Volume 108, Issue 8

Apatite is a common accessory phase in igneous and metamorphic rocks. Its stability in magmatic-hydrothermal and hydrothermal systems is known to be a key control on the mobility of rare earth elements (REE). To better constrain how apatite is altered during fluid-rock interaction at comparably low temperatures, batch-type apatite dissolution experiments were conducted at 150 and 250 °C at saturated water vapor pressure in acidic to mildly acidic (pH of 2–4) aqueous fluids having variable salinities (0, 0.5, and 5 wt% NaCl). The study reveals the dominance of apatite dissolution textures with the formation of micrometer-scale etch pits and dissolution channels developing prominently along the c-axis of the apatite crystals. Backscattered electron imaging shows an increase in apatite dissolution with increasing temperature and upon reacting the crystals with more acidic and higher salinity starting fluids. This study also demonstrates an increase in dissolved REE in the experimental fluids corroborating with the observed apatite dissolution behavior. Backscattered electron imaging of secondary minerals formed during apatite dissolution and scanning electron microscopy-based energy dispersive spectrometry peaks for Ca, P, and REE support the formation of monazite-(Ce) and minor secondary apatite as deduced from fluid chemistry (i.e., dissolved P and REE concentrations). The studied apatite reaction textures and chemistry of the reacted fluids both indicate that the mobility of REE is controlled by the dissolution of apatite coupled with precipitation of monazite-(Ce), which are enhanced by the addition of NaCl in the starting fluids. This coupled process can be traced by comparing the REE to P ratios in the reacted fluids with the stoichiometry of the unreacted apatite crystals. Apatite metasomatized at temperatures <300 °C is therefore controlled by dissolution rather than dissolution-reprecipitation reactions commonly observed in previous experiments conducted above 300 °C. Furthermore, this study demonstrates that the presence of NaCl plays a crucial role in increasing the solubility of apatite, which controls the availability of REE to form secondary phosphates even in mildly acidic aqueous fluids. This implies that both the effects of acidity/alkalinity of the fluids and the role of dissolved alkalis (NaCl and KCl), need to be considered for understanding the controls on REE in magmatic-hydrothermal systems. Lastly, the experiments of this study expand the known conditions at which apatite is susceptible to be overprinted by hydrothermal alteration from 900 °C down to 150 °C and highlights the necessity of appropriately screening apatite grains using backscattered electron and cathodoluminescence imaging for signs of hydrothermal alteration textures in igneous apatite.

Apatite is a ubiquitous phase in granite plutons and in most adjacent country rocks, thus contamination of a granite magma with wall-rock material results in two genetic types of apatite in the magma: cognate and foreign. These two textural and chemical varieties of apatite undergo textural and compositional changes to reach physical and chemical equilibrium (perfect assimilation) in the melt. Our experiments replicate the conditions in such contaminated granites. The starting materials consist of a peraluminous synthetic SiO 2 -Al 2 O 3 -Na 2 O-K 2 O (SANK 1.3) granite gel with A/NK of 1.3, synthetic F-apatite, synthetic Cl-apatite, and natural Durango apatite. Initial experiments in cold-seal hydrothermal pressure vessels at magmatically realistic temperatures of 750 °C and pressures of 200 MPa produced negligible reactions, even after run times of 2000 h. Instead, we used an argon-pressurized internally heated pressure vessel with a rapid-quench setup at temperatures of 1200 °C, pressure of 200 MPa, and run durations of 192 h. An advantage of this high temperature is that it exceeds the liquidus for quartz and feldspar; therefore, apatite is the only solid phase in the run products. The starting composition of each run was 90 wt% SANK 1.3 granite gel and 10 wt% crushed apatite (consisting of one, two, or three varieties), with and without 4 wt% added H 2 O. Run products were examined by SEM for texture and by EMPA and LA-ICP-MS for composition. The starting synthetic granite composition contains no Ca, F, Cl, or REEs thus, in every run, apatite was initially undersaturated in the melt. In all experiments, most large apatite grains consisted of anhedral shards with rounded corners, most small apatite grains were round, and a small proportion of apatite grains developed one or more crystal faces. In experiments with two or three apatite compositions, the run-product apatite grains had compositions intermediate between those of the starting-material grains, and they were homogeneous with respect to Cl, and probably F, but not with respect to REEs. The processes to reach textural equilibrium consist of dissolution until the melt is saturated in apatite, followed by Ostwald ripening to eliminate small grains and to develop crystal faces on larger ones. The processes to reach chemical equilibrium consist of dissolution of apatite, diffusion of cations (Ca, P, REE) and anions (F, Cl, OH) through the silicate melt, and solid-state diffusion in the undissolved apatite grains. The halogens approached chemical equilibrium in all experiments, but in the experiments containing Durango apatite, the REEs have not. Models involving radial diffusion into spherical apatite grains at the temperatures of the experiments show complete re-equilibration of the halogens, but changes in the REE concentrations affecting only the outer few micrometers. We conclude that the rate of chemical equilibrium for the halogens is greater than the rate of physical equilibrium for texture, which in turn is greater the rate of chemical equilibrium for REEs. We illustrate these processes with a natural example of contaminated granite from the South Mountain Batholith in Nova Scotia. Given that all granites are contaminated rocks, we propose that future petrogenetic studies focus on developing techniques for a minerals-based quantitative estimation of contamination (QEC).

