Search

Mineral dehydration in the subduction zone enormously affects Earth's geodynamics and the global geochemical cycles of elements. This work uses Raman spectroscopy and X-ray diffraction to investigate the dehydration process of antigorite under compression and shear loading conditions in a rotational diamond anvil cell (RDAC) at room temperature. In order to compare the shear effects, T301 stainless steel and Kapton plastic are applied as the gasket materials. In the experiment using a high-strength T301 stainless steel gasket, two new broad OH-stretching peaks of H2O and H3O2− appear at 3303 and 3558 cm−1, respectively, at 1.7 GPa. The original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 remain, while the central pressure is increased to 8.0 GPa, and the largest pressure gradient is about 2.5 GPa in the sample chamber. In another experiment with a low-strength gasket of Kapton plastic, two new OH-stretching broad peaks of H2O and H3O2− also start to appear at 3303 and 3558 cm−1, respectively, at a lower pressure of 0.3 GPa, but the original sharp OH-stretching peaks of antigorite at 3668 and 3699 cm−1 almost completely vanish as the central pressure reaches 3.0 GPa, with the largest pressure gradient at around 4.8 GPa. The comparison between the two experiments shows that antigorite is easier to dehydrate in the chamber of a Kapton plastic gasket with a larger gradient of shear stress. However, its axial compression stress is lower. The high-pressure Raman spectra of MgO2(OH)4 octahedron and SiO4 tetrahedron in the low wavenumber zones (100–1200 cm−1) combined with the micro-beam X-ray diffraction spectrum of the recovered product strongly support the structural breakdown of antigorite. This investigation reveals that the water-bearing silicate minerals have strong shear dehydration in the cold subduction zone of the plate, which has important applications in predicting the physical and chemical properties of subduction zones and deducing the rate of plate subduction. [ABSTRACT FROM AUTHOR]

Hydrous minerals in the subducting slabs are potential water carriers into the deep mantle, and thus the synthesis of new hydrous phases is significant in our understanding of water circulation throughout the Earth's interior. In this study, we report the two new hydrous phases, Al2SiO6H2 and Al5.5Si4O18H3.5 (hereafter referred to simply as phases Psi and Phi, respectively), which are synthesized in the Al2O3-SiO2-H2O system at 15.5 GPa, 1400°C and 17.5 GPa, 1600°C by using Sakura 2500-ton multi-anvil apparatus. The luminescence spectra of Cr3+ show the phase Psi has characteristic peaks at 687, 693 and 705 nm, while phase Phi has characteristic peaks at 691, 696 and 708 nm. Single-crystal X-ray diffraction (SCXRD) refinements yield a monoclinic structure of both phases (space group P21) with ideal chemical formulae of Al2SiO6H2 and Al5.5Si4O18H3.5, respectively. The determined lattice parameters for phase Psi are a=9.4168±0.0016 Å, b=4.3441±0.0007 Å, c=9.4360±0.002 Å, and β=119.726±0.005° at ambient pressure and 300 K, while the phase Phi has a=7.2549±0.0018 Å, b=4.3144±0.001 Å, c=8.0520±0.002 Å, and β=01.740±0.009° at ambient pressure and 250 K. Electron microprobe analyses (EPMA) show the chemical compositions of phases Psi and Phi to be Al1.99Si0.85O6H2.62 and Al5.58Si2.81O18H8.03, respectively, which slightly deviate from the ideal formulae inferred from SCXRD measurements. This may result from the disorder or substitution of Al and Si by H in the crystal structures under our synthesis conditions. Our study suggests that phases Psi and Phi are the two potential water carriers at the upper part of the mantle transitions zone, providing new insights into how deep water is stored in this region. [ABSTRACT FROM AUTHOR]

δ-AlOOH is regarded as a potential water carrier that is stable in the Earth's lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth's interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A1 mode, which corresponds to a lattice vibration of the AlO6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm−1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P21nm-to-Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P21nm-to-Pnnm transition. The continuous decrease in R2-R1 splitting with pressure indicated that the distortion of the AlO6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron difraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa. [ABSTRACT FROM AUTHOR]

