糀谷 浩

High-pressure phase transitions in CaRhO3 were examined using a multianvil apparatus up to 27 GPa and 1930 degrees C. CaRhO3 perovskite transforms to post-perovskite via a monoclinic intermediate phase with increasing pressure. Volume changes for the transitions of perovskite - intermediate phase and of intermediate phase - post-perovskite are -1.1 and -0.7 %, respectively. CaRhO3 post-perovskite is the fourth quenchable post-perovskite oxide found so far. By high-temperature calorimetric experiments, enthalpy of the perovskite - post-perovskite transition in CaRuO3 was measured as 15.2 +/- 3.3 kJ/mol. Combining the datum with those of CaIrO3, it is shown that CaIrO3 perovskite is energetically less stable than CaRuO3 perovskite. This is consistent with the fact that orthorhombic distortion of CaIrO3 perovskite is larger than CaRuO3, as indicated with the tilt-angle of octahedral framework of perovskite structure. The transition pressure from perovskite to post-perovskite in CaBO3 (B = Ru, Rh, Ir) increases almost linearly with decreasing the tilt-angle, suggesting that the perovskite - post-perovskite transition may result from instability of the perovskite structure with pressure.

High-pressure high-temperature phase relation experiments in MgAl2O4 were performed in the pressure and temperature ranges of 18-27 GPa and 1400-2500 degrees C using a Kawai-type multi-anvil high-pressure apparatus. It was clarified that MgAl2O4 spinel directly transforms to the Mg2Al2O5 + Al2O3 assemblage at about 20 GPa and temperature higher than 2100 degrees C. The experimental results indicates that the phase assemblage of Mg2Al2O5 + Al2O3 is stable in the P-T region of 20 < P < 25 GPa and T > 2000 degrees C. The crystal structure of the Mg2Al2O5 phase was refined by the Rietveld method using a structure model based on that of ludwigite. The calculated density of rho(calc) = 3.801(1) g/cm(3) for the Mg2Al2O5 phase is very consistent with the phase relations determined by the high-pressure high-temperature experiments.

High-pressure phase transitions of CaRhO3 perovskite were examined at pressures of 6-27 GPa and temperatures of 1,000-1,930A degrees C, using a multi-anvil apparatus. The results indicate that CaRhO3 perovskite successively transforms to two new high-pressure phases with increasing pressure. Rietveld analysis of powder X-ray diffraction data indicated that, in the two new phases, the phase stable at higher pressure possesses the CaIrO3-type post-perovskite structure (space group Cmcm) with lattice parameters: a = 3.1013(1) , b = 9.8555(2) , c = 7.2643(1) , V (m) = 33.43(1) cm(3)/mol. The Rietveld analysis also indicated that CaRhO3 perovskite has the GdFeO3-type structure (space group Pnma) with lattice parameters: a = 5.5631(1) , b = 7.6308(1) , c = 5.3267(1) , V (m) = 34.04(1) cm(3)/mol. The third phase stable in the intermediate P, T conditions between perovskite and post-perovskite has monoclinic symmetry with the cell parameters: a = 12.490(3) , b = 3.1233(3) , c = 8.8630(7) , beta = 103.96(1)A degrees, V (m) = 33.66(1) cm(3)/mol (Z = 6). Molar volume changes from perovskite to the intermediate phase and from the intermediate phase to post-perovskite are -1.1 and -0.7%, respectively. The equilibrium phase relations determined indicate that the boundary slopes are large positive values: 29 +/- A 2 MPa/K for the perovskite-intermediate phase transition and 62 +/- A 6 MPa/K for the intermediate phase-post-perovskite transition. The structural features of the CaRhO3 intermediate phase suggest that the phase has edge-sharing RhO6 octahedra and may have an intermediate structure between perovskite and post-perovskite.

High-pressure phase transitions of CaRhO3 perovskite were examined at pressures of 6-27 GPa and temperatures of 1,000-1,930A degrees C, using a multi-anvil apparatus. The results indicate that CaRhO3 perovskite successively transforms to two new high-pressure phases with increasing pressure. Rietveld analysis of powder X-ray diffraction data indicated that, in the two new phases, the phase stable at higher pressure possesses the CaIrO3-type post-perovskite structure (space group Cmcm) with lattice parameters: a = 3.1013(1) , b = 9.8555(2) , c = 7.2643(1) , V (m) = 33.43(1) cm(3)/mol. The Rietveld analysis also indicated that CaRhO3 perovskite has the GdFeO3-type structure (space group Pnma) with lattice parameters: a = 5.5631(1) , b = 7.6308(1) , c = 5.3267(1) , V (m) = 34.04(1) cm(3)/mol. The third phase stable in the intermediate P, T conditions between perovskite and post-perovskite has monoclinic symmetry with the cell parameters: a = 12.490(3) , b = 3.1233(3) , c = 8.8630(7) , beta = 103.96(1)A degrees, V (m) = 33.66(1) cm(3)/mol (Z = 6). Molar volume changes from perovskite to the intermediate phase and from the intermediate phase to post-perovskite are -1.1 and -0.7%, respectively. The equilibrium phase relations determined indicate that the boundary slopes are large positive values: 29 +/- A 2 MPa/K for the perovskite-intermediate phase transition and 62 +/- A 6 MPa/K for the intermediate phase-post-perovskite transition. The structural features of the CaRhO3 intermediate phase suggest that the phase has edge-sharing RhO6 octahedra and may have an intermediate structure between perovskite and post-perovskite.

Phase relations in the system NaAlSiO4-MgAl2O4 were determined at 11-30CPa at 1273-1873K, using multianvil apparatus. At 1873K, calcium-ferrite solid solution in the compositional range of (1-x)NaAlSiO4 center dot xMgAl(2)O(4) (0 <= x <= 0.3) is formed above 17 GPa, and hexagonal aluminous phase is stable in the compositional range of 0.5 <= x <= 0.7 above 13.5 GPa. The hexagonal aluminous phase becomes nonstoichiometric with increasing MgAl2O4 component from x = 0.5 due to substitution mechanisms involving cation vacancy. In the composition of 0.3 <= x <= 0.5, Na-rich calcium-ferrite and Mg-rich hexagonal aluminous phase coexist. In 50%NaAlSiO4 center dot 50%MgAl2O4 composition (mol%), MgAl2O4 spinel + NaAlSi2O6 jadeite + alpha-NaAlO2 reacts to form a single hexagonal phase (see Reaction (I)) at 13-14 GPa at 1273-1873 K. In 67%MgAl2O4 center dot 33%CaAl2O4 composition, MgAl2O4 spinel+CaAl2O4 calcium ferrite changes to a single hexagonal phase (see Reaction (2)) at 13-14GPa at 1273-1673K. The two hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 are stable up to at least 30 GPa at 1873 K. By high-temperature drop-solution calorimetry, enthalpies at 298 K of Reactions (I) and (2) to form hexagonal phases were obtained to be 54.6 +/- 1.6 and 36.8 +/- 2.3 kJ/mol, respectively. Isobaric heat capacities (C-p) and entropies (S-298 degrees) of hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 were calculated by Kieffer models, based on Raman spectra and C-p measured by a differential scanning calorimeter. The calculated S-298 degrees of hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 are 86.7 and 88.0 J/(mol K), respectively. Using the above enthalpies and entropies, P-T boundaries for formation of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 hexagonal phases from the low-pressure phase assemblages were calculated. The calculated boundaries are generally consistent with high-pressure experimental data within the errors. The measured enthalpies and molar volumes suggest that hexagonal phase of Na0.5Mg0.5Al1.5Si0.5O4 would transform to calcium ferrite at pressure in the upper half of the lower mantle. (C) 2008 Elsevier B.V. All rights reserved.

