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The single crystals of both calcite and rhodochrosite used in the present work are natural samples. The calcite sample is Iceland spar and the pink rhodochrosite is from an unspecified locality in Mexico. The chemical composition of the latter was confirmed by electron probe analysis [(Mn 0.98 Mg 0.01 Ca 0.01 )CO 3 ]. Both samples can be readily oriented using crystal morphology. ABSTRACTThe single-crystal elastic moduli of natural samples of both calcite (CaCO 3 ) and rhodochrosite (MnCO 3 ) have been measured by Brillouin spectroscopy under ambient condition. GPa for CaCO 3 and MnCO 3 , respectively. Our data for calcite are in good agreement with earlier data obtained by ultrasonic experiments. The off-diagonal elastic constants (C 12 , C 13 , and C 14 ) for rhodochrosite have systematically larger values than the trend defined by other isostructural carbonates, in all of which the divalent cations are alkaline-earth metals. This is a distinctive signature of transition-metal-bearing oxides, which is present in silicates and simple oxides as well.

Both natural and synthetic pyrope have been compressed to pressures above 300 kbar in a diamond‐anvil press and heated to temperatures above 800°C by a continuous YAG laser. After quenching and release of pressure, X‐ray diffraction shows that the natural pyrope transforms to a single phase with perovskite‐like structure whereas the synthetic pyrope disproportinates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are ao = 4.816 ± 0.004 Å, bo = 4.973 ± 0.004 Å, and co = 6.997 ± 0.006 Å with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao = 4.818 ± 0.005 Å, bo = 4.869 ± 0.005 Å, and co = 6.906 ± 0.007 Å with Z=4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm³, or 3.7% denser than the isochemical mixed oxides. These experiments are the first demonstration of static high pressure phase transformations in silicate garnets and the first examples of a pure silicate existing in the perovskite structure.

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The existence of a cubic fluorite-type SnO(2) and a hexagonal TiO(2) (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO(2) (fluorite) is a = 4.925 +/- 0.005 angstroms and those for TiO(2) (fluorite-related) are a = 9.22 +/- 0.01 angstroms and c = 5.685 +/- 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about -8 percent for SnO(2) and -10.5 percent for TiO(2) at transition. Upon release of pressure, both the fluorite-type SnO(2) and the TiO(2) reverted to the alpha-PbO(2) structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO(2) to possess the fluorite structure or its related forms at high pressure, as shown for SnO(2) and TiO(2) in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here.

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