Department of Chemistry

Titanoclinohumite, titanochondrodite, and associated Na-bearing tremolite occurring as crystal fragments in the Buell Park kimberlite are not likely crystallization products of a kimberlite magma. They more likely formed as phases in hydration assemblages of peridotite at temperatures below 700~ and pressures below 18 kbar. The crystals were dispersed in kimberlite as rock fragments were comminuted during transport to the surface.

Accurate measurements of the volume change of the synthetic garnets pyrope and almandine were performed under hydrostatic compression up to about 100 kbar by means of the X ray diffraction technique using a cubic anvil type of high-pressure apparatus. A least squares calculation using the Birch-Murnaghan equation gives the isothermal bulk modulus Kr = 1.71 + 0.03 Mbar for pyrope and Kr = 1.75 4-0.07 Mbar for almandine. INTRODUCTION Pyrope-rich garnet is generally accepted as one of the major constituent minerals in the earth's upper mantle. Ringwood and Major [1966] first suggested that pyroxene reacts with garnet and transforms to a complex garnet solid solution within the pressure range of the mantle transition zone. The pyroxenegarnet transformation has recently been established by systematic high-pressure high-temperature experiments on the systems enstatiterpyrope and ferrosilite-almandine [Liu, 1977; Akaogi and Akimoto, 1977]. The following mineralogic model for the mantle transition zone is proposed by them: (1) the region at depths between about 400 and 500 km is dominated by modified spinel (/•) and garnet with a minor amount of pyroxene, and (2) mixturesof/• phase and garnet and of Spinel (3•) and garnet are present in •the regions 'between about 500 and 600 km. Sinc e such a model suggests that the transition

Low-temperature isobaric heat capacities (C p ) of MgSiO 3 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 MgSiO 3 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 MgSiO 3 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.

We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg3A12SiaOa2 (pyrope) -MggSigOa2 (magnesium majorite). The vibrational spectra of Mg-majorite show a large number of additional weak peaks compared with the spectra of cubic pyrope garnet, consistent with tetragonal symmetry for the MgSiO3 garnet phase. The Raman bands for this phase show no evidence for line broadening, suggesting that Mg and Si are ordered on octahedral sites in the garnet. The bands for the intermediate garnet compositions are significantly broadened compared with the end-members pyrope and Mg-majorite, indicating cation disorder in the intermediate phases. Solid state 27A1 NMR spectroscopy for pyrope and two intermediate compositions show that AI is present only on octahedral sites, so the cation disorder is most likely confined to Mg-A1-Si mixing on the octahedral sites. We have also obtained a Raman spectrum for a natural, shock-produced (Fe,Mg) majorite garnet. The sharp Raman peaks suggest little or no cation disorder in this sample.

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO 3 . ZnGeO 3 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 GeO 6 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 GeO 6 octahedra in the ilmenite-perovskite transition. The correlation of quenchability with rotation angle of GeO 6 octahedra for other germanate perovskites is also discussed.

Topotactic reaction of the n = 2 RuddlesdenÀPopper phase Sr 3 Fe 2 O 7Àδ (δ ≈ 0.18) with polytetrafluoroethylene (PTFE) yields a highly fluorinated phase Sr 3 Fe 2 O 5+x F 2Àx (x ≈ 0.44), compared with Sr 3 Fe 2 O 6 F 0.87 prepared by the reaction of Sr 3 Fe 2 O 6 and F 2 gas. Structure analyses based on powder neutron diffraction, synchrotron powder diffraction, and 57 Fe M€ ossbauer spectroscopy measurements demonstrate that the new oxyfluoride perovskite has no anion deficiencies and adopts the tetragonal structure (space group I4/mmm) with the lattice constants a = 3.87264(6) Å and c = 21.3465(6) Å at room temperature. The fluoride ions preferentially occupy the terminal apical anion sites with oxide ions in a disordered manner, which results in square pyramidal coordination around iron. The present compound also shows an antiferromagnetic order with a N eel temperature (T N ) of 390 K, in sharply contrast to Sr 3 Fe 2 O 6 F 0.87 , which has a T N value that is lower than room temperature.