Phase relations in the system NaAlSiO4-MgAl2O4 were determined at 11-30CPa at 1273-1873K, using multianvil apparatus. At 1873K, calcium-ferrite solid solution in the compositional range of (1-x)NaAlSiO4 center dot xMgAl(2)O(4) (0 <= x <= 0.3) is formed above 17 GPa, and hexagonal aluminous phase is stable in the compositional range of 0.5 <= x <= 0.7 above 13.5 GPa. The hexagonal aluminous phase becomes nonstoichiometric with increasing MgAl2O4 component from x = 0.5 due to substitution mechanisms involving cation vacancy. In the composition of 0.3 <= x <= 0.5, Na-rich calcium-ferrite and Mg-rich hexagonal aluminous phase coexist. In 50%NaAlSiO4 center dot 50%MgAl2O4 composition (mol%), MgAl2O4 spinel + NaAlSi2O6 jadeite + alpha-NaAlO2 reacts to form a single hexagonal phase (see Reaction (I)) at 13-14 GPa at 1273-1873 K. In 67%MgAl2O4 center dot 33%CaAl2O4 composition, MgAl2O4 spinel+CaAl2O4 calcium ferrite changes to a single hexagonal phase (see Reaction (2)) at 13-14GPa at 1273-1673K. The two hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 are stable up to at least 30 GPa at 1873 K. By high-temperature drop-solution calorimetry, enthalpies at 298 K of Reactions (I) and (2) to form hexagonal phases were obtained to be 54.6 +/- 1.6 and 36.8 +/- 2.3 kJ/mol, respectively. Isobaric heat capacities (C-p) and entropies (S-298 degrees) of hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 were calculated by Kieffer models, based on Raman spectra and C-p measured by a differential scanning calorimeter. The calculated S-298 degrees of hexagonal phases of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 are 86.7 and 88.0 J/(mol K), respectively. Using the above enthalpies and entropies, P-T boundaries for formation of Na0.5Mg0.5Al1.5Si0.5O4 and Mg0.67Ca0.33Al2O4 hexagonal phases from the low-pressure phase assemblages were calculated. The calculated boundaries are generally consistent with high-pressure experimental data within the errors. The measured enthalpies and molar volumes suggest that hexagonal phase of Na0.5Mg0.5Al1.5Si0.5O4 would transform to calcium ferrite at pressure in the upper half of the lower mantle. (C) 2008 Elsevier B.V. All rights reserved.

High-pressure high-temperature experiments of Ca2AlSiO5.5 were performed at 7.5-23 GPa and 1473-1873 K to clarify the high-pressure phase relations. Rhombohedral perovskite phase was recovered in runs at pressure higher than about 14GPa. Samples synthesized below 14GPa showed a powder X-ray diffraction pattern different from that of the rhombohedral perovskite phase. A phase boundary between the rhombohedral perovskite phase and the low-pressure phase was determined tightly to be at 13-14GPa over the experimental temperatures. By measuring drop-solution enthalpies of the Ca2AlSiO5.5 rhombohedral perovskite, Ca2AlSiO5.5 low-pressure phase and brownmillerite-type Ca2Al2O5, their formation enthalpies from constituent oxides were obtained to be -67.98 +/- 5.88, -94.64 +/- 4.16, -28.10 +/- 3.81 kJ/mol, respectively. The formation enthalpy of Ca2AlSiO5.5 rhombohedral perovskite suggests that the enthalpy of reaction from 1/2Ca(2)Si(2)O(6) (cubic perovskite) + 1/2Ca(2)Al(2)O(5) (brownmillerite) is about -70 kJ/mol. This implies that disordering of Si, Al and oxygen defects may contribute to reduce energy of Ca2AlSiO5.5 rhombohedral perovskite. (C) 2009 Elsevier B.V. All rights reserved.

A tetragonal perovskite PbMnO3 was obtained by treating the 6H hexagonal perovskite phase at 15 GPa and 1273 K. Structural analysis using synchrotron X-ray diffraction suggested that PbMnO3 crystallizes in the centrosymmetric space group P4/mmm, unlike PbTiO3 and PbVO3 which have a polar structure in space group P4mm. Iodometric titration revealed the presence of the oxygen deficiency of x = 0.06 for PbMnO3-x. The hexagonal 6H and the 3C perovskite phases exhibited antiferromagnetic ordering at 155 and 20 K, respectively.

Phase relations in CaAl4Si2O11 were examined at 12-23 GPa and 1000-1800 degrees C by multianvil experiments. A three-phase mixture of grossular, kyanite and corundum is stable below about 13 GPa at 1000-1800 degrees C. At higher pressure and at temperature below about 1200 degrees C, a Mixture of grossular, stishovite and corundum is stable, indicating the decomposition of kyanite. Above about 1200 degrees C, CaAl4Si2O11 CAS phase is stable at pressure higher than about 13 GPa. The triple point is placed at 14.7 GPa and 1280 degrees C. The equilibrium boundary of formation of CAS phase from the mixture of grossular, kyanite and corundum has a small negative slope, and that from the mixture of grossular, stishovite and corundum has a strongly negative slope, while the decomposition boundary of kyanite has a small positive slope. Enthalpies of the transitions were measured by high-temperature drop-solution calorimetry. The enthalpy of formation of CaAl4Si2O11 CAS phase from the mixture of grossular, kyanite and corundum was 139.5 +/- 15.6 kJ/mol, and that from the mixture of grossular, stishovite and corundum was 94.2 +/- 15.4 kJ/mol. The transition boundaries calculated using the measured enthalpy data were consistent with those determined by the high-pressure experiments. The boundaries in this study are placed about 3 GPa higher in pressure and about 200 degrees C lower in temperature than those by Zhai and Ito [Zhai, S., Ito, E., 2008. Phase relations of CaAl4Si2O11 at high-pressure and high-temperature with implications for subducted continental crust into the deep mantle. Phys. Earth Planet. Inter. 167,161-167]. Combining the thermodynamic data measured in this study with those in the literature, dissociation boundary of CAS phase into a mixture of Ca-perovskite, corundum and stishovite and that of grossular into Ca-perovskite plus corundum were calculated to further constrain the stability field of CAS phase. The result suggests that the stability of CAS phase would be limited at the bottom of transition zone and top of the lower mantle, when sediments are subducted into the deep mantle. It is also suggested that CAS phase may be stable at the depth of the upper part of the lower mantle, when partial melting of basalt occurs at the depth. (c) 2008 Elsevier B.V. All rights reserved.