Iron oxides and oxyhydroxides show promise as superconductor materials and as repositories of paleo-environmental information. However, there are no microscale non-destructive analytical techniques to characterize their combined mineralogy, chemical composition, and crystal properties. We address this by developing cathodoluminescence mounted on a scanning electron microscope (SEM-CL) as an in situ, non-destructive method for the crystallographic and petrographic study of iron oxides and oxyhydroxides. We show that goethite, hematite, and magnetite display different SEM-CL spectra, which may be used for mineral identification. We further show that different formation pH, manganese substitution for iron in goethite and hematite, and titanium substitution for iron in magnetite cause shifts in the SEM-CL spectra of these minerals. These spectral shifts are not always detectable as a change in the emission color but are easily discernable by quantitative analysis of the spectra. Together with subtle but observable variations in the SEM-CL spectra of natural goethite and hematite, we suggest that these dependences of the SEM-CL spectra on pH and chemical composition may be used as a means of identifying multiple episodes of mineralization and recrystallization. We apply the newly developed SEM-CL methods to two polished sections of natural samples and show that quantitative analysis of the spectra obtained allows the identification of differences between varieties of the same mineral that are not observable by other means. Like the application of SEM-CL to geologic samples in this study, we suggest that this approach may be used to explore the in situ chemistry and crystallinity of various natural and manufactured iron oxides and oxyhydroxides.

Olivine compositions are widely used to constrain magmatic thermodynamic conditions such as magmatic temperature, oxygen fugacity, and H 2 O content. However, elemental diffusion may change the initial compositions and lead to large uncertainty on the estimation of these thermodynamic conditions. In this study, we conducted LA-ICP-MS elemental mapping and EPMA analysis of olivine phenocrysts and olivine-hosted spinel from the Jiagedaqi (JGD) alkaline basalts in northeast China to evaluate the influence of elemental difusion on olivine-composition-based geothermometry, oxybarometry, and hygrometry. The JGD olivines show normal Fo [Mg/(Mg + Fe) × 100 in moles] zoning, with cores having Fo of 77–87 and rims having Fo of 67–73. The constant P contents from core to rim indicate that these compositional zonings were caused mainly by difusion. Because Al is a slow-diffusing element and its content is relatively constant from core to rim, the temperature calculated by the Al-in-olivine thermometer is not influenced by elemental difusion and preserves the JGD olivine crystallization temperature up to 1150 °C. The temperatures calculated using the Sc/Y-in-olivine thermometer, the oxygen fugacity calculated using the olivine–spinel oxybarometer, and the H 2 O content calculated on the basis of Ca partitioning between olivine and melt are strongly influenced by the diffusion of Fo, Sc/Y, and Ca. However, the compositional plateaus in olivine cores, which were not influenced by elemental diffusion, preserve the magmatic temperature (1150 °C), oxygen fugacity (QFM+1.4), and H 2 O content (4 wt%) that applied during the formation of the JGD olivines. Together, these findings suggest that the mantle source of the JGD basalts was metasomatized by fluids released from the subducted slab. This study highlights that elemental difusion in olivine phenocrysts can strongly afect the application of olivine-composition-based geothermometers, oxybarometers, and hygrometers. However, primitive olivine cores that have not been influenced by difusion preserve the initial magmatic thermodynamic conditions.