High-temperature drop calorimetry in the temperature range of 139% 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" above the solidus of anhydrous mantle peridotite in the CaO-MgO-Al,O,-SiO, system. From the differences between the heat contents (H, -Hzg8) of liquid and that of crystal mixture at the liquidus temperature, melting enthalpies of the samples of 1.1, 3 and 4 GPa-primary melt compositions were determined at 1 atm to be 531 + 39 J. g-' at 1583 K, 604 & 21 J. g-' at 1703 K, 646 + 21 J. g-' 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 MgzSiO, liquid, Geophys.

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°C) and Jd (1870°C) for that pressure were estimated experimentally. The melting temperature for the Di 50 Jd 50 composition at 15.5 GPa is 2270°C. The appearance of garnet is clearly dependent on initial clinopyroxene composition: at 1600°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 3perovskite (Ca-Pv) in Ca-rich systems, whereas Maj is associated with almost pure Jd up to a pressure of 21.5 GPa. Above $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 GPa. In the Di 70 Jd 30 system at 24 GPa an assemblage of Maj + Ca-Pv + MgSiO 3 with ilmenite structure (Mg-Il) 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 range 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.

Calorimetric measurements of fusion enthalpies for Ni 2 SiO 4 and Co 2 SiO 4 olivines were carried out using a high-temperature calorimeter, and Ni and Co partitioning between olivine and silicate liquid was analyzed using the measured heats of fusion. The fusion enthalpy of Co 2 SiO 4 olivine measured by transposedtemperature drop calorimetry was 103 Ϯ 15 kJ/mol at melting point (1688 K). The fusion enthalpy of Ni 2 SiO 4 olivine was calculated based on the enthalpies of liquids in the system An 50 Di 50 -Ni 2 SiO 4 measured by transposed-temperature drop calorimetry at 1773 K, and was 221 Ϯ 26 kJ/mol at its metastable melting point (1923 K). The fusion enthalpy of Ni 2 SiO 4 is the largest among those of olivine group, this is caused by the large crystal field stabilization energy of six-coordinated Ni 2ϩ in olivine. The larger fusion enthalpy of Ni 2 SiO 4 can account for the large and variable partition coefficient of Ni between olivine and silicate liquid. Based on the comparison between partition coefficients calculated from thermodynamic data and those observed in partition experiments, it is considered that the magnitude of partition coefficients is primarily dependent on the heats of fusion of the components. Furthermore, the activity coefficients for Ni-, Co-and Mn-bearing components in magmatic liquid are nearly of the same magnitude.

The garnet-perovskite phase transformation in CaGe03 was investigated in the pressure-temperature region to 6.5 GPa and 1200øC using a cubic anvil type of high-pressure apparatus combined with synchrotron radiation. In-situ measurements with an energy dispersive x-ray diffraction system enable us to carry out dynamical observation of the transformation. The equilibrium phase boundary between the garnet and perovskite phases was determined as P(GPa)= 6.9 -0.0008T(øC).

Compositions of natural olivine tholeiites were simplified in the system CaO-MgO-A1203-SiO 2, 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, forsteritc 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.