A tetragonal perovskite PbMnO3 was obtained by treating the 6H hexagonal perovskite phase at 15 GPa and 1273 K. Structural analysis using synchrotron X-ray diffraction suggested that PbMnO3 crystallizes in the centrosymmetric space group P4/mmm, unlike PbTiO3 and PbVO3 which have a polar structure in space group P4mm. Iodometric titration revealed the presence of the oxygen deficiency of x = 0.06 for PbMnO3-x. The hexagonal 6H and the 3C perovskite phases exhibited antiferromagnetic ordering at 155 and 20 K, respectively.

Phase relations in CaAl4Si2O11 were examined at 12-23 GPa and 1000-1800 degrees C by multianvil experiments. A three-phase mixture of grossular, kyanite and corundum is stable below about 13 GPa at 1000-1800 degrees C. At higher pressure and at temperature below about 1200 degrees C, a Mixture of grossular, stishovite and corundum is stable, indicating the decomposition of kyanite. Above about 1200 degrees C, CaAl4Si2O11 CAS phase is stable at pressure higher than about 13 GPa. The triple point is placed at 14.7 GPa and 1280 degrees C. The equilibrium boundary of formation of CAS phase from the mixture of grossular, kyanite and corundum has a small negative slope, and that from the mixture of grossular, stishovite and corundum has a strongly negative slope, while the decomposition boundary of kyanite has a small positive slope. Enthalpies of the transitions were measured by high-temperature drop-solution calorimetry. The enthalpy of formation of CaAl4Si2O11 CAS phase from the mixture of grossular, kyanite and corundum was 139.5 +/- 15.6 kJ/mol, and that from the mixture of grossular, stishovite and corundum was 94.2 +/- 15.4 kJ/mol. The transition boundaries calculated using the measured enthalpy data were consistent with those determined by the high-pressure experiments. The boundaries in this study are placed about 3 GPa higher in pressure and about 200 degrees C lower in temperature than those by Zhai and Ito [Zhai, S., Ito, E., 2008. Phase relations of CaAl4Si2O11 at high-pressure and high-temperature with implications for subducted continental crust into the deep mantle. Phys. Earth Planet. Inter. 167,161-167]. Combining the thermodynamic data measured in this study with those in the literature, dissociation boundary of CAS phase into a mixture of Ca-perovskite, corundum and stishovite and that of grossular into Ca-perovskite plus corundum were calculated to further constrain the stability field of CAS phase. The result suggests that the stability of CAS phase would be limited at the bottom of transition zone and top of the lower mantle, when sediments are subducted into the deep mantle. It is also suggested that CAS phase may be stable at the depth of the upper part of the lower mantle, when partial melting of basalt occurs at the depth. (c) 2008 Elsevier B.V. All rights reserved.

Phase transitions in MgAl2O4 were examined at 21-27GPa and 1400-2500 degrees C using a multianvil apparatus. A Mixture of MgO and Al2O3 corundum that are high-pressure dissociation products of MgAl2O4 spinel combines into calcium-ferrite type MgAl2O4 at 26-27GPa and 1400-2000 degrees C. At temperature above 2000 degrees C at pressure below 25.5 GPa, a Mixture of Al2O3 corundum and a new phase with Mg2Al2O5 composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg2Al2O5 phase indicated that the phase represented a new structure type with orthorhornbic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6)angstrom, b=12.1952(6)angstrom, c=2.7916(2)angstrom, V=319.03(3) angstrom(3),Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O-6 octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O-6 octahedra. The structure is related with that of ludwigite (Mg, Fe2+)(2)(Fe3+, Al)(BO3)O-2. The molar volume of the Mg2Al2O5 phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al2O3 corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl2O4 among various A(2+)B(2)(3+)O(4) compounds. (C) 2008 Elsevier Inc. All rights reserved.

A high-quality polycrystalline sample of the correlated 4d post-perovskite CaRhO3 (Rh4+: 4d(5), S-el = 1/2) was attained under a moderate pressure of 6 GPa. Since the post-perovskite is quenchable at ambient pressure/temperature, it can be a valuable analogue of the post-perovskite MgSiO3 (stable higher than 120 GPa and unstable at ambient pressure), which is a significant key material in earth science. The sample was subjected for measurements of charge-transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below similar to 90 K in an unusual way, in striking contrast to what was observed for the perovskite phase. The post-perovskite CaRhO3 offers future opportunities for correlated electrons science as well as earth science.

Phase transitions in MgAl2O4 were examined at 21-27GPa and 1400-2500 degrees C using a multianvil apparatus. A Mixture of MgO and Al2O3 corundum that are high-pressure dissociation products of MgAl2O4 spinel combines into calcium-ferrite type MgAl2O4 at 26-27GPa and 1400-2000 degrees C. At temperature above 2000 degrees C at pressure below 25.5 GPa, a Mixture of Al2O3 corundum and a new phase with Mg2Al2O5 composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg2Al2O5 phase indicated that the phase represented a new structure type with orthorhornbic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6)angstrom, b=12.1952(6)angstrom, c=2.7916(2)angstrom, V=319.03(3) angstrom(3),Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O-6 octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O-6 octahedra. The structure is related with that of ludwigite (Mg, Fe2+)(2)(Fe3+, Al)(BO3)O-2. The molar volume of the Mg2Al2O5 phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al2O3 corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl2O4 among various A(2+)B(2)(3+)O(4) compounds. (C) 2008 Elsevier Inc. All rights reserved.

A high-quality polycrystalline sample of the correlated 4d post-perovskite CaRhO3 (Rh4+: 4d(5), S-el = 1/2) was attained under a moderate pressure of 6 GPa. Since the post-perovskite is quenchable at ambient pressure/temperature, it can be a valuable analogue of the post-perovskite MgSiO3 (stable higher than 120 GPa and unstable at ambient pressure), which is a significant key material in earth science. The sample was subjected for measurements of charge-transport and magnetic properties. The data clearly indicate it goes into an antiferromagnetically ordered state below similar to 90 K in an unusual way, in striking contrast to what was observed for the perovskite phase. The post-perovskite CaRhO3 offers future opportunities for correlated electrons science as well as earth science.

