糀谷 浩

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.

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.

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.

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.

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.

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.

KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 +/- 7.4 kJ mol(-1) for sanidine --> wadeite + kyanite + coesite and 99.3 +/- 3.6 kJ mol(-1) for wadeite + kyanite + coesite --> hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160-700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as C-p=3.896 x 10(2)-1.823 x 10(3)T(-0.5)-1.293 x 10(7)T(-2)+1.631 x 10(9)T(-3) (C-p in J mol(-1) K-1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite --> hollandite intersects with the coesite-stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase-equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.

Raman spectroscopy of calcium ferrite type MgAl2O4 and CaAl2O4 and heat capacity measurement of CaAl2O4 calcium ferrite were performed. The heat-capacity of CaAl2O4 calcium ferrite measured by a differential scanning calorimeter (DSC) was represented as C-P(T) = 190.6-1.116 x 10(7) T-2 + 1.491 x 10(9) T-3 above 250 K (T in K). The obtained Raman spectra were applied to lattice dynamics calculation of heat capacity using the Kieffer model. The calculated heat capacity for CaAl2O4 calcium ferrite showed good agreement with that by the DSC measurement. A Kieffer model calculation for MgAl2O4 calcium ferrite similar to that for CaAl2O4 calcium ferrite was made to estimate the heat capacity of the former. The heat capacity of MgAl2O4 calcium ferrite was represented as C-P(T) = 223.4-1352T(-0.5) - 4.181 x 10(6) T-2 + 4.300 x 10(8) T-3 above 250 K. The calculation also gave approximated vibrational entropies at 298 K of calcium ferrite type MgAl2O4 and CaAl2O4 as 97.6 and 114.9 J mol(-1) K-1, respectively.

[1] MgSiO3-rich perovskite is expected to dominate Earth's lower mantle (pressures >25 GPa) with iron and aluminum as significant substituents. The incorporation of trivalent ions, M3+, may occur by two competing mechanisms: Mg-A + Si-B = M-A + M-B and Si-B = Al-B + 0.5 (vacancy)(O). Phase synthesis studies show that both substitutions do occur and the nonstoichiometric or defect substitution is prevalent along the MgSiO3-MgAlO2.5 join. Lattice parameters associated with the first substitution ( stoichiometric) show more rapid increases with increasing Al content than those for the second substitution ( nonstoichiometric), consistent with the differences in size of substituting ions. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO3 (perovskite) + 0.05 Al2O3 (corundum) = Mg0.95Al0.10Si0.95O3 (perovskite), has an enthalpy of - 0.8 +/- 2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al2O3 (corundum) = MgSi0.9Al0.1O2.95 (perovskite) has a small positive enthalpy of 8.5 +/- 4.6 kJ/mol. Configurational T DeltaS terms play a role in both substitutions. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowustite and may significantly affect the elasticity, rheology, and water retention of silicate perovskite in Earth.

In the Earth's mantle, the mechanism(s) of solid solution of Al in MgSiO3 perovskite strongly impacts its thermodynamic and transport properties. We present Al-27 NMR data for perovskite samples of nominal composition Mg(Si0.9Al0.1)O-2.95, to test a mechanism by which Al3+ substitutes at the octahedral Si4+ sites, leaving a corresponding number of O-site vacancies. We find evidence for this process in a significantly greater peak area for Al at B (Si) sites vs. A (Mg) sites in the structure, and the possible identification of a small concentration of five-coordinated Al adjacent to such vacancies. However, substitution of Al3+ at the A sites remains significant. As in perovskite-type technological ceramics, O-atom vacancies may play an important role in enhancing ion mobility and the dissolution of water.