Ion adsorption-type rare earth element (REE) ore deposits in South China are a major source of heavy rare earth elements (HREE) around the world, which are of considerable economic and strategic significance. In these ores, REE is enriched in the clay minerals, specifically kaolinite and halloysite, which are derived from their parent granitoid by the weathering process. However, the mechanisms of supergene REE mineralization remain unclear. We investigated the nature and origin of supergene REE mineralization, based on a nanoscale study of a typical REE-mineralized granite regolith profile (ΣREE max = 1201 ppm) in the Dazhou super-large, ion adsorption-type REE deposit, Guangxi Province, South China. Bulk mineralogical and geochemical analyses, coupled with novel nano-characterization techniques [i.e., hollow fiber flow field-flow fractionation inductively coupled plasma-mass spectrometry (HF5-ICP-MS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)], were used to determine the nature of the nano-minerals and nanoparticles in the regolith samples. X-ray diffraction and SEM-EDS analyses revealed that ion-adsorption clay minerals are dominated by platy-shaped kaolinite and rod-like halloysite (10 and 7 Å) within the regolith. The average clay mineral contents decreased from 38 to 15% from the fully weathered horizon to the semi-weathered horizon, whereas the proportion of halloysite increased in the clay mineral fraction in the deep horizons. The REE-bearing nanoparticles consist predominantly of macromolecules of organic matter (2–5 nm) and clay minerals (5–40 and 40–80 nm) according to the HF5-ICP-MS analysis. There is a close association between REE and Al contents in particles with sizes of 5–40 nm in the semi-weathered horizons and 40–80 nm in the highly weathered horizons, which indicates that nanoscale clay minerals (halloysite and kaolinite, respectively) are important REE carriers. In addition, nanoscale secondary REE mineral phases, including oxide, silicate, and phosphate, were identified by the SEM and TEM observations. These phases are typically adsorbed onto the surfaces of clay minerals, specifically rod-like halloysite, but have different occurrences in the regolith profile. Cesium-oxide (cerianite) and Ce-silicate (cerite) occur mainly in the upper horizon of the regolith profile, whereas low-crystallinity REE phosphates [rhabdophane-(La)] occur mainly in the lower horizon of the profile. Our results indicate that nano-minerals and nanoparticles affect REE enrichment and fractionation during granite weathering. Migration and accumulation of REE-bearing nano-minerals were caused by leaching and neoformation of REE-bearing nano-minerals during secondary precipitation. These processes contribute to the formation of supergene REE mineralization in granite regolith.

To investigate the role of atomic-scale structure on the frictional properties of gibbsite, a dioctahedral-type aluminum hydroxide, we calculated the atomic-scale interlayer shear properties using the first-principles method based on density functional theory. We found that the presence of vacant sites within the octahedral sheet of gibbsite enables hydroxyls to move to more stable positions and reduce the repulsive force, leading to a lower atomic-scale shear stress of gibbsite compared with brucite, a trioctahedral-type magnesium hydroxide. We also estimated the macroscopic single-crystal friction coefficient of gibbsite with the assumption that only the atomic-scale interlayer friction controls macroscopic friction. The estimated single-crystal friction coeficient for gibbsite is 0.36(6), which is clearly lower than the experimentally obtained friction coeficient of the powdered gouge of gibbsite (0.74). This diference between the interlayer friction coeficient and gouge friction coeficient suggests the presence of additional mechanisms that afect the frictional strength, such as microstructures within a fault gouge.

