Volume 108, Issue 4

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 Námaskarð hill, reveal different patterns of alteration depending on water-rock ratio, degree of oxidation, and substrate composition and age. The focus of this study is on the mineral deposits at and near hot springs at Hverir and Námaskarð. Surface samples, and samples collected from shallow pits in the alteration aprons adjacent to hot springs, were analyzed by X-ray diffraction (XRD) and X-ray fluorescence (XRF) to constrain the differences in composition with both distance and depth. Fluids were analyzed in the field for their environmental parameters and sampled for cation and anion analysis. Fluid analyses revealed uniformly acidic conditions but with site-to-site variation in other parameters such as temperature, salinity, and conductivity. Solid phases identified include amorphous silica, pyrite, elemental sulfur, and kaolinite in the muds, surrounded by Fe 2+ -sulfate and then Fe 3+ -sulfate efflorescence, following a redox gradient pattern involving the oxidation of sulfur and then iron with increasing distance. Shallow pits excavated near two Námaskarð hot springs reveal a shallow oxidation front, with sulfide-rich materials below a thin surface of sulfates and elemental sulfur. Silica phases include amorphous silica and quartz. Quartz likely reflects diagenetic maturation of earlier-formed amorphous silica, under surface hydrothermal conditions. The high iron content of the substrate basalt and the prevalence of Fe-sulfates and Fe-oxides among the alteration products make this geothermal area an especially useful analog for potential martian hydrothermal environments. In particular, these sulfate-rich deposits adjacent to volcanic, acidic hot springs could provide a helpful comparison for sulfur-rich soils in the Columbia Hills on Mars, where some of the same minerals have been identified (e.g., ferricopiapite) or inferred (e.g., rhomboclase).

Nitrogen is an essential element for life, one of the most abundant volatiles in the atmosphere, and an important component in the Earth’s interior, where iron nitride is an essential host of deep nitrogen. Here, we investigate the pressure-induced electronic spin-pairing transition of iron in siderazot (Fe 3 N 1.2 ) at pressures up to 45.8 GPa at room temperature, using diamond-anvil cell techniques coupled with synchrotron X-ray emission spectroscopy. The integrated intensity of the satellite emission peak ( K ′β) decreases upon compression but remains unchanged at pressures greater than 30.5 GPa. In other words, the high-spin to low-spin transition of iron in Fe 3 N 1.2 starts immediately at very low pressures and completes at ~30.5 GPa. The iron spin transition completion pressures increase with the nitrogen concentration of hexagonal close-packed iron nitrides (i.e., Fe 3 N 1.2 , Fe 7 N 3 , and Fe 2 N). Moreover, the identity and concentration of light elements in binary iron-rich compounds such as Fe 3 N, Fe 3 C, Fe 3 P, Fe 3 S, Fe 7 C 3 , and Fe 7 N 3 , together with their crystal structure, could affect the iron spin transition pressures. The spin transition of iron-rich alloys could alter the bonding nature and the physical properties, including the thermal and electrical conductivity, thereby influencing the thermal state and evolution of planetary interiors.

The density of the Earth’s core is several percent lower than that of iron-nickel alloy under conditions of pressure and temperature equivalent to the Earth’s core. Hydrogen is one of the most promising constituents accounting for the density deficit, but hydrogen occupation sites and density decrease of iron-nickel alloy caused by hydrogenation have never been investigated. In this study, the phase relation and crystal structure of Fe 0.9 Ni 0.1 H x (D x ) at high pressures and temperatures up to 12 GPa and 1000 K were clarified by in situ X-ray diffraction and neutron diffraction measurements. Under the P-T conditions of the present study, no deuterium atoms occupied tetragonal ( T ) sites of face-centered cubic (fcc) Fe 0.9 Ni 0.1 D x , although the T-site occupation was previously reported for fcc FeH x (D x ). The deuterium-induced volume expansion per deuterium v D was determined to be 2.45(4) and 3.31(6) Å 3 for fcc and hcp Fe 0.9 Ni 0.1 D x , respectively. These v D values are significantly larger than the corresponding values for FeD x . The v D value for fcc Fe 0.9 Ni 0.1 D x slightly increases with increasing temperature. This study suggests that only 10% of nickel in iron drastically changes the behaviors of hydrogen in metal. Assuming that v D is constant regardless of pressure, the maximum hydrogen content in the Earth’s inner core is estimated to be one to two times the amount of hydrogen in the oceans.