Ti[O.sub.2] solubility in rutile-saturated felsic melts and coexisting minerals was determined at 1.5-3.5 GPa, 750-1250[degrees]C, and 5-30 wt% [H.sub.2]O. Ti[O.sub.2] solubility in the melt primarily increases with temperature and melt basicity; it increases slightly with water content in the melt, and it decreases with pressure. A general Ti[O.sub.2] solubility model was obtained and is expressed as: ln[(Ti[O.sub.2]).sub.melt] = ln[(Ti[O.sub.2]).sub.rutile] + 1.701 - (9041/T) - 0.173P + 0.348FM + 0.016[H.sub.2]O, where Ti[O.sub.2] and [H.sub.2]O are in wt%, T is in Kelvin, P in GPa, and FM is the melt composition parameter given by FM = (1/Si)*[Na + K + 2(Ca + Fe + Mn + Mg)]/ A1, in which the chemical symbols represent cation fractions. Ti[O.sub.2] solubility in amphibole, garnet, and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived. These data were used to assess the protolith Ti[O.sub.2] content required for futile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition. The results show that only 0.8-1.0 wt% Ti[O.sub.2] is required for rutile saturation during low-degree ( Archean tonalites-trondhjemites-granites (TTG) are widely accepted to be the products of low-degree melting of metabasalts at the amphibolite to eclogite transition, with rutile being present in the residue. Comparison of natural TTG compositions with our experimental rutile solubility data indicates that the dominant TTG magmas were produced at temperatures of 750-950[degrees]C, which requires that the partial melting occurred at hydrous conditions. Models involving melting at the base of oceanic plateaus are inadequate to explain TTG genesis because the plateau root zones are likely dominated by anhydrous cumulates. A slab-melting model satisfies the requirement of a hydrous metabasalt, which during subduction would melt to produce voluminous TTG melts under high Archean geothermal gradients. The geothermal gradients responsible are estimated to be between 10 and 19[degrees]C/km based on a pressure range of 1.5-2.5 GPa for the amphibolite to eclogite transition. Keywords: Rutile, amphibolite to eclogite transition, partial melting, TTG, Archean subduction

The structural stability of gibbsite γ-Al(OH)3 was investigated under high pressure and shear stress conditions by using a rotating diamond anvil cell (RDAC) combining with micro-laser Raman spectroscopy and micro-beam X-ray diffraction. As increasing pressure to 1.5 GPa and rotating to 180°, gibbsite triggers a new structural change at room temperature. In the range of high-wavenumbers, the four peaks of hydroxyl vibration (3 363, 3 434, 3 524 and 3 618 cm-1) disappear gradually, and two new peaks with different intensities appear at 3 303 and 3 560 cm-1. In the range of low-wavenumbers, the intensities of Raman spectra are getting weaker and weaker, but the broad peaks of amorphism are not observed. Both of the double peaks (568, 539 cm-1) of Al-O-Al deformation vibration and the shoulder peaks (321 and 307 cm-1) of Al-O stretching vibration merge into one peak, respectively. However, the four vibration peaks of hydroxyl deformation vibration (1 052, 1 018, 981 and 922 cm-1) still remain. In addition, further increasing pressure to 3.5 GPa and rotating to 360°, and finally decreasing to ambient pressure, two new peaks of hydroxyl stretching vibration in high-wavenumbers, and the peaks of Al-O-Al deformation vibration and Al-O stretching vibration in low-wavenumbers are still observed. Compared to the phase transition of -Al(OH)3 under quasi-hydrostatic pressure conditions, the Raman spectra and phase transition pressure in this study reveal that the -Al(OH)3 take place another new structure change under high pressure and shear stress conditions. The micro-beam X-ray diffraction spectrum of the quenched product reveals that the framework of (OH)-Al-(OH) octahedra of this new phase is still remain, but has a shorter distance between the layers of (OH)-Al-(OH) and a higher symmetry. This newfound structure change of gibbsite in this study mainly due to the inhomogeneous pressure distribution in the sample chamber (from 0.5 to 4.5 GPa). The investigation on structural stability of -Al(OH)3 under high pressure and shear stress conditions is vital for us to identify the stability of hydrous minerals in the cold subduction slabs, and to derive the physical and chemical properties of slab and its subduction rates. [ABSTRACT FROM AUTHOR]

