Volume 108, Issue 3

Iceland’s Námafjall geothermal area exhibits a range of alteration environments. Geochemical and mineralogical analyses of fumaroles and hot springs interacting with Holocene basaltic lavas at Hverir, and with Pleistocene hyaloclastites atop nearby Námaskar∂, reveal different patterns of alteration depending on the water/rock ratio, degree of oxidation, and substrate composition and age. The focus of this study is a transect of a Hverir fumarole that has formed a bullseye pattern of alteration of a Holocene basaltic lava flow. Surface samples and samples collected from shallow pits were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) to constrain changes in mineral assemblage and major elemental composition with both distance and depth. Elemental sulfur is concentrated near the vent, with leached deposits with amorphous silica and anatase nearby and kaolinite, hematite, and jarosite/alunite-group sulfate minerals farther out, with smectites and less altered material at the margins, though smaller-scale mineralogical diversity complicates this pattern. Silica phases include amorphous silica (most samples), cristobalite (some samples in the leached part of the apron), and quartz (minor constituent of a few samples). The silica was concentrated through residual enrichment caused by leaching and is accompanied by a significant enrichment in TiO 2 (in anatase). The presence of abundant cristobalite in a surface fumarole-altered Holocene basaltic lava flow most likely reflects cristobalite formed during the devitrification of volcanic glass or precipitation from fumarolic vapors, rather than high-temperature processes. Minor, localized quartz likely reflects diagenetic maturation of earlier-formed amorphous silica, under surface hydrothermal conditions. Natroalunite, natrojarosite, and jarosite are all present and even exhibit compositional zonation within individual crystals, showing that under surface hydrothermal conditions, these minerals can form a significant solid solution. The high iron content of the substrate basalt and the prevalence of Fe-sulfates and Fe-oxide spherules among the alteration products makes this geothermal area an especially useful analog for potential martian hydrothermal environments. The residual enrichment of silica in the leached deposits of the Hverir fumarole apron could serve as an acid-sulfate leaching model in which amorphous silica forms without appreciable sulfur-bearing phases in many samples, a possible analog for silica-rich soils in the Columbia Hills on Mars. The coexistence of hematite spherules and jarosite-group minerals serves as an intriguing analog for a volcanic/hydrothermal model for hematite and jarosite occurrences at Meridiani Planum.

Investigations of planetary processes using phosphate minerals often focus on igneous, recrystallized, or potentially metasomatized minerals, likely as a result of the minerals commonly available for study within meteorites and lunar samples. However, Mars is a relatively phosphorus-rich planet and possesses abundant evidence of past aqueous surface interactions. Therefore, secondary phosphate phases may be important on the martian surface. Giniite [ Fe2+Fe43+(PO4)4(OH)2⋅2H2O ] $\left[ \text{F}{{\text{e}}^{2+}}\text{Fe}_{4}^{3+}{{\left( \text{P}{{\text{O}}_{4}} \right)}_{4}}{{\left( \text{OH} \right)}_{2}}\cdot 2{{\text{H}}_{2}}\text{O} \right]$ is a secondary phosphate mineral that has been suggested as a potentially significant phase at locations in Gusev Crater and Meridiani Planum on Mars. Although relatively rare as a natural mineral on Earth, giniite has gained attention as an important mineral in industry and technology, especially the lithium battery industry, and the ferrian version of the mineral is often synthesized. This suggests giniite may be important as an in situ resource utilization (ISRU) target for future extended human missions to Mars. Despite this, there are few data available on the natural mineral and the last characterization of the structure was over 40 years ago. There has also been confusion in the literature as to whether giniite is orthorhombic or monoclinic. In this work we revisit and document the chemistry and crystal structure of natural giniite from the type locality at the Sandamab pegmatite in Namibia using updated techniques. Our results refine and update what was previously known regarding the structure and chemistry of giniite and support the potential of the mineral as a possible martian scientific and resource target for further study to aid future missions.

