Showing papers by "Donald J. Weidner published in 1996"

Thermal equation of state of CaSiO3 perovskite

Abstract: A comprehensive pressure-volume-temperature data set has been obtained for CaSiO3 perovskite up to 13 GPa and 1600 K, using synchrotron X ray diffraction with a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, nonhydrostatic stress is determined from the relative shift in the diffraction lines of NaCl, within which the sample was embedded. Heating to above 973 K greatly reduced the strength of NaCl (to below 0.05 GPa), making the measurements hydrostatic. At room temperature the cubic perovskite structure remains metastable at pressures as low as 1 GPa, below which the sample transforms into an amorphous phase as indicated by a large background, a marked decrease in diffraction signals, and an anomalous volume decrease of the remaining crystalline phase. Because our experimental uncertainties are significantly smaller than those in previous measurements, the new data provide a tighter constraint on the zero pressure bulk modulus for CaSiO3 perovskite. A new set of room temperature equation of state parameters are identified so that both our data and the diamond cell data of Mao et al. [1989] are compatible [KT0 = 232(8) GPa, K′T0 = 4.8(3), and V0 = 45.58(4) A3]. Volume measurements along several isotherms under both stable and metastable pressure conditions allow isochoric and isobaric interpolations within the range of experimental pressure and temperature conditions. Analyses using various approaches yielded consistent results for (∂KT/∂T)P of −0.033(8) GPa K−1, and (∂α/∂P)T of −6.3 × 10−7 GPa−1 K−1, with a zero-pressure thermal expansion α0 of 3.0 × 10−5 K−1. The thermal pressure is found to be virtually independent of volume, and thus the Anderson-Gruneisen parameter δT = K′T0 = 4.8. These results are used to predict the bulk modulus and density of CaSiO3 perovskite under lower mantle conditions. Along an adiabat with the foot temperature of 2000 K, the density of the perovskite agrees with that of the preliminary reference Earth model (PREM) within 1% throughout the lower mantle. The bulk modulus shows a smaller pressure dependence along the adiabat; it matches that of PREM at the top of the lower mantle but is about 10% too low near the core-mantle boundary.

The symmetry of garnets on the pyrope (Mg3Al2Si3O12)-majorite (MgSiO3) join

Abstract: Garnets with compositions between majorite and pyrope, Mj38, Mj48, Mj75 and Mj79 were synthesized at high pressures and temperatures in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) and investigated using high resolution synchrotron X-ray powder diffraction and transmission electron microscopy. The results from both techniques are consistent with the tetragonal field for these garnets extending to a majorite composition just below Mj75. The cubic-tetragonal structural phase transition in garnet along the majorite-pyrope join is sensitive to both composition and temperature and is expected to result in anomalous behavior in elastic shear moduli. This phase transition may occur in the transition zone of the earth's mantle and will have important effects on the elastic and rheological properties of this region where these garnets are stable phases.

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract: A phase transition from Ca(OH)2 I (portlandite) to Ca(OH)2 II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO1); it is monoclinic (P21/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.

Elasticity of superhydrous B

Abstract: The adiabatic single-crystal elastic moduli of superhydrous B, Mg10Si3O14(OH)4, have been measured at ambient conditions using Brillouin spectroscopy. This material is the first hydrous phase found to be stable at the extreme conditions of 20 GPa and 1400 °C. The single-crystal moduli, in GPa, are: C 11=280.0±1.5, C 22=307.4±1.6, C 33=293.4±1.4, C 44=90.0±1.1, C 55=99.2±0.8, C 66=89.6±0.6, C 12=66.1±2.2, C 13=105.6±2.6, C 23=81.8±2.6. With aggregate elastic properties of K VRH =154.0±4.2 and μ VRH =97.0±0.7 GPa, superhydrous B is approximately 16% suffer than forsterite and 20% softer than magnesium silicate spinel; it is also considerably more elastically isotropic than forsterite. The single-crystal moduli are compared to those of forsterite, magnesium silicate spinel and periclase, materials that are both structurally and compositionally similar to superhydrous B. The longitudinal moduli of superhydrous B and forsterite follow similar trends and appear to be dominated by the incompressibility and rotation of silicon tetrahedra. The shear and off-diagonal moduli more closely resemble those of periclase and spinel and may reflect the properties inherent to layers of magnesium octahedra.

Pressure-induced ordering in (ni,mg)2sio4 olivine

Abstract: An imaging-plate detector interfaced to a large-volume high-pressure device allows quantitative in situ powder X-ray diffraction measurements at the National Synchrotron Light Source (NSLS). High-quality diffraction patterns were recorded from an (Ni,Mg)2Si04 olivine solid-solution sample at 4 GPa and 800°C as a function of time using 3 min exposures at 40, 63, and 109 min. Refinement of the crystal structure indicated an increase in the ordering of Ni2+ and Mg2+ cations over the Ml and M2 sites at high pressure. The unit-cell volume was found to decrease with increasing ordering. Kinetic phenomena associated with the cation redistribution were observed on the time scale of tens of minutes. Extrapolation based on an exponential law of ordering relaxation toward equilibrium gave a distribution coefficient (KD)of 10.7(1) at 4 GPa and 800°C; the starting sample, which was annealed at 800 °c and room pressure, gave KD = 8.3(1).