Low-temperature isobaric heat capacities (C-p ) of MgSiO3 ilmenite and perovskite were measured in the temperature range of 1.9-302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C-p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C-p , standard entropies at 298.15 K of MgSiO3 ilmenite and perovskite were determined to be 53.7 +/- 0.4 and 57.9 +/- 0.3 J/mol K, respectively. The positive entropy change (4.2 +/- 0.5 J/mol K) of the ilmenite-perovskite transition in MgSiO3 is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite-perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20-23 GPa at 1,000-2,000 K with a Clapeyron slope of -2.4 +/- 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.

Phase relations on the diopside (Di)-hedenbergite (Hd)-jadeite (Jd) system modeling mineral associations of natural eclogites were studied for the compositions (mol %) Di(70)Jd(30), Di(50)Jd(50), Di(30)Jd(70), Di(20)Hd(80), and Di(40)Hd(10)Jd(50) using a toroidal anvil-with-hole (7 GPa) and a Kawai-type 6 - 8 multianvil apparatus (12 - 24 GPa). We established that Di, Hd, and Jd form complete series of solid solutions at 7 GPa, and melting temperatures of pure Di (1980 degrees C) and Jd (1870 degrees C) for that pressure were estimated experimentally. The melting temperature for the Di(50)Jd(50) composition at 15.5 GPa is 2270 degrees C. The appearance of garnet is clearly dependent on initial clinopyroxene composition: at 1600 degrees C the first garnet crystals are observed at 13.5 GPa in the jadeite-rich part of the system (Di(30)Jd(70)), whereas diopside-rich starting material (Di(70)Jd(30)) produces garnet only above 17 GPa. The proportion of garnet increases rapidly above 18 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites are produced from diopside-rich starting materials. In all experiments, garnet coexists with stishovite (St). At a pressure above 18 GPa, pyroxene is completely replaced by an assemblage of majorite (Maj) + St + CaSiO(3) - perovskite (Ca-Pv) in Ca-rich systems, whereas Maj is associated with almost pure Jd up to a pressure of 21.5 GPa. Above similar to 22 GPa, Maj, and St are associated with NaAlSiO(4) with calcium ferrite structure (Cf). We established that an Hd component also spreads the range of pyroxene stability up to 20 CiPa. In the Di(70)Jd(30) system at 24 CiPa an assemblage of Maj + CaPv + MgSiO(3) with ilmenite structure (Mg-II) was obtained. The experimentally established correlation between Na, Si, and Al contents in Maj and pressure in Grt(Maj)-pyroxene assemblages, may be the basis for a "majorite" geobarometer. The results of our experiments are applicable to the upper mantle and the transition zone of the Earth (400 - 670 km), and demonstrate a wide ranee of transformations from eclogite to perovskite-bearing garnetite. In addition, the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle. (c) 2008 Elsevier Ltd. All rights reserved.

Phase relations on the diopside (Di)-hedenbergite (Hd)-jadeite (Jd) system modeling mineral associations of natural eclogites were studied for the compositions (mol %) Di(70)Jd(30), Di(50)Jd(50), Di(30)Jd(70), Di(20)Hd(80), and Di(40)Hd(10)Jd(50) using a toroidal anvil-with-hole (7 GPa) and a Kawai-type 6 - 8 multianvil apparatus (12 - 24 GPa). We established that Di, Hd, and Jd form complete series of solid solutions at 7 GPa, and melting temperatures of pure Di (1980 degrees C) and Jd (1870 degrees C) for that pressure were estimated experimentally. The melting temperature for the Di(50)Jd(50) composition at 15.5 GPa is 2270 degrees C. The appearance of garnet is clearly dependent on initial clinopyroxene composition: at 1600 degrees C the first garnet crystals are observed at 13.5 GPa in the jadeite-rich part of the system (Di(30)Jd(70)), whereas diopside-rich starting material (Di(70)Jd(30)) produces garnet only above 17 GPa. The proportion of garnet increases rapidly above 18 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites are produced from diopside-rich starting materials. In all experiments, garnet coexists with stishovite (St). At a pressure above 18 GPa, pyroxene is completely replaced by an assemblage of majorite (Maj) + St + CaSiO(3) - perovskite (Ca-Pv) in Ca-rich systems, whereas Maj is associated with almost pure Jd up to a pressure of 21.5 GPa. Above similar to 22 GPa, Maj, and St are associated with NaAlSiO(4) with calcium ferrite structure (Cf). We established that an Hd component also spreads the range of pyroxene stability up to 20 CiPa. In the Di(70)Jd(30) system at 24 CiPa an assemblage of Maj + CaPv + MgSiO(3) with ilmenite structure (Mg-II) was obtained. The experimentally established correlation between Na, Si, and Al contents in Maj and pressure in Grt(Maj)-pyroxene assemblages, may be the basis for a "majorite" geobarometer. The results of our experiments are applicable to the upper mantle and the transition zone of the Earth (400 - 670 km), and demonstrate a wide ranee of transformations from eclogite to perovskite-bearing garnetite. In addition, the mineral associations obtained from the experiments allowed us to simulate parageneses of inclusions in diamonds formed under the conditions of the transition zone and the lower mantle. (c) 2008 Elsevier Ltd. All rights reserved.

High-pressure phase relations in CaRuO3 were examined at 13-27 GPa and 900-1200 degrees C using a multi-anvil apparatus. At about 21-25 GPa at 900-1200 degrees C, CaRuO3 with orthorhombic perovskite structure transformed to CaIrO3-type post-perovskite structure. The phase boundary of the post-perovskite transition in CaRuO3 was determined as P(GPa) =0.010 x T(degrees C)+ 12.4. The post-perovskite phase of CaRuO3 was quenchable to ambient conditions like that of CaIrO3. Rietveld refinement confirmed that CaRuO3 post-perovskite and perovskite have the structures of CaIrO3-type post-perovskite (space group Cmcm) and GdFeO3-type orthorhombic perovskite (Pbnm), respectively. Lattice parameters and unit cell volume of CaRuO3 post-perovskite were determined to be a=3.1150(1) angstrom, b=9.8268(1) angstrom, c=7.2963(1) angstrom and V=223.34(1) angstrom(3), and those of CaRuO3 perovskite a=5.3635(2) angstrom, b = 5.5261(2) angstrom, c = 7.6668(2) angstrom and V= 227.24(1) angstrom(3). The structural features of CaRuO3 post-perovskite and perovskite are similar to those of the polymorphs of CaIrO3 and MgSiO3. The post-perovskite transition in CaRuO3 is consistent with the general tendency that orthorhombic perovskites with relatively large tilting of the octahedral framework transform to post-perovskite structure at high pressure. CaRuO3 would be a low-pressure, quenchable analogue material suitable for investigation on the post-perovskite phase transition of MgSiO3. (c) 2007 Elsevier B.V. All rights reserved.