The solid solution between CaSiO 3 and MgSiO 3 perovskites is an important control on the properties of the lower mantle but the effect of one of the most important impurity elements (iron) on this solution is largely unknown. Using density functional theory (DFT), ferrous iron’s influence on the reciprocal solubility of MgSiO 3 and CaSiO 3 perovskite (forming a single Ca-Mg mixed perovskite phase) was calculated under pressures and temperatures of 25–125 GPa and 0–3000 K, respectively. Except at iron-rich conditions, ferrous iron preferentially partitions into the mixed perovskite phase over bridgmanite. This is a small effect (partitioning coefficient K D ~0.25–1), however, when compared to the partitioning of ferrous iron to ferropericlase, which rules out perovskite phase mixing as a mechanism for creating iron-rich regions in the mantle. Iron increases the miscibility of Ca and Mg perovskite phases and reduces the temperature at which the two perovskite phases mix but this effect is highly nonlinear. We find that for a pyrolytic mantle [Ca% = 12.5 where Ca% = Ca/(Ca+Mg)] a perovskite ferrous iron concentration of ~13% leads to the lowest mixing temperature and the highest miscibility. With this composition, 1% ferrous iron in a pyrolytic composition would lead to mixing at ~120 GPa along the geothermal gradient, and 6.25% ferrous iron leads to mixing at ~115 GPa and 13% ~110 GPa. At high iron concentrations, Fe starts to impair miscibility, with 25% ferrous iron leading to mixing at ~120 GPa. Thus, in normal pyrolytic mantle, iron could induce a small amount of Ca-pv and Mg-pv mixing near the D″ layer but it generally partitions to ferropericlase instead and does not impact mixing. Extremely iron rich parts of the lower mantle such as ULVZs or the CMB (potentially) are also not a likely source of phase mixed perovskites due to the nonlinear effect of ferrous iron on phase mixing.

Trace element analyses of silicate materials by secondary ion mass spectrometry (SIMS) typically normalize the secondary ion count rate for the isotopes of interest to the count rate for one of the silicon isotopes. While the great majority of SIMS analyses use the signal from Si + , some laboratories have used a multiply charged ion (Si 2+ or Si 3+ ). We collected data and constructed calibration curves for lithium, beryllium, and boron using these different normalizing species on synthetic basaltic glass and soda-lime silicate glass standards. The calibrations showed little effect of changing matrix when Si + was used, but larger effects (up to a factor of ~2) when using Si 2+ or Si 3+ are a warning that care must be taken to avoid inaccurate analyses. The smallest matrix effects were observed at maximum transmission compared to detecting ions with a few tens of eV of initial kinetic energy (“conventional energy filtering”). Normalizing the light element ion intensities to Al 3+ showed a smaller matrix effect than multiply-charged Si ions. When normalized to 16 O + (which includes oxygen from the sample and from the primary beam), the two matrices showed distinct calibration curves, suggesting that changing sputter yields (atoms ejected per primary atom impact) may play a role in the probability of producing multiply charged silicon ions.

Cubic boron nitride ( c- BN) has the same structure as diamond, and it shows very inert reaction activity in different chemical environments, even under high-pressure ( P ) and high-temperature ( T ) conditions. Furthermore, the P- and T -dependent Raman shift of c- BN (e.g., TO mode) can be distinguished from that of the diamond anvil ( c- BN at ~1054 cm –1 vs. diamond at ~1331 cm –1 at ambient conditions), making c- BN a potential P - T sensor for diamond-anvil cell (DAC) experiments. However, the Raman shift of c- BN has not been well studied at high P - T conditions, especially at temperatures above 700 K. In this study, we systematically calibrated the Raman shift of the TO mode (ν TO ) for synthetic c- BN grains at high- P and high- T conditions up to 15 GPa and 1300 K. Both ruby (Mao et al. 1986) and Sm 2+ :SrB 4 O 7 (Datchi et al. 2007) were used as internally consistent standards for calibration of c- BN P - T sensor. Our results show that the Raman shift of c- BN is negatively correlated with temperature [∂ν TO /∂ T = –0.02206(71)] but positively correlated with pressure [∂ν TO /∂ P = –3.35(2)]. More importantly, we found that the P - T cross derivative for the Raman shift of c- BN [∂ 2 ν TO /∂ P ∂ T = 0.00105(7)] cannot be ignored, as it was assumed in previous studies. Finally, we calibrated a Raman shift P - T sensor of c- BN up to 15 GPa and 1300 K as follows: P = A T − A T 2 + 0.2194 B T , Δ ν 0.1097 $$P=\frac{\text{A}\left( T \right)-\sqrt{\text{A}{{\left( T \right)}^{2}}+0.2194\text{B}\left( T,\Delta \nu \right)}}{0.1097}$$ where A( T ) = 3.47(6) + 0.00105(7) T , B( T , Δν TO ) = 2.81(51) – 0.0053(16) T – 1.78(11) × 10 –5 T 2 – Δν TO . The c -BN Raman shift P - T sensor in this study fills the P - T gap ranging from previously performed externally resistance-heated to laser-heated DAC experiments. The effect of c -BN grain size and Raman system laser power on the calibration were also tested for the P - T sensor. In addition, we conducted three sets of high- P - T experiments to test the practicability of c -BN P - T sensor for water-rock interaction experiments in DAC. Testing experiments showed c -BN has very stable chemical activity in water and clear Raman signal at high- P - T conditions in comparison with other P - T sensors (e.g., ruby, Sm 2+ :SrB 4 O 7 , and quartz). Hence, the Raman shifts of c -BN may serve as an ideal P - T sensor for studying water-rock interactions in a DAC, especially at high- P and high- T conditions relevant to subduction zones.