The Yilan crater is 1.85 km in diameter and is located in the northeast of China's Heilongjiang Province. The crater is exposed in the Early Jurassic granite of the regional Paleozoic–Mesozoic granite complexes. The southern third of the crater rim is missing, but other rim sections are well preserved, with a maximum elevation above the present crater floor of 150 m. A drillcore from the center of the structure shows that the crater fill consists of 110 m thick lacustrine sediments underlain by a 319 m thick brecciated granite unit mainly composed of unconsolidated granite clasts and fragments. Melt products derived from the target granite, which include melted (and recrystallized) granite clasts, vesicular glass, and teardrop‐shaped glass, were found in the brecciated granite unit at 218–237 m depth. Petrographic investigations of unmelted granite clasts in the brecciated granite unit from this depth interval show the presence of multiple sets of planar deformation features (PDFs) in quartz. Orientation measurements for 79 PDF sets in 38 quartz grains with a U‐stage indicate the dominance of the ω{101¯3} and π{101¯2} orientations with a relative frequency of 39% and 18%, respectively. Only 7.6% of the observed PDFs remain unindexed. The observations of PDFs with the appropriate orientations are clear evidence of shock metamorphism and thus of an impact origin of the Yilan structure. Crystallite aggregates of coesite embedded in silica glass were found in the impact‐melted granite clasts. The carbon‐14 dates of possibly impact‐produced charcoal and lacustrine sediments from the crater fill suggest a young age for the impact event of 0.0493 ± 0.0032 Ma. [ABSTRACT FROM AUTHOR]

The high-pressure behavior of manganese diselenide MnSe2 was investigated by synchrotron angle-dispersive X-ray diffraction (ADXRD) and infrared reflection spectroscopy equipped with a diamond-anvil cell. It was found that MnSe2 with a pyrite-type structure undergoes a transformation into a disordered intermediate phase at ~ 12.5 GPa, with a ground state composed of an arsenopyrite-type structure, as confirmed by laser-heating treatment. The pyrite to arsenopyrite phase transition was found to be coupled to a large collapse in the unit-cell volume (∆V ~ 19%) and an electronic transition from a high-spin to low-spin state for manganese cations (Mn2+). With a fixed value for the pressure derivation of the bulk modulus K' = 4, fitting of the pressure–volume data to a second-order Birch–Murnaghan equation of state yielded isothermal bulk modulus values of K0 = 56.1(9) GPa and K0 = 93.1(4) GPa for the pyrite-type and arsenopyrite-type phases, respectively. The measured infrared reflectivity (Rsd) for MnSe2 showed a drastic increase at pressures between 13 and 20 GPa, but became insensitive to pressure under further compression, implying a pressure-induced transition from an insulator to metallic state. [ABSTRACT FROM AUTHOR]

Rhenium tablet is a frequently used gasket material at the ultra-high pressures in Diamond anvil cell (DAC) experiment. Water in deep Earth is the link between material exchange and energy circulation in the Earth's interior. It is greatly scientific and technical significance on the study of chemical reaction of Re- H2O system at high pressures and temperatures. Microscopic observations and Raman measurements show that the Re-H2O system takes place the redox reaction ... under the conditions of high pressures and high temperatures, and produce rhenium oxide (β-ReO2) with Re4+ and atomic hydrogen (H). Observed fourteen characteristic Raman peaks of oxidation product ReO2 have a continuous unequal shift to lower Raman frequencies with the release of pressure. Reduction product H does not further take place interreaction with the water molecules, rhenium metals and their reaction products β-ReO2 and atomic H under high pressures. But the hydrogen molecules are formed when the pressure is released to near atmospheric pressure. The chemical reaction of Re-H2O system under the conditions of high-pressure and temperature reveals that water (hydroxyl) can decompose to produce atomic hydrogen in the Earth's interior with the high pressure, high temperature and reductive material. This discovery not only provides a new experimental evidence for the conversion of water to hydrogen in deep Earth, but also gives important basis for exploring the possible geochemical behaviors of water in the Earth's interior. [ABSTRACT FROM AUTHOR]

Pyrite structure CuS2 was synthesized in diamond anvil cell at high pressures and high temperatures. Using Raman spectroscopy and synchrotron X-ray diffraction, the pyrite-type CuS2 was found to be stable in 0-30 GPa without any phase transition. Raman spectroscopy show that all observed Raman frequencies increasemonotonously with increasing pressures. Fitting experimental pressure and volume data of X-ray diffraction with Birch-Murnaghan equation of state, gives V0 = 193.8(5) ų, K0 = 99(2) GPa and K0' = 4 (fix). The dependencies of Raman frequencies and unit-cell volumes with pressures are coincident with the results of first-principles calculation. The results of calculation properly depict that of experiments. Compared with other pyrite structure transition-metal disulfides MS2(M = Mn, Fe, Co, Ni), the length of M--S dominates the unit-cell volume and compressibility of MS2, and the Cu cation tends to be +2 valance in the CuS2. This study makes up for the lack of high-pressure Raman and XRD research of CuS2, and confirms structural stability of pyrite-type CuS2 at high pressures and high temperatures. The results are important for comprehending the physical and chemical properties of CuS2 and realizing the unified law of pyrite structure materials. It's also meaningful in discussion of the valance and distribution of copper in deep Earth. [ABSTRACT FROM AUTHOR]