This paper presents the proceedings of the data analysis of the year and country of mineral discoveries with their Nickel-Strunz classes and rarity to enrich our knowledge of the evolution of mineral discoveries and their spatial distribution during different periods. Based on the dynamic of mineral discovery, three principal periods were identified: (1) Ancient period (up to 1800) of irregular mineral records; (2) Sustainable development period (1800–1949) with regular records and a moderate increase in the total number of minerals; and (3) Modern period (1950–present) of rapid development. It is pointed out that the timeline of mineral discoveries exhibits local anomalies. The positive anomalies were linked to the publications of mineralogical encyclopedias and classifications, while the negative ones were caused mainly by historical events, suppressing scientific activity. The majority of rock-forming and widespread minerals were discovered before the 1980s, while the discovery rate of rare and endemic species still progresses due to the study of hard-to-reach locations and the introduction of high-resolution analytical methods. A comparison of Nickel-Strunz class counts throughout mineral history revealed that the fraction of carbonates, oxides, and elements have drastically decreased during the Sustainable development period and the Modern period with a minor increase of elements during the last period. However, opposite behavior is observed for the phosphates, sulfates, and sulfides, with a sudden decrease in sulfates during the Modern period. On the other hand, the fraction of borates, halides, and silicates remained unchanged during all periods. Spatial analysis of the data showed that the distribution of mineral discoveries on the world map depends not only on the country’s geology but also on the area, population, economic development, and general interest in science.

Clay minerals are among the most important reactive components of soil systems, acting as a bridge linking organic and inorganic components. Lithology is a key factor in clay-mineral genesis and transformation, yet it has received scant attention to date at the nanoscale. Inferences regarding pedogenic clay-mineral transformations based on X-ray diffraction (XRD) are sometimes speculative, whereas mineralogic relationships documented by high-resolution transmission electron microscopy (HRTEM) are more robust due to direct evidence from lattice-fringe observations. In this contribution, the mineralogical and geochemical characteristics of four soils derived from different parent rock types (a gneiss, an Fe-rich siltstone, a sandstone, and a dolostone) from subtropical China were determined using HRTEM, XRD, and geochemical elemental data. The predominance of 2:1 clay minerals and kaolinite in the investigated soils is typical of subtropical climatic settings. Lattice-fringe images suggest the prevalence of topotactic transformations during clay-mineral alteration. Two distinct alteration pathways were observed in the investigated soils, one starting with chlorite and the other with illite, with convergence of mineralogic compositions toward kaolinite and crystalline iron and aluminum (oxyhydr)oxides. In the early stages of weathering, chlorite transformed into expandable clays through a continuous, solid-state mechanism with corrensite and/or randomly interstratified chlorite-vermiculite/chlorite-smectite as intermediate products. Unlike chlorite, which tends to form a 1:1 regularly interstratified phase, the weathering of illite commonly starts at layer edges. Under subtropical monsoonal climates, the precursor minerals in host rocks and aeolian materials determine the starting composition and, to a certain extent, the trajectory of clay-mineral transformation over time. With advanced weathering, mineralogic convergence toward kaolinite and Fe/Al-(oxyhydr)oxides tends to obscure the initial substrate composition. This study advances our understanding of the role of parent lithology in clay-mineral evolution at the nanoscale.