Hydrogen is the most abundant element in the solar system and has been considered one of the main light elements in the Earth’s core. The hydrogen content in the Earth’s core is determined normally by matching the volume expansion caused by the incorporation of hydrogen into FeH x to the Earth’s core density deficit. The magnitude of this volume expansion at the pressure ( P ) and temperature ( T ) conditions of the Earth’s core is still unknown, and the effect of spin transition in FeH x at high pressure is usually ignored. In this study, we simulate the Fe spin transition, equation of state, and hydrogen-induced volume expansion in Fe-H binaries at high P-T conditions using density functional theory (DFT) calculations. Our results indicate that hydrogen could stabilize the magnetic properties of fcc Fe from ~10 to ~40 GPa. A volume expansion induced by hydrogen is linear with pressure except at the Fe spin transition pressure, where it collapses significantly (~30%). The fcc FeH lattice is predicted to expand at an average rate of ~1.38 and 1.07 Å 3 per hydrogen atom under the Earth’s outer and inner core P-T conditions, where the hydrogen content is estimated to be ~0.54–1.10 wt% and ~0.10–0.22 wt%, respectively. These results suggest that the Earth’s core may be a potentially large reservoir of water, with up to ~98 times as much as oceans of water being brought to the Earth’s interior during its formation. Based on our predicted hydrogen content in the Earth’s core, we propose that the presence of hydrogen would induce a relatively lower core temperature, ~300–500 K colder than it has been previously speculated.

Thermodynamic parameters have been measured for synthetic analogs of the mimetite- group minerals Pb 5 (AsO 4 ) 3 X (X = OH, Cl, Br, I) belonging to the apatite supergroup. Phases precipitated from aqueous solutions under ambient conditions with well characterized structures and compositions were studied. For each phase, dissolution enthalpy was experimentally determined by oxide melt drop solution calorimetry in a molten solvent of sodium molybdate (3Na 2 O·4MoO 3 ) at 976 K. The enthalpy of formation from the elements Δ H f , e l ∘ $\Delta H_{\mathrm{f}, \mathrm{el}}^{\circ}$was calculated using thermochemical cycles and was −3030.6 ± 11.5, −3026.6 ± 15.8, −2967.6 ± 25.0, and −2993.1 ± 12.2 kJ/mol for Pb 5.00 (AsO 4 ) 3.00 OH 0.86 (CO 3 ) 0.07 , Pb 5.00 (AsO 4 ) 3.00 Cl 0.80 (CO 3 ) 0.10 , Pb 5.00 (AsO 4 ) 3.00 Br 0.80 (CO 3 ) 0.10 , and Pb 5.00 (AsO 4 ) 3.00 I 0.45 OH 0.35 (CO 3 ) 0.10 , respectively. These Δ H f , e l ∘ $\Delta H_{\mathrm{f}, \mathrm{el}}^{\circ}$values exhibit typical trends for apatites: they increased (were less negative) with the increasing molar mass and ionic radius of X and decreased with the electronegativity and ionization energy of X. The compilation and comparison of data for Ca-, Pb-, P-, and As-apatites revealed correlations indicating that thermodynamic enthalpic stability is largely influenced by chemical factors (e.g., differences in electronegativities of the elements, ionization energy, or ionic characteristics of the bonds) and to a lesser extent by physical and geometric parameters in the crystal structure related to the mass and size of the X anion. Using the correlations, it was possible to estimate the value of hitherto unknown Δ H f , e l ∘ $\Delta H_{\mathrm{f}, \mathrm{el}}^{\circ}$for Pb 5 (AsO 4 ) 3 F, −3144.3 ± 66.5 kJ/mol. The observed relationships apply to the entire apatite supergroup and can be used to predict the values of Δ H f , e l ∘ $\Delta H_{\mathrm{f}, \mathrm{el}}^{\circ}$for phases that have not been studied experimentally. The new data on environmentally significant phases will contribute to the modeling of mineral-water interactions, particularly for potential use in the remediation of soils and wastes contaminated with Pb and As and in the immobilization of radioactive waste containing I-129.

Heavy meteorite impacts on Earth’s surface produce melt and vapor that are quenched rapidly and scattered over wide areas as natural glasses with various shapes and characteristic chemistry, which are known as tektites and impact glasses. Their detailed formation conditions have long been debated using mineralogical and geochemical data and numerical simulations of impact melt formations. These impact processes are also related to the formation and evolution of planets. To unravel the formation conditions of impact-induced glasses, we performed shock recovery experiments on a tektite. Recovered samples were characterized by X-ray diffraction, Raman spectroscopy, and X-ray absorption fine structure spectroscopy on the Ti K -edge. Results indicate that the densification by shock compression is subjected to post-shock annealing that alters the density and silicate-framework structures but that the local structures around octahedrally coordinated Ti ions remain in the quenched glass. The relationship between the average Ti-O distance and Ti K pre-edge centroid energy is found to distinguish the valance state of Ti ions between Ti 4+ and Ti 3+ in the glass. This relationship is useful in understanding the formation conditions of impact-derived natural glasses. The presence of Ti 3+ in tektites constrains the formation conditions at extremely high temperatures or reduced environments. However, impact glasses collected near the impact sites do not display such conditions, but instead relatively mild and oxidizing formation conditions. These different formation conditions are consistent with the previous numerical results on the crater size dependence.

