Search

Fe,Al-bearing MgSiO3 perovskite (bridgmanite) is considered to be the most abundant mineral in Earth's lower mantle, hosting ferric iron in its structure as charge-coupled (Fe2O3 and FeAlO3) and vacancy components (MgFeO2.5 and Fe2/3SiO3). We examined concentrations of ferric iron and aluminum in the perovskite phase as a function of temperature (1700–2300 K) in the MgSiO3-FeAlO3-MgO system at 27 GPa using a multi-anvil high-pressure apparatus. We found a LiNbO3-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,Fe3+,□)(Al,Fe3+)2O4 composition (□= vacancy). The FeAlO3 component in the perovskite phase decreases from 69 to 65 mol% with increasing temperature. The Fe2O3 component in the perovskite phase remains unchanged at ~1 mol% with temperature. The A-site vacancy component of Fe2/3SiO3 in the perovskite phase exists as 1–2 mol% at 1700–2000 K, whereas 1 mol% of the oxygen vacancy component of MgFeO2.5 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 FeAlO3 charge-coupled component is dominant. [ABSTRACT FROM AUTHOR]

Large single crystals of stishovite were successfully synthesized at 11 GPa from a silica solution in water. The potential of both slow cooling and thermal gradient methods were examined. The thermal gradient method provided crystals of 0.8 x 0.8 x 1.3 mm in size grown at 1350 [degrees]C and a thermal gradient of 50 [degrees]C/mm using stishovite as a silica source. The use of quartz as a source resulted in the appearance of numerous stishovite crystals in the solution interior resulting in diminished space for the growth of large crystals. This can be explained by a significant difference in the solubility of metastable quartz and stishovite in water, estimated to be 85.3 and 5.6 wt% Si[O.sub.2] at 1000 [degrees]C and 11 GPa, respectively. Crystals up to 0.8 x 1.3 x 1.5 mm were grown by the slow cooling method in the system Si[O.sub.2] + 14.7 wt% [H.sub.2]O as temperature was decreased from 1600 to 1000 [degrees]C with a cooling rate of 2 [degrees]C/min. The size of single crystals obtained was large enough to carry out silicon self-diffusion experiments, which were performed at a pressure of 14 GPa and temperatures from 1400 to 1800 [degrees]C. The lattice diffusion coefficients along the [110] and [00l] directions can be expressed as [D.sub.[110]] ([m.sup.2]/s) 4.10 x [10.sup.-12] exp [-322 (kJ/mol)/RT] and [D.sub.[001]] ([m.sup.2]/s) = 5.62 x [10.sup.-12] exp [- 334 (kJ/mol)/RT], respectively, where R is the gas constant and T is the absolute temperature. Keywords: Stishovite, single-crystal growth, high pressure, aqueous fluid, silica solubility, diffusion DOI: 10.2138/am.2010.3255

We have synthesized large (0.7-1.0 mm) crystals of anhydrous, water-bearing, and Fe-bearing wadsleyite by means of growth from solution in the thermal gradient field. Nearly anhydrous ( Keywords: [Mg.sub.2]Si[O.sub.4], wadsleyite, single crystal growth, high pressure, thermal gradient method

Phase equilibria of [Fe.sub.2][O.sub.3] have been studied up to 58 GPa and 1400 K using the Kawai-type multi anvil apparatus equipped with sintered diamond anvils. Identification of phases and pressure determination has been carried out by means of in situ X-ray observation using synchrotron radiation at SPring-8. Hematite (phase I) successively transforms to the [Rh.sub.2][O.sub.3](II)-type structure (phase II) and then to an orthorhombic structure (phase III) with increasing pressure. The transformations of hematite into high-pressure phases have been observed only at temperatures higher than 500 K, which is not concordant with previous results obtained by using the diamond anvil cell. Volume changes accompanied by the I-II and II-III transformations are calculated to be -2.8 and -5.0%, respectively. The phase boundary between I and II phases and that between II and III have been proposed to be P (GPa) = -0.015 T (K) + 44.2 and P (GPa) = -0.005 T (K) + 48.7, respectively. Possible correlation between a Mort transition and the phase stabilities may be concealed at room temperature due to slow reaction kinetics of the structural transformations. Keywords: High-pressure phase equilibria, [Fe.sub.3][O.sub.3], Kawai-cell, sintered diamond, in situ X-ray observation

Large single crystals of MgSi[O.sub.3] perovskite were successfully synthesized by a thermal gradient method at 24 GPa and 1500 [degrees]C. This was achieved by an improvement of high-pressure synthesis technique that allowed us to grow single crystals under such ultrahigh-pressure conditions in relatively large volumes (e.g., 10 [mm.sup.3]). Since crystal growth is hindered by neighboring crystals, the nucleation density was suppressed by reducing the thermal gradient to 20 [degrees]C/mm, permitting an increase in free space for large crystal growth. KHC[O.sub.3]-Mg[(OH).sub.2] solvent can be used to grow perovskite crystals. However, the carbonate solvent produces melt inclusions. Silicate sources with MgSi[O.sub.3] composition produce stishovite inclusions, which in turn cause splitting of perovskite crystals. The formation of these inclusions is avoided by using [H.sub.2]O as a solvent and 85MgSi[O.sub.3]-15[Mg.sub.2]Si[O.sub.4] as a silicate source. The [H.sub.2]O also allows homogeneous crystal growth, probably because of its low viscosity and high silicate solubility. High-quality single crystals larger than 1 mm were successfully synthesized through these technical developments. Keywords: MgSi[O.sub.3], perovskite, single-crystal growth, high pressure, temperature gradient method, growth from solution