The structural stability of cubic Ho2O3 under high pressure has been investigated by angle-dispersive x-ray diffraction (ADXD) in a diamond anvil cell up to 63.0 GPa at room temperature. The diffraction data reveal two structural transformations on compression. The structural transformation from a cubic to a monoclinic structure starts at 8.9 GPa and is complete at 16.3 GPa with a ∼8.1% volume collapse. A hexagonal phase begins to appear at ∼14.8 GPa and becomes dominant at 26.4 GPa. This high-pressure hexagonal phase with a small amount of retained monoclinic phase is stable up to the highest pressure of 63.0 GPa in this study. After release of pressure, the hexagonal phase transforms to a monoclinic structure. A third-order Birch-Murnaghan fit yields zero pressure bulk moduli (B0) of 206(3), 200(7) and 204(19) GPa and their pressure derivatives (B0') of 4.8(4), 2.1(4), 3.8(5) for the cubic, monoclinic and hexagonal phases, respectively. Comparing with other rare-earth sesquioxides, it is suggested that the transition pressure from cubic to monoclinic phase, as well as the bulk modulus of the cubic phase, increases with the decreasing of the cation radius of rare-earth sesquioxides. [ABSTRACT FROM AUTHOR]

High-pressure transformation and compression behavior of Gd2O3 were investigated using synchrotron radiation x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. The structural transformation from a cubic to a monoclinic phase occurred during the sample precompression process. Phase transitions from both the cubic and the monoclinic polymorphs to a hexagonal structure were observed. The hexagonal phase was stable up to the highest pressure in this study and was not quenchable and transformed to a monoclinic phase after pressure release. The bulk moduli of Gd2O3 for the cubic, monoclinic, and hexagonal phases were obtained by fitting the compression data to the Birch–Murnaghan equation of state. Moreover, an anomaly of the hexagonal type Gd2O3 was observed. [ABSTRACT FROM AUTHOR]

In situ X-ray diffraction and Raman scattering experiments using a diamond anvil cell revealed that Im-3-type KSbO3 remains stable up to 40.5 GPa with a bulk modulus K0 = 101.6 (7) GPa. Rietveld structure refinements and mode Grüneisen parameters suggested that the stability mechanism of this three-dimensional cubic tunnel structure was attributed to the isotropic compression for all types of Sb-O bonding in the unit of SbO6 octahedron. Isotropic structure adjustment with external pressure reflected the nature that Im-3-type KSbO3 model structure has a high ionic tolerance with a change in the chemical pressure in the isomorphous substitutions. [ABSTRACT FROM AUTHOR]

The high-pressure minerals of reidite and coesite have been identified in the moderately shock-metamorphosed gneiss (shock stage II, 35-45 GPa) and the strongly shock-metamorphosed gneiss (shock stage III, 45-55 GPa), respectively, from the polymict breccias of the Xiuyan crater, a simple impact structure 1.8 km in diameter in China. Reidite in the shock stage II gneiss displays lamellar textures developed in parental grains of zircon. The phase transformation of zircon to reidite likely corresponds to a martensitic mechanism. No coesite is found in the reidite-bearing gneiss. The shock stage III gneiss contains abundant coesite, but no reidite is identified in the rock. Coesite occurs as acicular, dendritic, and spherulitic crystals characteristic of crystallization from shock-produced silica melt. Zircon in the rock is mostly recrystallized. The postshock temperature in the shock stage III gneiss is too high for the preservation of reidite, whereas reidite survives in the shock stage II gneiss because of relatively low postshock temperature. Reidite does not occur together with coesite because of difference in shock-induced temperature between the shock stage II gneiss and the shock stage III gneiss. [ABSTRACT FROM AUTHOR]

BaWO4-II has been synthesized at 5 GPa and 610°C. Its high pressure behavior was studied by in situ synchrotron X-ray diffraction measurements at room temperature up to 17 GPa. BaWO4-II retains its monoclinic structure. Bulk and axial moduli determined by fitting a third-order Birch–Murnaghan equation of state to lattice parameters are: K 0=86.2±1.9 GPa, K 0(a)=56.0±0.9 GPa, K 0(b)=85.3±2.4 GPa, and K 0(c)=146.1±3.2 GPa with a fixed K′=4. Analysis of axial compressible modulus shows that the a-axis is 2.61 times more compressible than the c-axis and 1.71 times more compressible than the b-axis. The beta angle decreases smoothly between room pressure and 17 GPa from 93.78° to 90.90°. [ABSTRACT FROM AUTHOR]