Bastnäsite contains considerable amounts of U and Th and has been widely used for U-Th-Pb dating. Hydrothermal alteration of bastnäsite is common in nature but its effects on U-Th-Pb dating are not currently well constrained. Hence the significance of U-Th-Pb ages obtained from altered bastnäsite cannot be evaluated. Here, we present a detailed geochronologic as well as micro- and nano-scale mineralogical study of altered bastnäsite in a Mo-REE deposit, Central China. The original bastnäsite grains were confirmed to have crystalized at 208 Ma but were variably overprinted by a hydrothermal event at 150 Ma. They commonly exhibit typical replacement textures that appear to have formed from a coupled dissolution-reprecipitation process, i.e., a primary unaltered domain surrounded by a porous altered domain. Micro- and nano-scale mineralogical observations strongly suggest that during the coupled dissolution-reprecipitation process, non-radiogenic (common) Pb was incorporated into the altered domains in the form of nanoscale galena inclusions. Such incorporation (even minor) has significantly affected the 206 Pb/ 238 U and 207 Pb/ 206 Pb ratios due to the low contents of U and its daughter isotopes in bastnäsite, resulting in highly variable, discordant U-Pb dates for the altered domains. In contrast, incorporation of the non-radiogenic Pb has very limited effects (<5%) on the Th-Pb system due to the remarkably high contents of Th and radiogenic 208 Pb in bastnäsite. Instead, the scattered 208 Pb/ 232 Th ages (208 to 150 Ma) of the altered domains were essentially affected by incomplete replacement, and thus can be used to approximate the lower age limit of the primary hydrothermal activity or the upper age limit of the secondary hydrothermal activity. The results from this study highlight that because of the different orders of magnitude between the U and Th contents in bastnäsite, the mobilization of radiogenic and non-radiogenic Pb during alteration may have significantly different impacts on the U-Pb and Th-Pb systems. Therefore, the two systems should be treated separately during the dating of bastnäsite resulting from secondary hydrothermal events.

Hydrated sulfates have been identified and studied in a wide variety of environments on Earth, Mars, and the icy satellites of the solar system. The subsurface presence of hydrous sulfur-bearing phases to any extent necessitates a better understanding of their thermodynamic and elastic properties at pressure. End-member experimental and computational data are lacking and are needed to accurately model hydrous, sulfur-bearing planetary interiors. In this work, high-pressure X-ray diffraction (XRD) and synchrotron Fourier-transform infrared (FTIR) measurements were conducted on szomolnokite (FeSO 4 ·H 2 O) up to ~83 and 24 GPa, respectively. This study finds a monoclinic-triclinic ( C 2/ c to P 1̅) structural phase transition occurring in szomolnokite between 5.0(1) and 6.6(1) GPa and a previously unknown triclinic-monoclinic ( P 1̅ to P 21) structural transition occurring between 12.7(3) and 16.8(3) GPa. The high-pressure transition was identified by the appearance of distinct reflections in the XRD patterns that cannot be attributed to a second phase related to the dissociation of the P 1̅ phase, and it is further characterized by increased H 2 O bonding within the structure. We fit third-order Birch-Murnaghan equations of state for each of the three phases identified in our data and refit published data to compare the elastic parameters of szomolnokite, kieserite (MgSO 4 ·H 2 O), and blödite (Na 2 Mg(SO 4 ) 2 ·4H 2 O). At ambient pressure, szomolnokite is less compressible than blödite and more than kieserite, but by 7 GPa both szomolnokite and kieserite have approximately the same bulk modulus, while blödite’s remains lower than both phases up to 20 GPa. These results indicate the stability of szomolnokite’s high-pressure monoclinic phase and the retention of water within the structure up to pressures found in planetary deep interiors.