High-pressure phase relations in CaRuO3 were examined at 13-27 GPa and 900-1200 degrees C using a multi-anvil apparatus. At about 21-25 GPa at 900-1200 degrees C, CaRuO3 with orthorhombic perovskite structure transformed to CaIrO3-type post-perovskite structure. The phase boundary of the post-perovskite transition in CaRuO3 was determined as P(GPa) =0.010 x T(degrees C)+ 12.4. The post-perovskite phase of CaRuO3 was quenchable to ambient conditions like that of CaIrO3. Rietveld refinement confirmed that CaRuO3 post-perovskite and perovskite have the structures of CaIrO3-type post-perovskite (space group Cmcm) and GdFeO3-type orthorhombic perovskite (Pbnm), respectively. Lattice parameters and unit cell volume of CaRuO3 post-perovskite were determined to be a=3.1150(1) angstrom, b=9.8268(1) angstrom, c=7.2963(1) angstrom and V=223.34(1) angstrom(3), and those of CaRuO3 perovskite a=5.3635(2) angstrom, b = 5.5261(2) angstrom, c = 7.6668(2) angstrom and V= 227.24(1) angstrom(3). The structural features of CaRuO3 post-perovskite and perovskite are similar to those of the polymorphs of CaIrO3 and MgSiO3. The post-perovskite transition in CaRuO3 is consistent with the general tendency that orthorhombic perovskites with relatively large tilting of the octahedral framework transform to post-perovskite structure at high pressure. CaRuO3 would be a low-pressure, quenchable analogue material suitable for investigation on the post-perovskite phase transition of MgSiO3. (c) 2007 Elsevier B.V. All rights reserved.

To map the stability field of calcium ferrite-type MgAl2O4-Mg2SiO4 solid solutions, high-pressure phase relations in the system MgAl2O4-Mg2SiO4 were studied in the compositional range of 0 to 50 Mot% Mg2SiO4. The calcium ferrite solid solutions are stable above 23 GPa at 1600 degrees C, and the maximum solubility of Mg2SiO4 component in MgAl2O4 calcium ferrite is 34 mot%. Lattice parameters and unit-cell volume of calcium ferrite-type MgAl2O4 (space group Pbnm) determined by Rietveld analysis are a = 9.9498(6) A, b = 8.6468(6) angstrom, c = 2.7901(2) angstrom, and V= 240.02(2) angstrom(3). Lattice parameters for the MgAl2O4-Mg2SiO4 solid solutions with the compositions of 14, 24, and 34 mot% Mg2SiO4 indicated the following compositional dependency of lattice parameters: a (angstrom) = 9.9498 + 0.1947 .X-Mg2SiO4, b (angstrom) = 8.6468 - 0.1097 .X-Mg2SiO4 and c (angstrom) = 2.7901 + 0.0086 X-Mg2SiO4 where X-Mg2SiO4 is the mole fraction Of Mg2SiO4 component. A linear extrapolation of the composition-molar volume relationship gave an estimated volume of 36.49(2) cm(3)/mol for the hypothetical calcium ferrite-type Mg2SiO4. This value is larger than that of the isochemical mixture of MgSiO3 perovskite and MgO, 35.72(l) cm(3)/mol. This implies that the mixture of MgSiO3, perovskite and MgO is more stable than the hypothetical calcium ferrite-type Mg2SiO4 under the lower mantle conditions.

To map the stability field of calcium ferrite-type MgAl2O4-Mg2SiO4 solid solutions, high-pressure phase relations in the system MgAl2O4-Mg2SiO4 were studied in the compositional range of 0 to 50 Mot% Mg2SiO4. The calcium ferrite solid solutions are stable above 23 GPa at 1600 degrees C, and the maximum solubility of Mg2SiO4 component in MgAl2O4 calcium ferrite is 34 mot%. Lattice parameters and unit-cell volume of calcium ferrite-type MgAl2O4 (space group Pbnm) determined by Rietveld analysis are a = 9.9498(6) A, b = 8.6468(6) angstrom, c = 2.7901(2) angstrom, and V= 240.02(2) angstrom(3). Lattice parameters for the MgAl2O4-Mg2SiO4 solid solutions with the compositions of 14, 24, and 34 mot% Mg2SiO4 indicated the following compositional dependency of lattice parameters: a (angstrom) = 9.9498 + 0.1947 .X-Mg2SiO4, b (angstrom) = 8.6468 - 0.1097 .X-Mg2SiO4 and c (angstrom) = 2.7901 + 0.0086 X-Mg2SiO4 where X-Mg2SiO4 is the mole fraction Of Mg2SiO4 component. A linear extrapolation of the composition-molar volume relationship gave an estimated volume of 36.49(2) cm(3)/mol for the hypothetical calcium ferrite-type Mg2SiO4. This value is larger than that of the isochemical mixture of MgSiO3 perovskite and MgO, 35.72(l) cm(3)/mol. This implies that the mixture of MgSiO3, perovskite and MgO is more stable than the hypothetical calcium ferrite-type Mg2SiO4 under the lower mantle conditions.

Al-containing MgSiO3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg0.975Si0.975Al0.05O3 and Mg0.95Si0.95Al0.10O3 considering only coupled substitution Mg2+ + Si4+ = 2Al(3+), and nonstoichiometric compositions of Mg0.99Si0.96Al0.05O2.985 and Mg0.97Si0.93Al0.10O2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si(4+) = 2Al(3+) + V-O. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter-composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the XAl range of 0-0.05, where X-Al is Al number on the basis of total cation of two (X-Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X-Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X-Al of 0.10, though they are the same as each other at XAl of 0.05. The Si(Al)-O1 distance, Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O distance of the nonstoichiometric perovskites keep almost constant up to X-Al of 0.05, and then the Si(Al)-O1 increases and both of the Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X-Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)-O1 distance and one of two independent Si(Al)-O2 distances in Si(Al)O-6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.