Six single crystals of [Mg.sub.3]([Mg.sub.x][Si.sub.x][Al.sub.2-2x][Si.sub.3][O.sub.12] with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 [degrees] C with a '6-8' type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors ([B.sub.eq]), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 [less than or equal to] x [less than or equal to] 0.24 show no birefringence, whereas those of x = 0.64 have a slight optical anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using synchrotron X-ray radiation is tetragonal with a slight deviation from cubic. On the basis of site splitting expected from compositional dependence of [B.sub.eq] obtained by cubic refinement, the most probable space group in the range 0.38 [less than or equal to] x [less than or equal to] 0.64 is I[4.sub.1]/acd (tetragonal), which is the maximal subgroup of the space group [Mathematical Expression Omitted] (cubic). Given that the previous reports that crystals with 0.8 [less than or equal to] x [less than or equal to] 1.0 have the tetragonal space group I[4.sub.1]/a, the majorite solid-solution in this system undergoes the series of symmetry changes, [Mathematical Expression Omitted], with increasing MgSi[O.sub.3] component. The symmetry changes from [Mathematical Expression Omitted] to I[4.sub.1]/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral ([Mg.sup.2+]) and tetrahedral ([Si.sup.4+]) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I[4.sub.1]/acd structure loses the center of symmetry with the symmetry reduction from [Mathematical Expression Omitted] to I[4.sub.1]/acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.

Pressless split-sphere apparatus (BARS) equipped with an 8-6 type multi-anvil system is used extensively for large (6 carats) gem quality diamond crystal growth and various mineralogical and geochemical studies at shallow mantle conditions (P ≤ 7.5 GPa). In this study, we extend the pressure range for this apparatus to the conditions of the Earth's transition zone (P = 20 GPa) using a 6-8 multi-anvil system. A scaled-up Kawai-cell consisting of 47 mm WC cubic anvils with 10 mm truncation and 18 mm MgO pressure medium was employed for the experiments. A 300 mm spherical multi-anvil block compressed by a hydraulic system with 2500 atm capacity was used to compress the Kawai-cell. A pressure of 15.5 GPa was successfully generated at the oil pressure of 1875 atm. The comparison of the BARS apparatus calibrations with those obtained using a uniaxial split-sphere press USSA-5000 suggests that pressure up to 17 and 20 GPa in compressed cell/sample volumes of 1200/60 and 400/18 mm³, respectively, can be generated using a combination of the BARS apparatus and the scaled-up Kawai-cell. This cell volume is much larger than that in conventional multi-anvil systems, which does not exceed 80 mm3 at the same conditions. [ABSTRACT FROM AUTHOR]

We found a new nickel sulfide that is isostructural with Fe3S. The synthesized nickel sulfide is a non-stoichiometric compound with a Ni deficiency and its composition is Ni2.90±0.5S. In situ synchrotron X-ray observations indicate that Ni3-xS forms above 5.1 GPa and melts incongruently into Ni+liquid up to 10 GPa. The bulk modulus of Ni3-xS at 300 K was determined to be 140 ± 2 GPa with a fixed K' = 4 by static compression with a liquid pressure medium. The eutectic point of the Ni-NiS system lies between Ni3S and Ni3S2 up to 10 GPa and its composition changes from Ni66.6S33.4 at 0.1 MPa to Ni70.7S29.3 at 10 GPa. The eutectic melting temperature of the Ni-NiS system decreases to 5.1 GPa and 720 K as the pressure increases where Ni and α-Ni3S2 are the eutectic solids. On the other hand, the eutectic melting temperature increases with a positive slope above 5.1 GPa where Ni3S+α-Ni3S2 is stable under subsolidus conditions. Intermediate compounds appear at a lower pressure in the Ni-NiS system than that for the Fe-FeS system. Ni3S2 is stable at atmospheric pressure and Ni3S forms at 5.1 GPa, whereas Fe3S2 and Fe3S form at 14 and 20 GPa, respectively. The addition of Ni complicates the melting relationship in the Fe-FeS system at high pressure because of the wider stability field of Ni3S. The low-melting temperature of the Fe-Ni-S system plays an important role in the percolative core-formation of planetesimals during planetary accretion. [ABSTRACT FROM AUTHOR]

To determine the phase boundary between the perovskite and post-perovskite structures in MnGeO3, in situ X-ray observations were carried out at pressures of 57-68 GPa and temperatures of 1000-1900 K using the Kawai-type high-pressure apparatus equipped with sintered diamond anvils interfaced with synchrotron radiation. The phase boundary was determined to be P (GPa) = 39.2 + 0.013T (K) based on Tsuchiya's (2003) gold pressure scale. The Clapeyron slope, dP/dT, of 13(+12/-5) MPa/K, determined in the present study is larger that of MgGeO3 and MgSiO3. [ABSTRACT FROM AUTHOR]