- The 1.8 km-diameter Xiuyan crater is an impact structure in northeastern China, exposed in a Proterozoic metamorphic rock complex. The major rocks of the crater are composed of granulite, hornblendite, gneiss, tremolite marble, and marble. The bottom at the center of the crater covers about 100 m thick lacustrine sediments underlain by 188 m thick crater-fill breccia. A layer of polymict breccia composed of clasts of granulite, gneiss, hornblendite, and fragments of glass as well as clastic matrix, occurs near the base, in the depth interval from 260 to 295 m. An investigation in quartz from the polymict breccia in the crater-fill units reveals abundant planar deformation features (PDFs). Quartz with multiple sets of PDFs is found in clasts of granulite that consist of mainly quartz and feldspar, and in fine-grained matrix of the impact-produced polymict breccia. A universal stage was used to measure the orientation of PDFs in 70 grains of quartz from five thin sections made from the clasts of granulite of polymict breccia recovered at the depth of 290 m. Forty-four percent of the quartz grains contain three sets of PDFs, and another 40% contain two sets of PDFs. The most abundant PDFs are rhombohedron forms of , , and with frequency of 33.5, 22.3, and 9.6%, respectively. A predominant PDF form of in quartz suggests a shock pressure >20 GPa. The occurrence of PDFs in quartz from the polymict breccia provides crucial evidence for shock metamorphism of target rocks and confirms the impact origin of this crater, which thus appears to be the first confirmed impact crater in China. [ABSTRACT FROM AUTHOR]

Abstract: Abundant coesite and quartz coexist with the shock-produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the Xiuyan crater. Quartz occurs as idiomorphic, needle-like and spherulitic crystals, and polycrystalline aggregates. Coesite occurs as granular or idiomorphic, dendritic and needle-like crystals, and polycrystalline aggregates of stringers. The occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt. During decompression, coesite firstly crystallizes from shock-produced dense silica melt at high pressure. As pressure decreases to the stability field of quartz, quartz crystallizes from the melt, whereas coesite remains metastable state. A growth rate of 10− 3 m/s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10ms in the coesite stability field for the crater. Higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially. The occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater. [ABSTRACT FROM AUTHOR]

Studies the hydrocarbon formation by reaction of graphite with water-containing minerals under high pressure and high temperature. Indication of gas chromatographic analysis; Formation of abiogenetic hydrocarbon; Preservation of graphite.

Investigates the mechanism of formation of abiogenetic hydrocarbons at the depth of the Earth. Open and non-equilibrium state of the reactions between carbonates and water or water-bearing minerals; Chromatographic analyses of the gas products.

[Display omitted] • Raman spectra of natural fluorapophyllite-(K) were collected in the interval of 0.0–50.7 GPa. • Two clear discontinuous changes in the Raman spectra indicate high-pressure-induced phase transition and amorphization. • The H 2 O structural units in fluorapophyllite-(K) are more prone to destabilization than the rigid SiO 4 units. • H 2 O groups in fluorapophyllite-(K) are sensitive local probes for the pressure-induced phase transition and amorphization. The high-pressure Raman spectra in the low-wavenumber region (50–1200 cm−1) and the hydroxyl stretching vibration region (2600–3800 cm−1) of fluorapophyllite-(K) were collected in the interval of 0.0–50.7 GPa. Experimental results show that fluorapophyllite-(K) undergoes a crystalline-crystalline phase transition from a tetragonal structure (P4/mnc) to an orthorhombic structure (Pnnm) and then amorphization under high pressure conditions. Discontinuous changes in Raman peaks in the hydroxyl vibration region (2600–3800 cm−1) and the low-wavenumber region (50–1200 cm−1) occur at different onset pressures, suggesting that the destabilization of sub-lattices in fluorapophyllite-(K) under high pressure is not synchronous; the weak interlayer H 2 O structural units are more prone to destabilization at lower pressures than the rigid SiO 4 tetrahedral layer units. Therefore, H 2 O groups in fluorapophyllite-(K) serve as sensitive local probes during the pressure-induced phase transition and amorphization. [ABSTRACT FROM AUTHOR]