MgSiO3 - rich perovskite is expected to dominate the Earth's lower mantle (pressures > 25 GPa), with iron and aluminum as significant substituents. The incorporation of trivalent ions, M3+, may occur by two competing mechanisms: Mg-A + Si-B = M-A + M-B and Si-B = Al-B + 0.5 V-O. Phase synthesis studies show that both substitutions do occur, and the nonstoichiometric or defect substitution is prevalent along the MgSiO3 - MgAlO2.5 join. Oxide melt solution calorimetry has been used to compare the energetics of both substitutions. The stoichiometric substitution, represented by the reaction 0.95 MgSiO3 (perovskite) + 0.05 Al2O3 (corundum) = Mg0.95Al0.10Si0.95O3 (perovskite), has an enthalpy of -0.8+/-2.2 kJ/mol. The nonstoichiometric reaction, 0.90 MgSiO3 (perovskite) + 0.10 MgO (rocksalt) + 0.05 Al2O3 (corundum) = MgSi0.9Al0.1O2.95 (perovskite) has a small positive enthalpy of 8.5+/-4.6 kJ/mol. The defect substitution is not prohibitive in enthalpy, entropy, or volume, is favored in perovskite coexisting with magnesiowustite, and may significantly affect the elasticity, rheology and water retention of silicate perovskite in the Earth.

Enthalpies of drop solution (DeltaH(drop-sol)) of CaGeO3, Ca(Si0.1Ge0.9)O-3, Ca(Si0.2Ge0.8)O-3, Ca(Si0.3-Ge0.7)O-3 perovskite solid solutions and CaSiO3 wollastonite were measured by high-temperature calorimetry using molten 2PbO .B2O3 solvent at 974 K. The obtained values were extrapolated linearly to the CaSiO3 end member to give DeltaH(drop-sol) of CaSiO3 perovskite of 0.2 +/-4.4 kJ mol(-1). The difference in DeltaH(drop-sol) between CaSiO3, wollastonite, and perovskite gives a transformation enthalpy (wo --> pv) of 104.4 +/-4.4 kJ mol(-1). The formation enthalpy of CaSiO3 perovskite was determined as 14.8 +/-4.4 kJ mol(-1) from lime + quartz or -22.2 +/-4.5 kJ mol(-1) from lime + stishovite. A comparison of lattice energies among A(2+)B(4+)O(3) perovskites suggests that amorphization during decompression may be due to the destabilizing effect on CaSiO3 perovskite from a large nonelectrostatic energy (repulsion energy) at atmospheric pressure. By using the formation enthalpy for CaSiO3 perovskite, phase boundaries between beta -Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where DeltaG(P, T) = 0] as the measured phase boundary. The calculations suggest that the phase equilibrium boundary occurs between 11.5 and 12.5 GPa around 1500 K. Its slope is still not well constrained.

High-temperature drop calorimetry in the temperature range of 1398-1785 K was performed for the samples of mixtures of synthetic anorthite (An), diopside (Di), enstatite (En) and forsterite (Fo) with the same compositions as those of primary melts generated at 1.1, 3 and 4 GPa at most 10 degrees above the solidus of anhydrous mantle peridotite in the CaO-MgO-Al2O3-SiO2 system. From the differences between the heat contents (H-T-H-298) of liquid and that of crystal mixture at the liquidus temperature, melting enthalpies of the samples of 1.1, 3 and 3 GPa-primary melt compositions were determined at 1 arm to be 531 +/- 39 J.g(-1) at 1583 K, 604 +/- 21 J.g(-1) at 1703 K, 646 +/- 21 J.g(-1) at 1753 K, respectively. These heat of fusion values suggest that mixing enthalpy of the melt in the An-Di-En-Fo system is approximately zero within the experimental errors when we use the heat of fusion of Fo by Richet et al. (P. Richet, F. Leclerc, L. Benoist, Melting of forsterite and spinel, with implications for the glass transition of Mg2SiO4 liquid, Geophys. Res. Lett. 20 (1993) 1675-1678). The measured enthalpies of melting at 1 atm were converted into those for melting reactions which occur under high pressures by correcting enthalpy changes associated with solid-state mineral reactions. Correcting the effects of pressure, temperature and FeO and Na2O components on the melting enthalpies at 1 atm, heat of fusion values of a representative mantle peridotite just above the solidus under high pressure were estimated to be 590 J at 1.1 GPa and 1523 K, 692 J at 3 GPa and 1773 K, and 807 J at 4 GPa and 1923 K for melting reactions producing liquid of 1 g, with uncertainties of 50 J. By applying these melting enthalpies to a mantle diapir model which generates present MORBs, a potential mantle temperature of 1533 K has been estimated, assuming an eruption temperature of magma of 1473 K. (C) 1997 Elsevier Science B.V.