The use of copper canisters in the Swedish KBS-3 concept for spent nuclear fuel disposal could result in the formation of copper-bearing uranyl phases should a canister suffer from defects or if the containment were to fail before reducing conditions are established in the repository. Most uranyl species would be expected to display higher solubility than the original uranium(IV) dioxide fuel, leading to enhanced release, though this would depend on the phase and prevailing groundwater conditions. Secondary alteration products may also be poorly crystalline or even amorphous, making characterization difficult during the pre-closure period owing to the high radiation field close to the canister. Vandenbrandeite, [CuUO 2 (OH) 4 ], is a rare mineral in nature but known to form by alteration of primary uraninite through interaction with oxidizing groundwater containing dissolved copper. Consequently, an attempt has been made to characterize two vandenbrandeite specimens of varying crystallinity by luminescence and multiple-laser Raman spectroscopy; techniques amenable to remote, robotic deployment and which have proved useful in discriminating other uranyl oxy-hydroxides, silicates, and phosphates. The first reported luminescence emission and excitation spectra for vandenbrandeite revealed near-negligible luminescence, with a slightly enhanced signal for the specimen displaying poorer crystallinity. This observation agrees well with density functional theory calculations. The simulated projected density of state and band structure show an unlikely transition from the U f-orbitals to Cu d-orbitals, or O states, would be required for luminescence to be detectable; this probably improves for poorly crystalline specimens as the spatial overlap between the orbitals increases. Furthermore, negligible differences in the number of peaks and peak positions were detected in the laser wavelength-dependent Raman spectra although again, variation in background noise and peak shape was observed based on the degree of crystallinity. Good agreement was obtained between experimental and simulated Raman spectra, particularly with the environmentally sensitive axial uranyl stretching modes, validating the crystal system derived in this study. The findings of this study suggest luminescence spectroscopy, when combined with Raman spectroscopy, may be able to both identify vandenbrandeite and distinguish between crystalline and amorphous forms based on their relative luminescence intensity.

The frequent occurrence of chlorides and carbonates in the form of microinclusions of melts or high-density fluid (HDF) in diamonds and igneous minerals of kimberlites worldwide generates genuine interest in their phase diagrams under pressure. Here, we present the first experimental results on the phase relations in the NaCl-CaCO 3 and NaCl-MgCO 3 systems at 6 GPa in the range 1000–1600 °C performed using a multi-anvil press. We found that both systems have the eutectic type of phase diagrams. The subsolidus assemblages are represented by halite + aragonite and halite + magnesite. Halite-aragonite eutectic is situated just below 1200 °C and has a composition of 40 wt% NaCl and 60 wt% CaCO 3 . Halite-magnesite eutectic is located at 1300 °C and has a composition of 72 wt% NaCl and 28 wt% MgCO 3 . The halite melting point was established at 1500 °C. Complete miscibility between carbonate and chloride liquids was observed up to 1600 °C. The results support the hypotheses that saline HDF is either a low-temperature derivative or precursor of mantle carbonatite HDF. The data also do not exclude an alternative hypothesis, according to which saline HDF are formed as a result of the reduction of the carbonate component of chloride-containing carbonatite melts to diamond.

Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli ( C ij ) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature ( P - T ) conditions. Here we report single-crystal C ij of (Al,Fe)-bearing bridgmanite, Mg 0.88 Fe 0.1 Al 0.14 Si 0.90 O 3 (Fe10-Al14-Bgm) with Fe 3+ /ΣFe = ~0.65, up to ~82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (–0.50, 0.05, –0.86) and (0.65, –0.59, 0.48), that are sensitive to deriving all nine C ij , are used for compressional and shear wave velocity ( ν P and ν S ) measurements as a function of azimuthal angles over 200° at each experimental pressure. Our results show that all C ij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1–2% (±1σ), except C 55 and C 23 , which have uncertainties of 3–4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: K S = 326 ± 4 GPa, μ = 211 ± 2 GPa, Ks′ $K_{s}^{\prime}$ = ′3.32 ± 0.04, and μ′ = 1.66 ± 0.02 GPa. The high-pressure aggregate ν S and ν P of Fe10-Al14-Bgm are 2.6–3.5% and 3.1–4.7% lower than those of MgSiO 3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P - T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models.