The Hannuoba basalt, located in the northern margin of the North China Craton, is a typical intra-continental basalt with ocean island basalt-like geochemical features and has been extensively studied. However, its origin and deep processes, such as magma mixing and crystallization conditions, are still unclear. To further understand the mechanisms leading to the compositional heterogeneity and magmatic processes of Hannuoba basalt at crustal and/or mantle depth, in situ major element, trace element, and 87 Sr/ 86 Sr compositional heterogeneity of four representative plagioclase crystals in three Hannuoba tholeiite samples, as well as whole-rock major and trace element data, are reported. According to the petrographic characteristics, the basalts are divided into fine-grained and coarse-grained groups. The anorthite content in plagioclase of samples varies in a small range (56–64%), but the content of trace elements in plagioclase from the coarse-grained samples is generally higher than that of the fine-grained samples. Clinopyroxene-melt equilibrium thermobarometer and plagioclase-clinopyroxene magnesium and rare earth element exchange thermometers show that the magma for the two types of basalt was stored and crystallized at a similar depth, and crystallized within a 20 °C (fine-grained basalt) and 50 °C (coarse-grained basalt) temperature window, which may be a reason for the grain size diferences between the two types of basalts. We found that 87 Sr/ 86 Sr of all the studied plagioclase crystals varied from 0.70333 ± 0.00018 (2SE) to 0.70556 ± 0.00031 (2SE), a much large range than the whole rock of Hannuoba basalts reported previously and consistent with that of Cenozoic basalts in North China. Therefore, at least two kinds of melts with significant differences in isotope and minor heterogeneity in major and trace elements are injected into each magma plumbing system. The content of trace elements in the Hanuoba tholeiite is between the Hanuoba alkaline basalt and the lower crust, which may be explained by the mixing of the alkaline basalt and the lower crust, but the low 87 Sr/ 86 Sr (<0.704) characteristics of plagioclase cannot be derived from alkaline basalts, because trace element abundances in the plagioclase are not in equilibrium with the alkaline basalt. Therefore, we believe that the compositional heterogeneity of Hannuoba tholeiitic basalt is caused by the mixing of heterogeneous lower crust rather than different mantle-derived melts. In turn this indicates that the contribution of the continental lower crust to the continental basalt is more complicated than previously recognized.

Control of oxygen fugacity during high-temperature phase equilibrium experiments is required to simulate the conditions that exist in natural systems. At high pressures, oxygen fugacity may be imposed using solid buffer equilibria via the classic “double capsule” technique. This design becomes untenable, however, at temperatures above the melting points of commonly used noble metal capsule materials and/or where buffer assemblages may alloy with the capsule or contaminate the sample. Here we introduce and test a modified double capsule approach that includes a solid metal-oxide buffer in close proximity to but separate from the sample of interest. Buffers used include (in order of most oxidized to reduced) Ni-NiO, Co-CoO, W-WO 3 , Fe-FeO, Mo-MoO 2 , Cr-Cr 2 O 3 , V-V 2 O 3 , Ta-Ta 2 O 5 , and Nb-NbO. At a fixed temperature, these buffers span a wide range—up to 10 log f O2 units. To demonstrate the buffering capacity of this double capsule approach, secondary redox equilibria and V-doped CaO-MgO-Al 2 O 3 -SiO 2 system glasses were studied in experiments using the double capsule geometry. The secondary equilibria provide an independent verification of the oxygen fugacity established in the double capsule environment. The glasses proved difficult to interpret, and our results provide guidance to future efforts to utilize the glass oxybarometer at reducing conditions. Application of this modified double capsule technique to studies of V valence in MgAl 2 O 4 spinels led to the recognition of several factors that will affect V valence in this system: temperature of equilibration, duration of experiment, and spinel bulk composition. We have synthesized V-bearing MgAl 2 O 4 spinel at the reduced conditions of the Cr-Cr 2 O 3 , (IW-3.51), Ta-Ta 2 O 5 , (IW-5.37), and Nb-NbO buffers (IW-5.44). This spinel exhibits a very small V 3+ pre-edge peak consistent with its reduced nature. The absence of evidence for V 2+ suggests that MgAl 2 O 4 spinel excludes V 2+ due to the preference of V for octahedral sites. This finding is supported by DFT calculations for spinels of variable composition, and in agreement with some other indirect evidence for preference for V 3+ in aluminous spinels (Bosi et al. 2016,; Paque et al. 2013,).