Heat capacity of MgSiO 3 perovskite has been measured in the temperature range 140-295 K by differential scanning calorimetry. The Kieffer model approach has been used to calculate the heat capacity beyond the measured temperature range. The estimated entropy of MgSiO 3 perovskite is 57.2+1.0 J/mol. K at 298 K which is higher than that of ilmenite, in accord with the negative pressuretemperature slope for the ilmenite-perovskite transition. From the heat capacity thermodynamic Debye temperature of MgSiO 3 perovskite is evaluated to be 980•_15 K at 298 K and 1030+20 K at temperatures above about 700 K. Thermodynamic Cmmeisen parameter of MgSiO 3 perovskite at 30 GPa and above 1000 K is also estimated as 1.45+0.15. An adiabatic temperature profile in the pyrolitic lower mantle is calculated with the average gradient of 0.32+0.04 K•m. The temperature at the top of the D" layer is estimated to be 2500+100 K. Introduction Magnesium-rich (Mg, Fe)SiO 3 perovskite is accepted to be the most abundant constituent mineral in the lower mantle [e.g. Ito and Takahashi, 1989]. Thermophysical and thermochemical properties of the silicate per0vskite are of essential importance to calculate the phase relations and to estimate the thermal state in the lower mantle. Although heat capacity is a fundamental thermodynamic quantity which allows us to calculate entropy, Debye temperature, Griineisen parameter etc., no experimental measurement has been made on the silicate perovskite. In this study we have measured directly the isobaric heat capacity (Cp) of MgSiO 3 perovskite in the temperature range of 140-295 K at 1 bar. The heat capacity of MgSiO 3 perovskite at 0-1000 K is also calculated by the Kieffer's lattice vibrational model. The entropy and thermodynamic Debye temperature of perovskite are calculated directly from the heat capacity. Thermodynamic Gruneisen parameter of perovskite at high. pressure is also estimated using the heat capacity, and is used to calculate an adiabatic geotherm in the lower mantle. Experimental Method MgSiO 3 orthorhombic perovskite of about 12 mg was synthesized at 25 GPa and 1950 K using a uniaxial splitsphere apparatus [Ito and Yamada, 1982] from a starting material of synthetic MgSiO 3 orthopyroxene provided by Dr. M. Ozima. The starting material was put into a tubular rhenium furnace. Microfocused x-ray diffraction analysis

Quartz-type GeO 2 was compressed at room temperature up to pressures of 11, 15 and 18 GPa, and the samples recovered to i atm were mostly amorphized. Measured densities of the amorphous phases increased with increasing synthesis pressure: the amorphous phase synthesized at 18 GPa was 32 % denser than GeO 2 quartz. Enthalpies of the amorphous phases relative to GeO2 quartz and glass prepared at I atm were measured by lead borate solution calorimetry. The enthalpies increased almost linearly with decreasing molar volumes from GeO 2 quartz to the 18 GPa-amorphous phase. The enthalpy of the 18GPa-amorphous phase was about 18 kJ/mol higher than GeO2 quartz, and about 4 kJ/mol higher than the I atm-glass. The higher enthalpy relative to GeO2 quartz is interpreted as the energy associated with disordering of the quartz structure and collapse of the tetrahedral framework which lead to octahedral coordination of germanium atoms.

High-pressure structural phase transitions in NaNiF 3 and NaCoF 3 were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO 3 type) started to transform into postperovskite phases (CaIrO 3 type) at about 11−14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF 3 broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF 3 . Elemental analysis of the recovered amorphous samples indicated that the NaCoF 3 postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF 3 even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF 3 , such a postpostperovskite phase was not found in the present compounds.

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

Isobaric heat capacities of •-quartz, coesite, and stishovite were measured by differential scanning calorimetry at 183-703 K. The heat capacity data of coesite represent a significant revision of the previous data. The heat capacities and entropies were also calculated using Kieffer's model. By high-temperature solution calorimetry, transition enthalpy for the 13-quartz-coesite at 979 K was measured to be 1.27ñ0.39 kJ/mol. The enthalpies for the •-quartz-coesite and coesite-stishovite transitions at 298 K were measured to be 3.40ñ0.56 and 33.62ñ1.01 kJ/mol, respectively, by differential drop-solution calorimetry. The enthalpy of the coesite-stishovite transition obtained in this study is about 10-15 kJ/mol smaller than those in the previous studies. Phase relations in SiO: at high pressures and high temperatures were calculated using these new thermodynamic data. The calculated boundary for the •-quartz-coesite transition is consistent with those determined experimentally. For the coesite-stishovite transition, the calculated boundary having a slope of 2.5ñ0.3 MPa/K disagrees with that by Yagi and Akimoto but is generally consistent with that by Zhang et al.'s new in situ X ray diffraction study.