Apatite is a mineral of widespread importance in Earth and planetary science. Here we examine the behavior of a natural fluorapatite (FAp) crystal from Durango (Mexico) under compression to 61 GPa. Single-crystal X-ray diffraction experiments were carried out in a diamond-anvil cell using a synchrotron source. The apatite structure persists up to 32.4 GPa. Birch-Murnaghan equation of state parameters were fit to the pressure-volume data for fluorapatite for two cases: fixing V 0 at its measured ambient value resulted in a bulk modulus, K 0T , of 97.0(8) GPa and a pressure derivative of the bulk modulus, K′ 0T , of 3.3(1), while fixing V 0 and K 0T at its ambient value 90.5 GPa (derived from ultrasonically measured elastic constants) resulted in a K′ 0T value of 4.1(1). At 35.6 GPa, fluorapatite transforms to a triclinic phase ( P 1, Z = 4), designated here as fluorapatite II (FAp-II). This phase persists up to at least 61 GPa. The major structural differences between FAp and FAp-II involve the buckling of the Ca polyhedra along the c -axis and changes in the number and coordination of the Ca sites. Our study extends the pressure range over which fluorapatite has been examined by more than a factor of three, providing new insights into its structural response to high-pressure conditions.

Fe,Al-bearing MgSiO 3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth’s lower mantle, hosting ferric iron in its structure as charge-coupled (Fe 2 O 3 and FeAlO 3 ) and vacancy components (MgFeO 2.5 and Fe 2/3 SiO 3 ). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO 3 -FeAlO 3 -MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO 3 -structured phase in the quenched run product, which was the perovskite phase under high pressures and high temperatures. The perovskite phase coexists with corundum and a phase with (Mg,Fe 3+ ,□)(Al,Fe 3+ ) 2 O 4 composition (□= vacancy). The FeAlO 3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe 2 O 3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A -site vacancy component of Fe 2/3 SiO 3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO 2.5 appears at higher temperatures, although the analytical errors prevent definite conclusions. The A -site vacancy component might be more important than the oxygen vacancy component for the defect chemistry of bridgmanite in slabs and for average mantle conditions when the FeAlO 3 charge-coupled component is dominant.

The acoustic velocities of three series of alkali-rich hydrous silicate glasses were determined at ambient conditions using ultrasonic interferometry. The sound velocities and calculated elastic properties are nearly linear functions of dissolved water content. The water content derivatives of both compressional [ d ( V P )/ d ( X H2O )] and shear wave velocity [ d ( V S )/ d ( X H2O )] decrease with increasing Na 2 O content, which suggests that increasing sodium content might weaken the effect of water on acoustic velocities. For each glass series, the shear modulus decreases with increasing water content, whereas the adiabatic bulk modulus ( K S ) varies little with water content. By comparing our results of K S to previously published data on hydrous alkalic (rhyolite, phonolite, and trachyte) glasses, we speculate that H 2 O may have a negligible effect on the compressibility of alkalic glasses/melts. The measured densities of each series of glasses were converted to molar volumes and extrapolated to the hypothetical pure water component to derive the partial molar volume of water V ¯ H 2 O . $\left( {{{\bar{V}}}_{{{H}_{2}}O}} \right).$The resulting V ¯ H 2 O $\left( {{{\bar{V}}}_{{{H}_{2}}O}} \right)$at room pressure and temperature was found to be independent of composition for the three series of glasses, with a value of 10.4 ± 0.5 cm 3 /mol, whereas the partial molar bulk modulus ( K S ) of water is composition-dependent, suggesting that V ¯ H 2 O $\left( {{{\bar{V}}}_{{{H}_{2}}O}} \right)$may depend on composition at elevated pressures. Furthermore, the Poisson’s ratio (ν) increases linearly with increasing water content for all three composition series. Therefore, the similar behavior of Al-free and Al-bearing glasses implies that water depolymerizes melts in both systems. The decrease of water content derivative of the Poisson’s ratio [ d (ν)/ d ( X H2O )] with increasing Al/(Al+Si) indicates that water interaction with Al-O bonds to produce Al-OH depolymerizes peralkaline silicate melts less effectively than with Si-O bonds to form Si-OH. Hence, we speculate that water prefers to interact with Si-O bonds rather than Al-O bonds in peralkaline silicate systems. The dissolution mechanisms of water between peraluminous and peralkaline melts might be different, which merits further research.