Al-containing MgSiO3 perovskites of four different compositions were synthesized at 27 GPa and 1,873 K using a Kawai-type high-pressure apparatus: stoichiometric compositions of Mg0.975Si0.975Al0.05O3 and Mg0.95Si0.95Al0.10O3 considering only coupled substitution Mg2+ + Si4+ = 2Al(3+), and nonstoichiometric compositions of Mg0.99Si0.96Al0.05O2.985 and Mg0.97Si0.93Al0.10O2.98 taking account of not only the coupled substitution but also oxygen vacancy substitution 2Si(4+) = 2Al(3+) + V-O. Using the X-ray diffraction profiles, Rietveld analyses were performed, and the results were compared between the stoichiometric and nonstoichiometric perovskites. Lattice parameter-composition relations, in space group Pbnm, were obtained as follows. The a parameters of both of the stoichiometric and nonstoichiometric perovskites are almost constant in the XAl range of 0-0.05, where X-Al is Al number on the basis of total cation of two (X-Al = 2Al/(Mg + Si + Al)), and decrease with further increasing X-Al. The b and c parameters of the stoichiometric perovskites increase linearly with increasing Al content. The change in the b parameter of the nonstoichiometric perovskites with Al content is the same as that of the stoichiometric perovskites within the uncertainties. The c parameter of the nonstoichiometric perovskites is slightly smaller than that of the stoichiometric perovskites at X-Al of 0.10, though they are the same as each other at XAl of 0.05. The Si(Al)-O1 distance, Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O distance of the nonstoichiometric perovskites keep almost constant up to X-Al of 0.05, and then the Si(Al)-O1 increases and both of the Si(Al)-O1-Si(Al) angle and minimum Mg(Al)-O decrease with further Al substitution. These results suggest that the oxygen vacancy substitution may be superior to the coupled substitution up to X-Al of about 0.05 and that more Al could be substituted only by the coupled substitution at 27 GPa. The Si(Al)-O1 distance and one of two independent Si(Al)-O2 distances in Si(Al)O-6 octahedra in the nonstoichiometric perovskites are always shorter than those in the stoichiometric perovskite at the same Al content. These results imply that oxygen defects may exist in the nonstoichiometric perovskites and distribute randomly.

The low-temperature isobaric heat capacities (C-p) of beta- and gamma-Mg2SiO4 were measured at the range of 1.8 - 304.7 K with a thermal relaxation method using the Physical Property Measurement System. The obtained standard entropies (S degrees(298)) of beta- and gamma-Mg2SiO4 are 86.4 +/- 0.4 and 82.7 +/- 0.5 J/mol K, respectively. Enthalpies of transitions among alpha-, beta- and gamma- Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the alpha-beta and beta - gamma transitions at 298 K (Delta H degrees(298)) in Mg2SiO4 are 27.2 +/- 3.6 and 12.9 +/- 3.3 kJ/ mol, respectively. Calculated alpha - beta and beta - gamma transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured Delta H degrees(298) and Delta S degrees(298) with selected data of in situ X-ray diffraction experiments at high pressure, the Delta H degrees(298) and Delta S degrees(298) of the alpha - beta and beta - gamma transitions were optimized. Calculation using the optimized data tightly constrained the alpha - beta and beta - gamma transition boundaries in the P, T space. The slope of alpha - beta transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of beta - gamma boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of gamma-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on gamma-Mg2SiO4 and available enthalpy and entropy data on MgSiO3 perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of - 2.6 +/- 0.2 MPa/K is located at pressure consistent with the 660 km discontinuity, considering the error of the thermodynamic data.

A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (3 1 in water depth) oil the fore-reef slope of le island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the delta O-18-derived temperature (T(delta)18(O)) of C. iejiniensis suggests that the species grows between April and July; (5) the T(delta)18(O) of C iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 40 and AD 1000 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene. (c) 2006 Published by Elsevier B.V.

A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (3 1 in water depth) oil the fore-reef slope of le island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the delta O-18-derived temperature (T(delta)18(O)) of C. iejiniensis suggests that the species grows between April and July; (5) the T(delta)18(O) of C iejimensis from cored sediments implies that there were two warmer intervals, at AD 340 40 and AD 1000 40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene. (c) 2006 Published by Elsevier B.V.

To study the ambient analog of the deep mantle MgSiO3 perovskite to post-perovskite phase transition, high-temperature drop calorimetry experiments of perovskite and post-perovskite phases of CaIrO3 system as well as high-pressure phase equilibrium experiments in the CaIrO3 system were made. The enthalpies for dissociation of CaIrO3 (298 K) to CaO + Ir + O-2 (1573 K) were 486.7 +/- 9.2 kJ/mol for post-perovskite and 454.5 +/- 12.5 kJ/mol for perovskite. From the difference between them, the phase transition enthalpy from perovskite to post-perovskite at 298 K is -32.2 +/- 15.5 kJ/mol. This Gives 2.7 +/- 15.6 kJ/mol as formation enthalpy of CaIrO3 perovskite from CaO + IrO2 at 298 K. Using the phase transition enthalpy and volume change of -0.48 +/- 0.02 cm(3)/mol determined in this study, the phase equilibrium boundary is calculated to be P (GPa) 0.040 T (K) - 67.1. The strongly positive slope agrees with that obtained in high-pressure experiments. This is consistent with a large positive Clapeyron slope of post-perovskite phase transition in MgSiO3 recently reported from experimental and theoretical studies.

To study the ambient analog of the deep mantle MgSiO3 perovskite to post-perovskite phase transition, high-temperature drop calorimetry experiments of perovskite and post-perovskite phases of CaIrO3 system as well as high-pressure phase equilibrium experiments in the CaIrO3 system were made. The enthalpies for dissociation of CaIrO3 (298 K) to CaO + Ir + O2 (1573 K) were 486.7 ± 9.2 kJ/mol for post-perovskite and 454.5 ± 12.5 kJ/mol for perovskite. From the difference between them, the phase transition enthalpy from perovskite to post-perovskite at 298 K is -32.2 ± 15.5 kJ/mol. This gives 2.7 ± 15.6 kJ/mol as formation enthalpy of CaIrO3 perovskite from CaO + IrO2 at 298 K. Using the phase transition enthalpy and volume change of -0.48 ± 0.02 cm3/mol determined in this study, the phase equilibrium boundary is calculated to be P (GPa) = 0.040 T (K) - 67.1. The strongly positive slope agrees with that obtained in high-pressure experiments. This is consistent with a large positive Clapeyron slope of post-perovskite phase transition in MgSiO3 recently reported from experimental and theoretical studies.

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO3. ZnGeO3 ilmenite transformed into perovskite at 30.0 GPa and 1300 +/- 150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite - perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO6 octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO6 octahedra in the ilmenite - perovskite transition. The correlation of quenchability with rotation angle of GeO6 octahedra for other germanate perovskites is also discussed.

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO3. ZnGeO3 ilmenite transformed into perovskite at 30.0 GPa and 1300 +/- 150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite - perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO6 octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO6 octahedra in the ilmenite - perovskite transition. The correlation of quenchability with rotation angle of GeO6 octahedra for other germanate perovskites is also discussed.