Anhydrite has become increasingly recognized as a primary igneous phase since its discovery in pumices from the 1982 eruption of El Chichón, Mexico. Recent work has provided evidence that immiscible sulfate melts may also be present in high-temperature, sulfur-rich, arc magmas. In this study we present partition coefficients for 37 trace elements between anhydrite, sulfate melt and silicate melt based on experiments at 0.2–1 GPa, 800–1200 °C, and f O2 > NNO+2.5. Sulfate melt–silicate melt partition coefficients are shown to vary consistently with ionic potential (the ratio of nominal charge to ionic radius, Z/r) and show peaks in compatibility close to the ionic potential of Ca and S. Partition coefficients for many elements, particularly REE, are more than an order of magnitude lower than previously published data, likely related to differences in silicate melt composition between the studies. Several highly charged cations, including V, W, and Mo are somewhat compatible in sulfate melt but are strongly incompatible in anhydrite. Their concentrations in quench material from natural samples may help to fingerprint the original presence of sulfate melt. Partition coefficients for 2+ and 3+ cations between anhydrite and silicate melt vary primarily as a function of the calcium partition coefficients (D Ca Anh-Sil ) and can be described in terms of exchange reactions involving the Ca 2+ site in anhydrite. Trivalent cations are dominantly charge-balanced by Na 1+ . Most data are well fit using a simple lattice-strain model, although some features of the partitioning data, including D La Anh-Sil > D Ca Anh-Sil , suggest the occurrence of two distinct anhydrite Ca-sites with slightly different optimum radii at the experimental conditions. The ratio D Sr Anh-Sil / D Ca Anh-Sil is shown to be relatively insensitive to silicate melt composition and should vary from 0.63–0.53 between 1200–800 °C, based on a simple, “one-site” lattice strain model. Comparison to D Sr Anh-Sil and D Ca Anh-Sil calculated for natural anhydrite suggests that in most cases, including the S-rich eruptions of Pinatubo and El Chichón, the composition of anhydrite is consistent with early crystallization of anhydrite close to the liquidus of silicate melt with a composition approximately that of the bulk erupted material. This illustrates how anhydrite (and perhaps sulfate melt) provides a mechanism to transport large quantities of sulfur from significant depth to the eruptive environment.

The presence of water may contribute to compositional heterogeneities observed in the deep lower mantle. Mg-rich ferropericlase (Fp) (Mg,Fe)O in the rock-salt structure is the second most abundant phase in a pyrolitic lower mantle model. To constrain water storage in the deep lower mantle, experiments on the chemical reaction between (Mg,Fe)O and H 2 O were performed in a laser-heated diamond-anvil cell at 95–121 GPa and 2000–2250 K, and the run products were characterized combining in situ synchrotron X-ray diffraction measurements with ex-situ chemical analysis on the recovered samples. The pyrite-structured phase FeO 2 H x ( x ≤ 1, Py-phase) containing a negligible amount of Mg (<1 at%) was formed at the expense of iron content in the Fp-phase through the reaction between (Mg,Fe)O and H 2 O, thus serving as water storage in the deepest lower mantle. The formation and segregation of nearly Mg-free Py-phase to the base of the lower mantle might provide a new insight into the deep oxygen and hydrogen cycles.

Solid iron-silicon alloys play an important role in planetary cores, especially for planets that formed under reducing conditions, such as Mercury. The CsCl (B2) structure occupies a considerable portion of the Fe-Si binary phase diagram at pressure and temperature conditions relevant for the core of Mercury, yet its thermodynamic and thermoelastic properties are poorly known. Here, we report in situ X-ray difraction measurements on iron-silicon alloys with 7–30 wt% Si performed in laser-heated diamond-anvil cells up to ~120 GPa and ~3000 K. Unit-cell volumes of the B2 phase at high pressures and high temperatures have been used to obtain a composition-dependent thermal equation of state of this phase. In turn, the thermal equation of state is exploited to determine the composition of the B2 phase in hcp+B2 mixtures at 30–100 GPa and to place constraints on the hcp+B2/B2 phase boundary, determined to vary between ~13–18 wt% Si in the considered pressure and temperature range. The hcp+B2/B2 boundary of Fe-Si alloys is observed to be dependent on pressure but weakly dependent on temperature. Our results, coupled with literature data on liquid equations of state, yield an estimation of the density contrast between B2 solid and liquid under Mercury’s core conditions, which directly relates to the buoyancy of the crystallizing material. While the density contrast may be large enough to form a solid inner core by the gravitational sinking of B2 alloys in a Si-rich core, the density of the B2 solid is close to that of the liquid at solidus conditions for Si concentration approaching ~10 wt% Si.

Primary water and oxygen isotope composition are important tools in tracing magma source and evolution. Metamictization of zircon due to U-Th radioactive decay may introduce external secondary water to the crystal, thereby masking the primary water and oxygen isotope signature. Recently, Raman-based screening has been established to select the low-degree metamict zircons. However, such an approach may not be appropriate for ancient samples, in which nearly all zircons are metamict. It was reported that thermal annealing can potentially heal crystals and retrieve primary water content and δ 18 O information from metamict zircons, given the weaker hydrogen bond of secondary water than that of primary water. Heating experiments at temperatures of 200–1000 °C over a period of 2–10 h reveal that annealing can effectively recover primary water and oxygen isotopes from metamict zircons. Primary water in crystalline and metamict zircons remains intact when heated at <700 °C, while secondary water can be effectively expelled from metamict zircons when heated at 600 °C for >4 h, which represent the optimal annealing treatment condition. Hydrothermally altered zircon is an exception. It only yields the minimum estimate of its primary water contents at 600 °C over a period of >4 h, probably due to partial primary water loss during metamictization for hydrothermal zircons. Moreover, the proportion of low-δ 18 O (<4.7‰) zircon grains that may be influenced by secondary water dropped from ~21% at <600 °C to ~9% when annealed at >700 °C. This study therefore provides the basis for applying zircon water and δ 18 O proxies to geologically ancient samples.