Iron phosphides with significant variations of Cr (up to 18 wt%) and V (up to 8.6 wt%) contents were detected in gehlenite-bearing breccia at the Hatrurim Complex, Negev desert, Israel. Investigations of the composition and structure of the Fe 2 P phosphides showed that when the V+Cr content is higher than 0.26 apfu (atoms per formula unit), a transition from the hexagonal barringerite ( P 6̅2 m ) to orthorhombic allabogdanite ( Pnma ) takes place. According to the experimental data, allabogdanite is a high-pressure (>8 GPa) polymorph of barringerite. Pseudowollastonite associated with Cr-V-bearing allabogdanite is an indicator of phosphide crystallization at high temperature (>1200 °C) and low pressure. Thus, at the low pressure close to ambient, when more than 13 at% Fe in Fe 2 P is substituted by Cr and V, the orthorhombic polymorph is stable. The orthorhombic phosphide with the highest Cr and V contents belongs to the andreyivanovite species with the FeCrP end-member formula. This is the first finding on Earth of that very rare mineral described from the Kaidun meteorite. Some Cr-V-bearing phosphides have an unusual morphology, which cannot be explained by crystallization from a melt. More probably, these phosphides can form in the process of replacing fish bone remains. We believe that sedimentary protolith was not thermally altered and contained a significant amount of bituminous organic matter and phosphorite inclusions. Injecting paralava into the sedimentary rocks determines the conditions for phosphide formation on the boundary of these rocks as a result of the high-temperature carbothermal reduction process.

Impact events modify and leave behind a complex history of rock metamorphism on terrestrial planets. Evidence for an impact event may be recorded in physical changes to minerals, such as mineral deformation and formation of high P - T polymorphs, but also in the form of chemical fingerprints, such as enhanced elemental diffusion and isotopic mixing. Here we explore laboratory shock-induced physical and chemical changes to zircon and feldspar, the former of which is of interest because its trace elements abundances and isotope ratios are used extensively in geochemistry and geochronology. To this end, a granular mixture of Bishop Tuff sanidine and Kuehl Lake zircon, both with well characterized Pb isotope compositions, was prepared and then shocked via a flat plate accelerator. The peak pressure of the experiment, as calculated by the impedance matching method, was ~24 GPa although a broader range of P - T conditions is anticipated due to starting sample porosity. Unshocked and shocked materials were characterized via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and Raman spectroscopy. These methods show that the starting zircon material had abundant metamict regions, and the conversion of the feldspar to glass in the post-shock material. Analyses of the shocked product also yielded multiple occurrences of the high-pressure ZrSiO 4 polymorph reidite, with some domains up to 300 μm across. The possibility of U-Pb system disturbance was evaluated via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS). The isotopic data reveal that disturbance of the U-Pb geochronometer in the reidite was minimal (<2% for the main U-Pb geochronometers). To better constrain the P - T conditions during the shock experiment, we complement impedance matching pressure calculations with iSALE2D impact simulations. The simulated results yield a range of P - T conditions experienced during the experiment and show that much of the sample may have reached >30 GPa, which is consistent with formation of reidite. In the recovered shocked material, we identified lamellae of reidite, some of which interlock with zircon lamellae. Reidite {112} twins were identified, which we interpret to have formed to reduce stress between the crystal structure of the host zircon and reidite. These two findings support the interpretation that shear transformation enabled the transition of zircon to reidite. The size and presence of reidite found here indicate that this phase is probably common in impact-shocked crustal rocks that experienced ~25 to ~35 GPa, especially when the target material has porosity. Additionally, shock loading of the zircon and transformation to reidite at these pressures in porous materials is unlikely to significantly disturb the U-Pb system in zircon and that the reidite inherits the primary U and Pb elemental and isotopic ratios from the zircon.

