ][H 0.06 V 10 O 28 ]·8H 2 O. Protocaseyite is triclinic, P 1, 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) Å 3 , and Z = 1. The crystal structure ( R 1 = 0.026 for 4032 I o > 2 σ I reflections) contains both the [V 10 O 28 ] 6− decavanadate polyoxoanion and a novel [Al 4 (OH) 6 (H 2 O) 12 ] 6+ polyoxocation.

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 ofset 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 diferent protocols give consistent results. The length data for apatites with diferent compositions could become comparable if their etch-time windows were linked to a compositional parameter.

The Tianshan orogenic belt hosts several world-class gold deposits and is one of the largest gold provinces on Earth. The Katbasu Au-Cu deposit in the Chinese Western Tianshan is hosted in a granite intrusion. Previous researchers have shown that the main gold ores formed much later than the ore-hosting granite. However, the formation age of Cu mineralization and its possible link to Au mineralization remain poorly understood. This paper reports detailed mineralogical studies, combined with zircon U-Pb, in situ hydrothermal monazite as well as rutile U-Pb ages to constrain the timing of Cu mineralization and its possible link to Au mineralization. The two main ore types in the Katbasu deposit include Cu-Au ores with pyrite-chalcopyrite veins that crosscut the granite and Au ores with massive pyrite and quartz as the main minerals. The Cu-Au ores are spatially associated with diorite that intruded the granite, and they are overprinted by massive gold ores. Detailed mineralogical studies show that chalcopyrite is the main Cu-bearing mineral in the Cu-Au ores, and it is closely associated with some native gold, monazite, and rutile. Secondary ion mass spectrometer (SIMS) U-Pb dating of zircon grains from the ore-hosting granite and mafic enclave yielded concordant ages of 354.1 ± 1.6 and 355.8 ± 1.7 Ma, respectively. The diorite that intruded the granite has a zircon U-Pb age of 352.0 ± 3.2 Ma. The trace element compositions of the monazite suggest they were formed by hydrothermal fluids rather than inherited from the ore-hosting granite. Hydrothermal monazite coexisting with chalcopyrite and native gold yielded a concordant age of 348.7 ± 2.3 Ma, and the W-rich hydrothermal rutile grains associated with the chalcopyrite yielded a U-Pb age of 345 ± 27 Ma, indicating an early Cu-Au mineralization event prior to the major Au mineralization (ca. 323–311 Ma). The formation time of early Cu-Au mineralization is consistent with the emplacement age of the diorite and may be of magmatic-hydrothermal origin, whereas the main Au has no genetic associations with magmatic rocks in the ore district and may belong to the orogenic type. Monazite geochronology provided a more reliable age constraint than rutile in the Katbasu Au-Cu deposit, and we suggest hydrothermal monazite has advantages over rutile in dating the complex mineralization ages of gold deposits.