A hydrothermal quartz vein with REE-U-Au mineralization in the Zimná Voda (Gemeric Unit, Western Carpathians, Slovakia) is associated with contact metamorphism between Permian granites and host phyllites and metaquartzites. It contains unique REE minerals of the monazite and xenotime groups. Monazite-(Ce), monazite-(Nd), monazite-(Sm), and Gd-dominant monazite [“monazite-(Gd)”], along with xenotime-(Y) to Gd-dominant xenotime [“xenotime-(Gd)”] and Gd-rich hingganite-(Y) show heterogeneous compositions and reflect a strong fractionation trend toward the enrichment of MREE (Sm to Dy), particularly Gd. Here, the gadolinium abundance reported in “monazite-(Gd)” (≤23.4 wt% Gd 2 O 3 ) and Gd-rich xenotime-(Y) to “xenotime-(Gd)” (≤28.7 wt% Gd 2 O 3 ) and accompanied by Gd-rich hingganite-(Y) (≤15.8 wt% Gd 2 O 3 ), is among the highest Gd concentrations ever reported in natural minerals. The Gd-richest compositions show the following formulas: (Gd 0.31 Sm 0.24 Nd 0.15 Ce 0.10 La 0.05 Dy 0.03 Y 0.03 ...) 0.98 PO 4 [“monazite-(Gd)”], (Gd 0.36 Y 0.32 Dy 0.13 Sm 0.08 Tb 0.05 ...) 0.98 (P 0.96 As 0.04 ) 1.00 O 4 [“xenotime-(Gd)”] and (Y 0.71 Gd 0.43 Dy 0.23 Sm 0.22 Tb 0.06 Er 0.04 Nd 0.06 ...Ca 0.06 ) 1.96 ( ◻ 0.87 Fe 0.13 2 + ) 1.00 ( B e 1.82 B 0.18 ) 2.00 ( S i 1.90 A s 0.10 ) 2.00 O 8 ( O H 1.70 O 0.30 ) 2.00 ${{({{\square }_{0.87}}\,\text{Fe}_{0.13}^{2+})}_{1.00}}{{(\text{B}{{\text{e}}_{1.82}}\,{{\text{B}}_{0.18}})}_{2.00}}\,{{(\text{S}{{\text{i}}_{1.90}}\text{A}{{\text{s}}_{0.10}})}_{2.00}}\,\,{{\text{O}}_{8}}{{(\text{O}{{\text{H}}_{1.70}}\,{{\text{O}}_{0.30}})}_{2.00}}$ [hingganite-(Y)]. The MREE-rich monazites, xenotimes, and hingganite-(Y) precipitated in response to the alteration of primary uraninite, brannerite, and fluorapatite by low-temperature hydrothermal fluids of heterogeneous compositions on a microscale. These are responsible for the strong enrichment of individual MREE, especially Gd in the secondary minerals. This is accompanied by the advancing development of the W-type tetrad effect on REE through monazite species. The substantial incorporation of Gd into both REE-selective monazite and xenotime structures that are accompanied by LREE vs. HREE segregation indicates the possibility of differently sized REE 3+ miscibility in REEPO 4 solid solutions, as well as the stabilization of the Gd-rich orthophosphate structure by substitution of the remaining A -site cations with smaller HREE+Y in the xenotime-type, and/or larger LREE in the monazite-type structure.

The ion-adsorption rare earth element (REE) deposit, a valuable type of REE deposit, has been thought to be derived from the release and enrichment of REE during granite weathering. Understanding the REE occurrence in regolith-hosted deposits is crucial for more eficient extraction. We investigated a weathering granite profile of a regolith-hosted REE deposit located in South China. X-ray difraction (XRD) and X-ray absorption near edge structure (XANES) analysis of the clay fractions reveal that the highest Ce(IV) content located in intensely weathered layers and cerianite nanoparticles (CeNPs) can be observed, besides invisible adsorbed REEs. Interestingly, most of the CeNPs scatter on halloysite basal surface and exhibit preferred orientation. Detailed analysis demonstrates that the diagonal plane of cerianite matches with the exposed basal surfaces (Si-O tetrahedron) of halloysite. Such a lattice match may contribute to the nucleation and growth of CeNPs after oxidation of the adsorbed Ce(III), which results in great REE enrichment by clays. The findings provide new insight into the mechanism of Ce precipitation and REE mineralization during granite weathering.

This issue of New Mineral Names provides a summary of five new species: pohlite, muonionalustaite, medvedevite, gysinite-(La), and nioboheftetjernite.