The Bin Yauri-Libata polymetallic ore district is a Sn and Au ore-bearing district in the Zuru schist belt, Northwestern Nigeria. The Libata Sn ore field is characterized by a set of cassiterite-bearing hydrothermal veins associated with Neoproterozoic Pan-African granites affected by deformation and low-grade metamorphism. The hydrothermal alteration associated with cassiterite-bearing quartz veins in the Libata deposit includes silicification, albitization, chloritization, and potassic alteration. In this study, geochemical and geochronological data from tourmalines and zircons from Sn-bearing lodes, unmineralized and altered granites is applied to reveal the timing, fluid composition, and source of ore-forming materials for tin mineralization in the Libata ore field. Zircon trace element and Hf isotopes [εHf(t) = +4.37 to +10.85] reveal a mantle-derived source with some crustal contribution for the melts forming the Libata Sn-bearing granites. LA-ICP-MS zircon U-Pb dating constrains the magmatic and hydrothermal ages to 650–646 and 649–646 Ma for the Libata granites. Overlapping zircon εHf(t) and 176 Hf/ 177 Hf but distinct 176 Lu/ 177 Hf and 176 Yb/ 177 Hf ratios from magmatic and hydrothermally altered zircons reveal a magmatic source for the hydrothermal fluids which triggered cassiterite deposition in the Libata ore field. Major element chemistry constrain tourmalines from the Libata ore field as schorls that show high alkalis, low-Ca contents, and moderate □ values (where □ is x-site vacancy). High Li, Zn, and Sn concentrations in tourmaline as well as Li/Sr and Ca-Fe-Mg ratios demonstrate that the tourmaline formed from granite-sourced fluid likely derived from the host Libata granites. Measured δ 11 B values from the Libata tourmaline range from –15.7‰ to –14.1‰. The δ 11 B of the mineralizing fluid is estimated to be –13.1 to –11.9‰ for the Libata tourmalines at 400–500 °C and overlaps with averages from fractionated granites worldwide. Therefore, our data show that tourmaline and zircon are useful tracers of magmatic-hydrothermal evolution in rare-metal-bearing granite systems.

The crystal structure of a birefringent garnet (~Adr 53 Grs 47 ) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/ a 1 2/ d (unconventional setting for Fddd ) space group symmetry, with unit-cell parameters of a = b = 11.966(3) Å, c = 11.964(3) Å, α = β = 90°, γ = 90.29(2)°, V = 1713.0(7) Å 3 , Z = 8. The orthorhombic garnet displays very high birefringence (δ ~0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y 1 and Y 2 sites, respectively. Difraction peaks (such as 101 and 103) violating the Ia 3̅ d symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between ~Adr 53 Grs 47 and ~Adr 100 ) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.

Jarosite and related subgroup minerals are of high importance in mineral processing, as sources and sinks for metals and acidity in the environment, and they have the potential to preserve elemental and isotopic biomarkers relevant to the search for life in the solar system. The crystal structures and chemistry of jarosite minerals affect their stability and reactivity and thus the roles they play in natural and engineered systems. Rhombohedral symmetry has been documented in natural and synthetic jarosites, whereas monoclinic symmetry has only been documented in synthetic jarosites. This research reports the occurrence of monoclinic symmetry in a natural natrojarosite sample investigated using synchrotron powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and electron backscatter diffraction (EBSD). Splitting of several rhombohedral PXRD peaks (e.g., 012, 027, and 033) into pairs of peaks was observed, with the magnitude of the splitting and the relative intensities of the pairs of peaks being almost identical to those reported for synthetic monoclinic jarosite. Rietveld refinement with room-temperature PXRD data shows an ordering of iron-site vacancies on the Fe1 site consistent with monoclinic symmetry, space group C2/ m . Conversion of monoclinic unit-cell parameters into pseudo-hexagonal unit-cell parameters, specifically β′, also supports the use of a monoclinic model to describe the natrojarosite structure. Structural analysis with increasing temperature is supportive of the thermal evolution previously described for synthetic monoclinic jarosite samples, with some indications of subtle differences between synthetic and natural materials including slower rates of thermal expansion and absence of FeOHSO 4 peaks for natural monoclinic jarosite. EBSD provides insight into the spatial–structural variation within the hand specimen from which the natrojarosite was sampled, demonstrating that there are areas of unambiguous monoclinic symmetry, but others where both monoclinic and rhombohedral natrojarosite coexist. The results of this study suggest that monoclinic symmetry in natural jarosites may be more prevalent than previous studies suggest. Monoclinic symmetry in jarosites is identifiable by an ordering of iron-site vacancies on the Fe1 site, splitting of specific rhombohedral XRD peaks into pairs of peaks, and an increase in jarosite symmetry (i.e., from monoclinic to rhombohedral) during heating. The splitting of peaks in monoclinic jarosites can be subtle so it is recommended that high-resolution XRD data are collected when studying the crystal structure of jarosites.

The systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L 2,3 -edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L 2,3 -edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10 Dq crystal-field parameter values for Nb 5+ , which range from 2.8 to 3.9 eV depending on Nb coordination and Nb‒O distances. Rather than a 10 Dq vs. R –5 relationship (where R represents the average Nb‒O bond distance) expected in a point-charge model, we find a R –3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb‒O bonds is equivalent, unlike more ionic 3 d metal‒O bonds. This systematic characterization of the L 2,3 -edges XANES spectral properties of Nb provides information on the mechanisms by which Nb 5+ substitutes for Fe 3+ , Ti 4+ , or Ce 4+ in oxides common in geological contexts. Whereas the substitution of Nb 5+ for Ce 4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb 5+ for Ti 4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb 5+ for Fe 3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L 2,3 - edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.

Mikecoxite, ideally (CHg 4 )OCl 2 , is the first mercury-oxide-chloride-carbide containing a C 4– anion coordinated by four Hg atoms (a permercurated methane derivative) to be described as a mineral species. It was found at the McDermitt open-pit mine on the eastern margin of the McDermitt Caldera, Humboldt County, Nevada, U.S.A. It is monoclinic, space group P 21/ n , Z = 4; a = 10.164(5), b = 10.490(4), c = 6.547(3) Å, V 698.0(5) Å 3 . Chemical analysis by electron microprobe gave Hg 86.38, Cl 11.58, Br 0.46, C 1.81, sum = 100.23 wt%, and O was detected but the signal was too weak for quantitative chemical analysis. The empirical formula, calculated on the basis of Hg + Cl + Br = 6 apfu, is (C 1.19 Hg 3.39 )(Cl 2.57 Br 0.05 ) Σ2.62 , and the ideal formula based on the chemical analysis and the crystal structure is (CHg 4 )OCl 2 . The seven strongest lines in the X-ray powder diffraction pattern are [ d (Å), I , ( hkl )]: 2.884, 100, (230); 2.989, 81, (301, 301, 112, 112, 131, 131); 2.673, 79, (122, 122, 212, 212); 1.7443, 40, (060, 432, 432); 5.49, 34, (101, 101); 4.65, 32, (120); 2.300, 30, (312, 312). The Raman spectrum shows three bands at 638, 675, and 704 cm –1 , well above the range characteristic of NHg 4 stretching vibrations between 540 and 580 cm –1 , that are assigned to CHg 4 stretching vibrations. Mikecoxite forms intergrowths of bladed crystals up to 100 μm long that occur on granular quartz or in vugs associated with kleinite. It is black with a submetallic to metallic luster and strong specular reflections and does not fluoresce under short- or long-wave ultraviolet light. Neither cleavage nor parting were observed, and the calculated density is 8.58 g/cm 3 . In the crystal structure of mikecoxite, ( C 4 − H g 4 2 + ) $(\mathrm C^{4-}\mathrm{Hg}_4^{2+})$groups link through O 2– ions to form three-membered rings that polymerize into corrugated [CHg 4 OCl] + layers with near-linear C 4– –Hg 2+ –O and C 4– –Hg 2+ –Cl linkages. The layers link in the third direction directly via weak Hg 2+ –O 2– and Hg 2+ –Cl – bonds to adjacent layers and also indirectly via interlayer Cl – . A bond-valence parameter has been derived for (Hg 2+ –C 4– ) bonds: R o = 2.073 Å, b = 0.37, which gives bond-valence sums at the C 4– ions in accord with the valence-sum rule. The source of carbon for mikecoxite in the volcanic high-desert environment of the type locality seems to be methane, with the reaction catalyzed by microbiota through full mercuration of carbon atoms, beyond the first stage that produces the volatile and highly mobile methylmercury, [CH 3 Hg] + , a potent neurotoxin that accumulates in marine food chains. Both the mineral and the mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2021-060). The mineral is named after Michael F. Cox (b. 1958), a founding member of the New Almaden Quicksilver County Park Association (NAQCPA) who was responsible for characterizing and remediating environmental mercury on-site and who recovered the rock containing the new mineral.