Ice-VII is a high-pressure polymorph of H 2 O ice and an important mineral widely present in many planetary environments, such as in the interiors of large icy planetary bodies, within some cold subducted slabs, and in diamonds of deep origin as mineral inclusions. However, its stability at high pressures and high temperatures and thermoelastic properties are still under debate. In this study, we synthesized ice-VII single crystals in externally heated diamond-anvil cells and conducted single-crystal X-ray difraction experiments up to 78 GPa and 1000 K to revisit the high-pressure and high-temperature phase stability and thermoelastic properties of ice-VII. No obvious unit-cell volume discontinuity or strain anomaly of the high-pressure ice was observed up to the highest achieved pressures and temperatures. The volume-pressure-temperature data were fitted to a high-temperature Birch-Murnaghan equation of state formalism, yielding bulk modulus K T0 = 21.0(4) GPa, its first pressure derivative ′ KT 0 = 4.45(6), d K /d T = –0.009(4) GPa/K, and thermal expansion relation α T = 15(5) × 10 –5 + 15(8) × 10 –8 × ( T – 300) K –1 . The determined phase stability and thermoelastic properties of ice-VII can be used to model the inner structure of icy cosmic bodies. Combined with the thermoelastic properties of diamonds, we can reconstruct the isomeke P - T paths of ice-VII inclusions in diamond from depth, ofering clues on the water-rich regions in Earth’s deep mantle and the formation environments of those diamonds.

Refractive indices of minerals and inorganic compounds can be calculated from their chemical compositions using the additivity rule for electronic polarizabilities and converting the sum of polarizabilities α using the Anderson-Eggleton relationship: α A E = n D 2 − 1 V m 4 π + 4 π 3 − 2.26 n D 2 − 1 $$\alpha_{\mathrm{AE}}=\frac{\left(n_{\mathrm{D}}^{2}-1\right) V_{\mathrm{m}}}{4 \pi+\left(\frac{4 \pi}{3}-2.26\right)\left(n_{\mathrm{D}}^{2}-1\right)} $$ with the molar volume V m solved for the mean refractive index n D at 589.3 nm. Whereas the polarizability of cations is a single parameter, the polarizability of anions is described by a two-parameter term α − = α − 0 ⋅ 10 − N o / V a n 1.20 ${\alpha}_{-}=\alpha_{-}^{0} \cdot 10^{-N_\mathrm{o} / V_{\mathrm{an}}^{1.20}}$with α – = anion polarizability, V an = anion molar volume, and the two least-squares parameters α − 0 $\alpha_{-}^{0}$ (corresponding to free-ion polarizability) and N o . For hydroxyls, Shannon and Fischer (2016) introduced different parameter sets for non-H-bonded hydroxyls α − o $\left(\alpha_{-}^{\mathrm{o}}\right.$ = 1.79 Å 3 , N o = 1.792 Å 3.6 ) and moderately strong H-bonded hydroxyls α − 0 $\left(\alpha_{-}^{0}\right.$ = 1.73 Å 3 , N o = 2.042 Å 3.6 ). In an effort to understand the lower polarizability of the H-bonded hydroxyl ions, we have evaluated observed and calculated polarizabilities, O-H, H∙∙∙O, O∙∙∙O distances, and O-H∙∙∙O angles in 10 minerals with non-hydrogen-bonded hydroxyls (mean <O∙∙∙O> distance 3.143 Å, mean <H∙∙∙O> distance 2.352 Å), in seven minerals with H-bonded-hydroxyls (<O∙∙∙O> = 2.739 Å, <H∙∙∙O> = 1.856 Å), and in 10 minerals with very strongly H-bonded hydroxyls (<O∙∙∙O> = 2.531 Å, <H∙∙∙O> = 1.525 Å). On the basis of quantum chemical cluster calculations using atomic parameters of well determined crystal structures of hydroxyl containing compounds, we found that calculated intrinsic polarizabilities of OH are correlated with the hydrogen bond lengths H∙∙∙O and O∙∙∙O between donor and acceptor of the H-bond. This is demonstrated for LiOH, brucite [Mg(OH) 2 ], portlandite [Ca(OH) 2 ], clinometaborite (β-HBO 2 ), sassolite (H 3 BO 3 ), archerite (KH 2 PO 4 ), kalicinite (KHCO 3 ), metaborite (γ-HBO 2 ), and NaPO 2 (OH) 2 . Thus, we find that these summed intrinsic polarizabilities for OH-bonds which are involved in H-bonding are significantly lower than the corresponding summed intrinsic polarizabilities for OH-bonds not involved in H-bonding. We attribute the reduction in polarizability of hydroxyl ions in clinometaborite, sassolite, archerite, kalicinite and metaborite, and the compound NaPO 2 (OH) 2 to the presence of H-bonds and a reduction of Hirshfeld atomic charge on the O atom.