Aluminum is an important minor constituent of a number of high-pressure mantle silicates in which it substitutes for octahedrally coordinated silicon. In several cases, its solid solution may be linked to the presence of oxygen vacancies; in others, to charge balance with H+. Here we present new data from high-resolution, high-field (18.8 Tesla) Al-27 NMR of aluminous stishovite and of a non-stoichiometric perovskite with nominal composition MgSi0.95Al0.05O2.975. For the stishovite, we characterize the local structure of the symmetrical, octahedral site for Al. These results, combined with Al-27{H-1} REDOR NMR, are consistent with hypothesized H+ charge balance, although the presence of a significant fraction of randomly distributed oxygen vacancies could remain undetected. As in a recent previous study of it related perovskite composition, the observed ratio of Al at symmetrical octahedral B sites to that of Al at large, central A sites is about 2:1, indicating file presence of oxygen vacancies to account for charge neutrality in this phase. Such vacancies are not preferentially associated with the Al octahedra, however, suggesting a random distribution in the structure.

Aluminum is an important minor constituent of a number of high-pressure mantle silicates in which it substitutes for octahedrally coordinated silicon. In several cases, its solid solution may be linked to the presence of oxygen vacancies; in others, to charge balance with H+. Here we present new data from high-resolution, high-field (18.8 Tesla) Al-27 NMR of aluminous stishovite and of a non-stoichiometric perovskite with nominal composition MgSi0.95Al0.05O2.975. For the stishovite, we characterize the local structure of the symmetrical, octahedral site for Al. These results, combined with Al-27{H-1} REDOR NMR, are consistent with hypothesized H+ charge balance, although the presence of a significant fraction of randomly distributed oxygen vacancies could remain undetected. As in a recent previous study of it related perovskite composition, the observed ratio of Al at symmetrical octahedral B sites to that of Al at large, central A sites is about 2:1, indicating file presence of oxygen vacancies to account for charge neutrality in this phase. Such vacancies are not preferentially associated with the Al octahedra, however, suggesting a random distribution in the structure.

Phase transitions in MgGeO3 and ZnGeO3 were examined up to 26 GPa and 2,073 K to determine ilmenite - perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite perovskite boundaries have negative slopes and are expressed as P(GPa) = 38.4 - 0.0082T(K) and P(GPa) = 27.4 - 0.0032T( K), respectively, for MgGeO3 and ZnGeO3. Enthalpies of SrGeO3 polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO3 pseudowollasonite - walstromite and walstromite - perovskite transitions at 298 K were determined to be 6.0 +/- 8.6 and 48.9 +/- 5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO3, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (Delta H-f degrees) of SrGeO3 perovskite from the constituent oxides at 298 K was determined to be - 73.6 +/- 5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite perovskite transition boundaries in MgGeO3 and ZnGeO3 and the boundary of formation of SrSiO3 perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The Delta H(f)degrees of MgGeO3, ZnGeO3 and SrSiO3 perovskites from constituent oxides were 10.2 +/- 4.5, 33.8 +/- 7.2 and - 3.0 +/- 2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A(2+) B4+ O-3 perovskites stable at both atmospheric and high pressures to explore the relationship between Delta H(f)degrees and ionic radii of eightfold coordinated A(2+) (R-A) and sixfold coordinated B4+ (R-B) cations. The results show that enthalpy of formation of A(2+) B4+ O-3 perovskite increases with decreasing R-A and R-B. The relationship between the enthalpy of formation and tolerance factor (t = R-A + R-o)/root 2(R-B + R-o), R-o: O2- radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A(2+) and B4+ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A(2+) and B4+ radii indicates a systematic change with equi-enthalpy curves. These relationships of Delta H(f)degrees with R-A and R-B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.

Phase transitions in MgGeO3 and ZnGeO3 were examined up to 26 GPa and 2,073 K to determine ilmenite - perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite perovskite boundaries have negative slopes and are expressed as P(GPa) = 38.4 - 0.0082T(K) and P(GPa) = 27.4 - 0.0032T( K), respectively, for MgGeO3 and ZnGeO3. Enthalpies of SrGeO3 polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO3 pseudowollasonite - walstromite and walstromite - perovskite transitions at 298 K were determined to be 6.0 +/- 8.6 and 48.9 +/- 5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO3, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (Delta H-f degrees) of SrGeO3 perovskite from the constituent oxides at 298 K was determined to be - 73.6 +/- 5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite perovskite transition boundaries in MgGeO3 and ZnGeO3 and the boundary of formation of SrSiO3 perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The Delta H(f)degrees of MgGeO3, ZnGeO3 and SrSiO3 perovskites from constituent oxides were 10.2 +/- 4.5, 33.8 +/- 7.2 and - 3.0 +/- 2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A(2+) B4+ O-3 perovskites stable at both atmospheric and high pressures to explore the relationship between Delta H(f)degrees and ionic radii of eightfold coordinated A(2+) (R-A) and sixfold coordinated B4+ (R-B) cations. The results show that enthalpy of formation of A(2+) B4+ O-3 perovskite increases with decreasing R-A and R-B. The relationship between the enthalpy of formation and tolerance factor (t = R-A + R-o)/root 2(R-B + R-o), R-o: O2- radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A(2+) and B4+ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A(2+) and B4+ radii indicates a systematic change with equi-enthalpy curves. These relationships of Delta H(f)degrees with R-A and R-B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.

High-pressure synthesis of a new SrSi2O5 phase was performed at 16 GPa and 900 degrees C by using a Kawai-type multianvil apparatus. The powder X-ray diffraction pattern of the compound was analyzed by Rietveld refinement based on the structure of a high-pressure polymorph of BaGe2O5, BaGe2O5 III. The structure is orthorhombic with space group Cmca and cell parameters of a = 5.2389( 1) angstrom, b = 9.2803(2) angstrom, c = 13.4406( 1) angstrom, V = 653.46( 2) angstrom(3) (Z = 8, rho(calc) = 4.549 g/cm(3)). The structure consists of layers containing SiO6 octahedra and SiO4 tetrahedra. In a unit layer, oxygen and strontium atoms are arranged in an approximation to hexagonal close-packing. The strontium atom is accommodated in a 12-coordinated site. Each SiO6 octahedron shares four corners with SiO4 tetrahedra and the other two corners with another SiO6 octahedra. The SiO6 octahedra are linked to each other to form SiO6 chains along the a-axis. This is the first known example of a silicate with a BaGe2O5 III-type structure.

High-pressure synthesis of a new SrSi2O5 phase was performed at 16 GPa and 900 degrees C by using a Kawai-type multianvil apparatus. The powder X-ray diffraction pattern of the compound was analyzed by Rietveld refinement based on the structure of a high-pressure polymorph of BaGe2O5, BaGe2O5 III. The structure is orthorhombic with space group Cmca and cell parameters of a = 5.2389( 1) angstrom, b = 9.2803(2) angstrom, c = 13.4406( 1) angstrom, V = 653.46( 2) angstrom(3) (Z = 8, rho(calc) = 4.549 g/cm(3)). The structure consists of layers containing SiO6 octahedra and SiO4 tetrahedra. In a unit layer, oxygen and strontium atoms are arranged in an approximation to hexagonal close-packing. The strontium atom is accommodated in a 12-coordinated site. Each SiO6 octahedron shares four corners with SiO4 tetrahedra and the other two corners with another SiO6 octahedra. The SiO6 octahedra are linked to each other to form SiO6 chains along the a-axis. This is the first known example of a silicate with a BaGe2O5 III-type structure.