The Neoproterozoic peridotites of Abu Dahr, Eastern Desert of Egypt, consist mainly of highly depleted harzburgites that have experienced multiple stages of serpentinization (lizarditization and antigoritization) and carbonation/listvenitization in a forearc environment. The Abu Dahr forearc harzburgites are more oxidized than oceanic mantle, with the oxygen fugacity ( f O2 ) values ranging from FMQ+0.41 to FMQ+1.20 (average = +0.60 FMQ), and were equilibrated at temperatures of 910–1217 °C and pressures of 4.1–7.8 kbar. This study has documented for the first time the presence of various Ni-rich Ni-Fe (-Co) sulfide and metal phases along with Fe-oxides/oxyhydroxides in serpentinized-carbonated peridotites of the Abu Dahr forearc. Here I concentrate on the relationship between redox state and Fe-Ni-Co-O-S minerals with emphasis on the role of hydrothermal processes in upgrading magmatic sulfide tenors, desulfurization (sulfur-loss) of magmatic pentlandite and hydrothermal upgrading of the sulfide phases in Abu Dahr forearc environment. The minerals involved are high-Ni pentlandite (Fe 4 Ni 5 S 8 ), cobaltian pentlandite (Fe 3.47 Ni 4.78 Co 0.75 S 8 ), heazlewoodite (Fe 0.07 Ni 2.93 S 2 ), godlevskite (Fe 0.26 Ni 8.73 Co 0.01 S 8 ), millerite (Fe 0.01 Ni 0.98 Cu 0.01 S), awaruite (Ni 75 Fe 21 ) and native Ni (Ni 93 Fe 5 ), and nickeliferous magnetite and goethite. Chalcopyrite is a rare mineral; other Cu-phases, Fe-sulfides and Ni-arsenides/phosphides are not present. Texturally, Ni-sulfide and alloy minerals occur as interstitial disseminated blebs of either solitary phases or composite intergrowths with characteristic replacement textures, documenting strong variations in oxygen and sulfur fugacities ( f O2 - f S2 ). Sulfide assemblages are divided into three main facies: (1) pentlandite-rich; (2) godlevskite-rich; and (3) millerite-rich. Textural relationships imply the following sequence: (a) primary pentlandite → cobaltian pentlandite, with partial replacement of the latter by awaruite and/or heazlewoodite along with magnetite; (b) heazlewoodite is replaced by godlevskite, which in turns is replaced by millerite; (c) Ni-rich awaruite breaks down to millerite; and finally, (d) magnetite is completely replaced by goethite. The sulfide mineralogy reflects the magmatic and post-magmatic evolution of the complex. The primary magmatic processes gave rise to pentlandite, whereas the secondary Ni-sulfides together with the metallic alloys formed in response to changing f O2 and f S2 conditions associated with post-magmatic serpentinization and carbonation. Serpentinization-related Ni-Fe-Co remobilization from magmatic olivines resulted in; (1) upgrading the Ni-Co tenors of pre-existing primary pentlandite, and desulfidation to form low-sulfur sulfides (mainly heazlewoodite) and awaruite under extremely low f O2 and f S2 conditions; (2) in situ precipitation of secondary Ni-sulfides in the presence of extra sulfur as aqueous H 2 S derived from the desulfurization of magmatic pentlandite or native Ni when f S2 approaches 0; (3) transformation from low-sulfur pentlandite- and godlevskite-rich assemblages to the high-sulfur millerite-rich assemblages related to later carbonation with increasing f O2 ; and (4) partial dehydration of antigorite serpentinites under high-pressure conditions (>1 GPa) generated Ni-rich awaruite in equilibrium with the prograde assemblage antigorite-metamorphic olivine at higher f O2 and f S2 within subduction channel. The mineralogical, chemical, and thermal similarities with other serpentinite-related Ni-sulfides worldwide suggest that Ni minerals in the Fe-Ni-Co-O-S system record changing f O2 and f S2 during progressive serpentinization and carbonation.

In this issue This issue of New Mineral Names provides a summary of new species that contain arsenic and lead. As of November 2022, there are 1219 minerals that contain constituent arsenic or lead, which is roughly 20% of all known mineral species. These two elements are an important component in many of the newly described minerals that typically form from hydrothermal or other diagenetic processes. Here we look at nitroplumbite, thorasphite, tennantite-(Cd), paradimorphite, tombstoneite, aldomarinoite, lomardoite, dobšináite, panskyite, yugensonite, and kufahrite.