Calorimetric data on heats of fusion of rocks have been very limited. In our investigation, high-temperature drop calorimetry was performed to measure heats of fusion of mantle rocks in the system CaO-MgO-Al₂O₃-SiO₂. Heats of fusion of natural mantle rocks under high pressure were estimated by correcting effects of FeO and Na₂O components, pressure and temperature on melting enthalpies to the observed heats of fusion. It is suggested that mixing enthalpy of silicate melt in the system CaO-MgO-Al₂O₃-SiO₂ is nearly zero by comparing the heats of fusion determined calorimetrically with those calculated by summing melting enthalpies of CaAl₂Si₂O₈, CaMgSi₂O₆, MgSiO₃ and Mg₂SiO₄.

Compositions of natural olivine tholeiites were simplified in the system CaO-Mgo-Al2O3-SiO2, and approximately eutectic composition in the system diopside(Di)- forsterite(Fo)- anorthite(An) (Di:Fo:An =49.0:7.5:43.5 wt%) was chosen as the model basalt composition. High-temperature drop calorimetry was performed at 1405 -1676K for the samples with the model basalt composition consisting of the mixture of synthetic diopside, forsterite and anorthite. The heat of fusion of the model basalt was obtained to be 506 +/- 38 J/g from the difference between the heat content of liquid and that of the mineral mixture at 1543K, which was the approximated eutectic temperature. This heat of fusion shows almost zero enthalpy of mixing in this system. Taking account of the pressure, temperature and composition effects on enthalpy, the heat of fusion of 490-560 J/g is estimated for generation of MORE (mid-ocean ridge basalt) in the upper mantle.

Compositions of natural olivine tholeiites were simplified in the system CaO‐MgO‐Al2O3‐SiO2, and approximately eutectic composition in the system diopside(Di)‐ forsterite(Fo) ‐ anorthite(An) (Di:Fo:An = 49.0:7.5:43.5 wt%) was chosen as the model basalt composition. High‐temperature drop calorimetry was performed at 1405–1676K for the samples with the model basalt composition consisting of the mixture of synthetic diopside, forsterite and anorthite. The heat of fusion of the model basalt was obtained to be 506±38 J/g from the difference between the heat content of liquid and that of the mineral mixture at 1543K, which was the approximated eutectic temperature. This heat of fusion shows almost zero enthalpy of mixing in this system. Taking account of the pressure, temperature and composition effects on enthalpy, the heat of fusion of 490–560 J/g is estimated for generation of MORB (mid‐ocean ridge basalt) in the upper mantle. Copyright 1995 by the American Geophysical Union.

Solution enthalpies of synthetic olivine solid solutions in the system Mg2SiO4 - Fe2SiO4 have been measured in molten 2 PbO . B2O3 at 979 K. The enthalpy data show that olivine solid solutions have a positive enthalpy of mixing and the deviation from ideality is approximated as symmetric with respect to composition, in contrast to the previous study. Applying the symmetric regular solution model to the present enthalpy data, the interaction parameter of ethalpy (W(H)) is estimated to be 5.3 +/- 1.7 kJ/mol (one cation site basis). Using this W(H) and the published data on excess free energy of mixing, the nonideal parameter of entropy (W(S)) of olivine solid solutions is estimated as 0.6 +/- 1.5 J/mol . K.

珪酸塩鉱物の熱力学的データは,地球のマントルにおける相平衡関係を計算するために不可欠なものである。本研究では,いくつかの珪酸塩鉱物の溶解,転移,及び融解のエンタルピーを測定するための高温熱量測定法を開発した。Mg₂SiO₄-Fe₂SiO₄オリビン固溶体の溶解エンタルピーは,ホウ酸鉛溶媒による溶解熱測定法で測定された。その結果は,オリビン固溶体が正の混合エンタルピーを持つことを示した。MgSiO₃オルソパイロキシン-ペロブスカイト転移のエンタルピーは,示差落下溶解熱測定法で得られた。CaMgSi₂O₆ディオプサイドの融解エンタルピーは,DSC法により測定された。これらの熱力学的データは,高圧力,高温下での相平衡境界を計算するためや,マグマの成因を議論するために用いられる。