The 3.65 Å phase [MgSi(OH) 6 ] is a hydrous phase that is predicted to be stable in a simplified MgO-SiO 2 -H 2 O (MSH) ternary system at pressures exceeding 9 GPa. Along cold subduction zones, it is likely to transport water, bound in its crystalline lattice, into the Earth’s interior. The 3.65 Å phase consists of Mg and Si octahedral sites attached to the hydroxyl group that forms a hydrogen bond and is predicted to undergo pressure-induced symmetrization of the hydrogen bond. Therefore, in this study, we investigate the high-pressure behavior of the 3.65 Å phase using Raman spectroscopy. We have conducted five distinct compressions up to ~60 GPa using two different pressure-transmitting media—alcohol mixture and neon. At ambient conditions, we identified vibrational modes using complementary first-principles simulations based on density functional perturbation theory. Upon compression, we note that the first derivative of the vibrational modes in the lattice region stifens, i.e., b i lattice  > 0. $b_{\mathrm{i}}^{\text {lattice }}>0.$In contrast, the hydroxyl region softens, i.e., b i O H > 0. $b_{\mathrm{i}}^{\mathrm{OH}}>0.$This is indicative of the strengthening of hydrogen bonding upon compression. We noticed a significant broadening of vibrational modes related to hydroxyl groups that are indicative of proton disorder. However, within the maximum pressures explored in this study, we did not find evidence for pressure-induced symmetrization of the hydrogen bonds. We used the pressure derivative of the vibrational modes to determine the ratio of the bulk moduli and their pressure derivative. We note that the smaller bulk moduli of hydrous phases compared to the major mantle phases are compensated by significantly larger pressure derivatives of the bulk moduli for the hydrous phases. This leads to a significant reduction in the elasticity contrast between hydrous and major mantle phases. Consequently, the detection of the degree of mantle hydration is likely to be challenging at greater depths.

Stishovite, a rutile-structured polymorph of SiO2, is a main component of subducted basaltic lithologies in the lower mantle. At mid lower-mantle depths, a second-order ferroelastic transition to orthorhombic CaCl 2 -type (post-stishovite) structure occurs, causing extensive elastic shear softening. Previous studies showed that Al incorporation can decrease the transition pressure, while it is still debated whether H has a similar effect. Here we report the equations of state, structural evolution, and phase transformation of Si 0.948 Al 0.052 O 1.983 H 0.018 (Al5) stishovite and Si 0.886 Al 0.114 O 1.980 H 0.074 (Al11) post-stishovite samples using diamond-anvil cells in combination with synchrotron X-ray diffraction and Raman spectroscopy. The Al5 sample transformed to the orthorhombic polymorph upon compression to 16 GPa, displaying a drop of ~12% in its bulk modulus across the transformation. The Al11 sample did not undergo any phase transition in the pressure range investigated. Single-crystal structural refinements and Raman spectroscopy measurements on the Al5 sample show that the soft optic mode B 1g is decoupled from the tetragonal-to-orthorhombic structural transformation and shows a plateau in the stability field of post-stishovite, between 20 and 30 GPa. This observation indicates that the transformation is not pseudo-proper ferroelastic as in SiO 2 stishovite and that existing Landau expansions are likely not applicable to H-rich Al-bearing silica samples. Using the equation of state parameters of orthorhombic Al5 and Al11 and literature data on SiO 2 post-stishovite we then discuss the possibility of non-ideal mixing along the SiO 2 -AlOOH join.

This issue of New Mineral Names provides a summary of the newly described minerals donowensite, mikehowardite, bortolanite, fluorsigaiite, alumolukrahnite, ferro-ferri-katophorite, tomsquarryite, and argentotetrahedrite-(Zn).