The presence of hexagonal silicate perovskite (6H-BaTiO3 type) was confirmed in the SrSiO3 compound by in-situ angle dispersive X-ray diffraction at high pressure. The perovskite was crystallized from pressure-induced amorphous SrSiO3 in a diamond anvil cell by laser heating at 35 GPa. On releasing the pressure, the perovskite also changed into an amorphous state as does CaSiO3 perovskite. This SrSiO3 perovskite, with a tolerance factor greater than unity, forms a face-sharing SiO6 octahedron, which leads to a structure with hexagonal symmetry. Incorporation of Sr into CaSiO3 perovskite in the early stage of the differentiation in the Earth's mantle might have influenced the symmetry of CaSiO3 perovskite in the present lower mantle. As far as we know, this is the first report suggesting the existence of hexagonal perovskite in silicates.

The presence of hexagonal silicate perovskite (6H-BaTiO3 type) was confirmed in the SrSiO3 compound by in-situ angle dispersive X-ray diffraction at high pressure. The perovskite was crystallized from pressure-induced amorphous SrSiO3 in a diamond anvil cell by laser heating at 35 GPa. On releasing the pressure, the perovskite also changed into an amorphous state as does CaSiO3 perovskite. This SrSiO3 perovskite, with a tolerance factor greater than unity, forms a face-sharing SiO6 octahedron, which leads to a structure with hexagonal symmetry. Incorporation of Sr into CaSiO3 perovskite in the early stage of the differentiation in the Earth's mantle might have influenced the symmetry of CaSiO3 perovskite in the present lower mantle. As far as we know, this is the first report suggesting the existence of hexagonal perovskite in silicates.

Phase transitions of CaMgSi2O6 diopside and CaSiO3 wollastonite were examined at pressures to 23GPa and temperatures to 2000degreesC, using a Kawai-type multiavil apparatus. Enthalpies of high-pressure phases in CaSiO3 and in the CaSi2O5-CaTiSiO5 system were also measured by high-temperature calorimetry. At 17-18GPa, diopside dissociates to CaSiO3-rich perovskite + Mg-rich (Mg,Ca)SiO3 tetragonal garnet (Gt) above about 1400degreesC. The solubilities of CaSiO3 in garnet and MgSiO3 in perovskite increase with temperature. At 17-18GPa below about 1400degreesC, diopside dissociates to Ca-perovskite + beta-Mg2SiO4 + stishovite. The Mg, Si-phases coexisting with Ca-perovskite change to gamma-Mg2SiO4 + stishovite, to ilmenite, and finally to Mg-perovskite with increasing pressure. CaSiO3 wollastonite transforms to the walstromite structure, and further dissociates to Ca2SiO4 larnite + CaSi2O5 titanite. The latter transition occurs at 9-11 GPa with a positive Clapeyron slope. At 1600degreesC, larnite + titanite transform to CaSiO3 perovskite at 14.6 +/- 0.6 GPa, calibrated against the alpha-beta transition pressure Of Mg2SiO4. The enthalpies of fort-nation of CaSiO3 walstromite and CaSi2O5 titanite from the mixture of CaOandSiO(2) quartz at 298 K have been determined as -76.1 +/- 2.8,and -27.8 +/- 2.1 kJ/mol, respectively. The latter was estimated from enthalpy measurements of titanite solid solutions in the system CaSi2O5-CaTiSiO5, because CaSi7O5 titanite transforms to a triclinic phase upon decompression. The enthalpy difference between titanite and the triclinic phase is only 1.5 +/- 4.8 kJ/mol. Using these enthalpies of formation and those of larnite and CaSiO3 perovskite, the transition boundaries in CaSiO3 have been calculated. The calculated boundaries for the wollastonite-walstromite-larnite + titanite transitions are consistent with the experimental determinations within the errors. The calculated boundary between larnite + titanite and Ca-perovskite has a slope of 1.3-1.8(+/-0.4) MPa/K, and is located at a pressure about 2 GPa higher than that determined by. (C) 2004 Elsevier B.V. All rights reserved.

Phase transitions of CaMgSi2O6 diopside and CaSiO3 wollastonite were examined at pressures to 23GPa and temperatures to 2000degreesC, using a Kawai-type multiavil apparatus. Enthalpies of high-pressure phases in CaSiO3 and in the CaSi2O5-CaTiSiO5 system were also measured by high-temperature calorimetry. At 17-18GPa, diopside dissociates to CaSiO3-rich perovskite + Mg-rich (Mg,Ca)SiO3 tetragonal garnet (Gt) above about 1400degreesC. The solubilities of CaSiO3 in garnet and MgSiO3 in perovskite increase with temperature. At 17-18GPa below about 1400degreesC, diopside dissociates to Ca-perovskite + beta-Mg2SiO4 + stishovite. The Mg, Si-phases coexisting with Ca-perovskite change to gamma-Mg2SiO4 + stishovite, to ilmenite, and finally to Mg-perovskite with increasing pressure. CaSiO3 wollastonite transforms to the walstromite structure, and further dissociates to Ca2SiO4 larnite + CaSi2O5 titanite. The latter transition occurs at 9-11 GPa with a positive Clapeyron slope. At 1600degreesC, larnite + titanite transform to CaSiO3 perovskite at 14.6 +/- 0.6 GPa, calibrated against the alpha-beta transition pressure Of Mg2SiO4. The enthalpies of fort-nation of CaSiO3 walstromite and CaSi2O5 titanite from the mixture of CaOandSiO(2) quartz at 298 K have been determined as -76.1 +/- 2.8,and -27.8 +/- 2.1 kJ/mol, respectively. The latter was estimated from enthalpy measurements of titanite solid solutions in the system CaSi2O5-CaTiSiO5, because CaSi7O5 titanite transforms to a triclinic phase upon decompression. The enthalpy difference between titanite and the triclinic phase is only 1.5 +/- 4.8 kJ/mol. Using these enthalpies of formation and those of larnite and CaSiO3 perovskite, the transition boundaries in CaSiO3 have been calculated. The calculated boundaries for the wollastonite-walstromite-larnite + titanite transitions are consistent with the experimental determinations within the errors. The calculated boundary between larnite + titanite and Ca-perovskite has a slope of 1.3-1.8(+/-0.4) MPa/K, and is located at a pressure about 2 GPa higher than that determined by. (C) 2004 Elsevier B.V. All rights reserved.