Kaichi Suito

Phase relations of CaCO3 at high pressure and high temperature

Abstract Phase transitions in calcite, a naturally occurring crystalline form of CaCO3, have been investigated by three different experimental techniques: (1) in-situ X-ray diffraction (XRD) using synchrotron radiation to 6 GPa and 1750 °C in a cubic anvil press; (2) Raman scattering to 10 GPa at room temperature using a diamond-anvil cell; and (3) post-compression XRD on samples retrieved after heat treatment at temperatures to 2000 °C and pressures to 9 GPa in an octahedral anvil press. At room temperature, calcite I transformed into calcite II at 1.7 GPa and then to calcite III at ~2 GPa. Calcite III persisted to at least 10 GPa. Elevation of temperature at 3, 4, and 6 GPa caused a sequence of transitions: calcite III → aragonite → disordered calcite → liquid, and aragonite was retained upon rapid cooling of the liquid. The melting curve of disordered calcite increased with pressure following a relation: Tm (°C) = 1338 + 82 P - 2.9 P2 where P is in units of GPa.

Synthesis of γ-Mg2SiO4

Abstract Magnesium orthosilicate with spinel structure (γ-Mg2SiO4) was synthesized at about 250 kbar and 1000°C. Unit cell dimension was established to be 8.076 ± 0.001A. X-ray powder diffraction pattern revealed a significant difference between γ-Mg2SiO4 and other γ-M2SiO4 spinels (M = Fe, Co, and Ni) in the intensities of (111) and (331) reflections, both of which are virtually absent in the Mg2SiO4 spinel. This feature could be thoroughly understood by the calculation of the intensities for several silicate spinels.

Thermoelastic properties of periclase and magnesiowüstite under high pressure and high temperature

Abstract A simple thermodynamic model for calculating the high-pressure and high-temperature properties of MgO consistently is presented. The model explains experimental equation of state (EOS) to ∼220 GPa at room temperature and shock-wave EOS to ∼200 GPa very well. The calculated Hugoniot temperature amounts to 3400 K at 200 GPa. The input parameters are the volume of the unit cell, V0, bulk modulus, K0, its pressure derivative, K0′, Debye temperature, Θ0, and a parameter relating to the shear modulus, f44, all in the static lattice at zero pressure which are estimated from experimental data at room temperature and zero pressure. The calculated thermal expansivity, α, and Anderson–Gruneisen parameters, δT and δS, at zero pressure are in agreement with experimental data to 1000–1250 K and those to 1800 K, respectively. Our model explains also experimental velocities vp and vs of compressional and shear waves to 1800 K at zero pressure very well. The pressure-dependence of α, vp and vs at room temperature agree reasonably with experimental data to 36 GPa at room temperature. Applying our model to magnesiowustite, we have calculated the thermal EOS and shock-wave EOS in agreement with experimental data. The implication of our result together with our previous one of magnesium-silicate perovskite for the composition of the lower mantle is briefly discussed.

The ZnSiO3 clinopyroxene-ilmenite transition: Heat capacity, enthalpy of transition, and phase equilibria

AbstractZnSiO3 clinopyroxene stable above 3 GPa transforms to ilmenite at 10–12 GPa, which further decomposes into ZnO (rock salt) plus stishovite at 20–30 GPa. The enthalpy of the clinopyroxene-ilmenite transition was measured by high-temperature solution calorimetry, giving ΔH0=51.71 ±3.18 kJ/mol at 298 K. The heat capacities of clinopyroxene and ilmenite were measured by differential scanning calorimetry at 343–733 and 343–633 K, respectively. The Cp of ilmenite is 3–5% smaller than that of clinopyroxene. The entropy of transition was calculated using the measured enthalpy and the free energy calculated from the phase equilibrium data. The enthalpy, entropy and volume changes of the pyroxene-ilmenite transition in ZnSiO3 are similar in magnitude to those in MgSiO3. The present thermochemical data are used to calculate the phase boundary of the ZnSiO3 clinopyroxene-ilmenite transition. The calculated boundary,

Photochromism of H2 and H3 centres in synthetic type Ib diamonds

P(Gpa) = 8.9( \pm 0.6) + 1.7(\bar + 0.5)

High-pressure synthesis of diamond from phenolic resin

. 10−3, is more tightly constrained than those determined previously by high-pressure high-temperature experiments, and can be used as a pressure calibration at 10–12 GPa and 1000–2000 K. The disproportionation of ZnSiO3 ilmenite to ZnO (rock salt) plus stishovite is calculated to occur at about 25 GPa at 1000–2000 K.

THERMAL EXPANSION STUDIES OF STISHOVITE AT 10.5 GPA USING SYNCHROTRON RADIATION

Abstract A method for calculating thermal equation of state (EOS) of solids is presented and is applied to CaSiO3 perovskite. The input parameters are the volume, V0, the bulk modulus, K0, its pressure derivative, K′0, and the Debye temperature, Θ0, all in the static lattice at zero pressure. These were determined from experimental data at room temperature. The present values of V0, K0 and K′0 agree well with recent theoretical values by Wentzcovitch et al. The present EOS is in good agreement with the high-temperature data to 1600 K and to 13 GPa by Wang et al. as well as the room-temperature data to 134 GPa by Mao et al., those to 112 GPa by Yagi et al. and those to 90 GPa by Tarrida and Richet. The calculated density and thermal expansivity of CaSiO3 perovskite under lower mantle conditions are in agreement, respectively, with PREM (Preliminary Reference Earth Model) within −1.2 −1.2% and with thermal expansivity of the lower mantle estimated by O.L. Anderson within 6.8 to −3.1% over the depth from 670 km to 2891 km. A method of calculating Lame constants, λS and μS, for an isotropic medium is also presented and is applied to CaSiO3 perovskite under lower mantle conditions. The calculated sound velocities, νp and νs, agree with PREM within 1.3–2.7% and −1.2 to −4.5%, respectively. The present results suggest that CaSiO3 perovskite behaves as an invisible component in the lower mantle.

Elastic properties of pressure-polymerized fullerenes

Photochromism has been studied for irradiated type Ib diamonds which are then annealed above 1500 degrees C. This mainly involves H2 and H3 centres. When the samples are illuminated with light of wavelength shorter than 600 nm, absorption of H2 centres is reduced while that of H3 centres is enhanced. In the dark the process is reversed. Several characteristic features have been examined for various illumination conditions and different samples. On the basis of the results a model is proposed: H2 and H3 centres are the same centre with different charge states and photoionization of H2 may be responsible for the photochromism.