Mixtures of CO2-NaCl-H2O contained in synthetic fluid inclusions are studied by laser Raman spectroscopy at high temperatures. With increasing temperature, the band splitting (X) of υ1-2υ2 diad of spectrum of CO2 presents more variations, and the intensity ratios of the hot bands to the υ1-2υ2 diad increase. For mixtures of gas phase rich in CO2 and liquid phase rich in H2O before homogenization, the strength of hydrogen bonding of water in the liquid phase decreases almost linearly with increasing temperature. When mixtures become homogeneous liquid phases, carbon dioxide content increases significantly as a result of the abrupt decrease in hydrogen bonds. Our results show that the hydrogen bonds change only slightly at higher temperatures above the homogeneous point, and a certain extent of the hydrogen bonds still remains at the highest temperature of 550°C of this work. The study is helpful to Raman spectroscopic analysis of natural fluid inclusions at high temperatures. [Copyright &y& Elsevier]

The high-pressure behavior of manganese diselenide MnSe2 was investigated by synchrotron angle-dispersive X-ray diffraction (ADXRD) and infrared reflection spectroscopy equipped with a diamond-anvil cell. It was found that MnSe2 with a pyrite-type structure undergoes a transformation into a disordered intermediate phase at ~ 12.5 GPa, with a ground state composed of an arsenopyrite-type structure, as confirmed by laser-heating treatment. The pyrite to arsenopyrite phase transition was found to be coupled to a large collapse in the unit-cell volume (∆V ~ 19%) and an electronic transition from a high-spin to low-spin state for manganese cations (Mn2+). With a fixed value for the pressure derivation of the bulk modulus K' = 4, fitting of the pressure–volume data to a second-order Birch–Murnaghan equation of state yielded isothermal bulk modulus values of K0 = 56.1(9) GPa and K0 = 93.1(4) GPa for the pyrite-type and arsenopyrite-type phases, respectively. The measured infrared reflectivity (Rsd) for MnSe2 showed a drastic increase at pressures between 13 and 20 GPa, but became insensitive to pressure under further compression, implying a pressure-induced transition from an insulator to metallic state. [ABSTRACT FROM AUTHOR]

High-pressure transformation and compression behavior of Gd2O3 were investigated using synchrotron radiation x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. The structural transformation from a cubic to a monoclinic phase occurred during the sample precompression process. Phase transitions from both the cubic and the monoclinic polymorphs to a hexagonal structure were observed. The hexagonal phase was stable up to the highest pressure in this study and was not quenchable and transformed to a monoclinic phase after pressure release. The bulk moduli of Gd2O3 for the cubic, monoclinic, and hexagonal phases were obtained by fitting the compression data to the Birch–Murnaghan equation of state. Moreover, an anomaly of the hexagonal type Gd2O3 was observed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrogenetic Fe-Mn micronodules have statistically high positive Ce anomalies. • REYs (except Ce) contents are lower in hydrogenetic micronodules than in crusts/nodules. • REYs (except Ce) prefer to migrate out from Fe-Mn micronodules to porewater. Ferromanganese micronodule, ubiquitous in aquatic environments, is a key authigenic phase in the rare earth elements and yttrium (REY) enriched deep-sea sediments. This is due to the high porosity and strong scavenging REY capacity of Fe-Mn (oxyhydr)oxides. In this study, micronodules were selected from surface sediments of the NW Pacific inter-seamount basins and the NE Pacific Clarion-Clipperton Zone (CCZ), both distal from oceanic ridges and largely impervious to hydrothermal activities. Hydrogenetic micronodules of this study have commonly consistent and highly positive Ce anomalies. Based on big data statistics, amounts of geochemical data of ferromanganese deposits (n > 1350) were compiled, and those of hydrogenetic ones (including micronodules, (macro)nodules and crusts) were selected. Comparative analyses reveal that both the hydrogenetic crusts (n = 289) and nodules (n = 159) have weaker positive Ce anomalies (avg. 2.23 and 2.97, respectively) than hydrogenetic micronodules (n = 204, avg. 6.27). Combined with the similar Ce content in all three hydrogenetic ferromanganese deposit types, and the lower concentrations of other REYs in hydrogenetic micronodules, the migration of REYs (except for Ce) from micronodules to porewater is indicated. While Ce is likely retained via oxidizing to insoluble Ce(IV), thus forming the highly positive Ce anomalies. Therefore, our study suggests that ferromanganese micronodules could serve as a buffer in the REY enrichment of pelagic sediments. [ABSTRACT FROM AUTHOR]