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1.
Paul McMillan Masaki Akaogi Eiji Ohtani Quentin Williams Ronald Nieman Robert Sato 《Physics and Chemistry of Minerals》1989,16(5):428-435
We have obtained infrared and Raman spectra for garnets synthesized at high (static) pressures and temperatures along the join Mg3Al2Si3O12 (pyrope) — Mg4Si4O12 (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 27Al NMR spectroscopy for pyrope and two intermediate compositions show that Al is present only on octahedral sites, so the cation disorder is most likely confined to Mg-Al-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. 相似文献
2.
Solution enthalpies of synthetic olivine solid solutions in the system Mg2SiO4-Fe2SiO4 have been measured in molten 2PbO·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 (WH) is estimated to be 5.3±1.7 kJ/mol (one cation site basis). Using this Wh and the published data on excess free energy of mixing, the nonideal parameter of entropy (Ws) of olivine solid solutions is estimated as 0.6±1.5 J/mol·K. 相似文献
3.
Infrared absorption spectra of the high-pressure polymorphs β-Mg2SiO4 and β-Co2SiO4 have been measured between 0 and 27 GPa at room temperature. Grüneisen parameters determined for 11 modes of β-Mg2SiO4 (frequencies of 300 to 1,050 cm?1) and 5 modes of β-Co2SiO4 (490 to 1,050 cm?1) range between 0.8 and 1.9. Averaging the mid-infrared spectroscopic data for β-Mg2SiO4 yields an average Grüneisen parameter of 1.3 (±0.1), in good agreement with the high-temperature thermodynamic value of 1.35. Similarly, we find a value of 1.05 (±0.2) for the average spectroscopic Grüneisen parameter of β-Co2SiO4. 相似文献
4.
Apurva Mehta Kurt Leinenweber Alexandra Navrotsky Masaki Akaogi 《Physics and Chemistry of Minerals》1994,21(4):207-212
A calorimetric study of the ilmenite and lithium niobate polymorphs of FeTiO3 was undertaken to assess the high-pressure stabilities of these phases. Ilmenite is known to be the stable phase at ambient pressure, but the lithium niobate form may be a quench phase from a perovskite form which has been previously observed in situ at high pressure.In this study, the lithium niobate phase of FeTiO3 was synthesized from an ilmenite starting material at 15– 16 GPa and 1473 K, using a uniaxial split-sphere high-pressure apparatus (USSA 2000). The energetics of the ilmenite to lithium niobate transformation were investigated through transposed-temperature drop calorimetry. The heat of back-transformation of lithium niobate to ilmenite was measured by dropping the sample in argon from ambient conditions to a temperature where the transformation occurs spontaneously. In drops made at 977 K, an intermediate x-ray amorphous phase was encountered. At 1273 K, the transformation went to completion. A value of -13.5±1.2 kJ/mol was obtained for the heat of transformation. 相似文献
5.
Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg2+Fe2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg2SiO4Fe2SiO4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe2+/(Mg + Fe2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe2+/(Mg + Fe2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg2+Fe2+ partition relation among coexisting minerals, an increase of about 1% in the Fe2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km. 相似文献
6.
The structural and elastic properties of the ilmenite and perovskite phases of MgSiO3 are investigated with a computational model based on energy minimization. The potential energies of these two crystals are approximated by the sum of Coulomb, van der Waals, and repulsion terms between atoms. Required energy parameters are derived by fitting the parameters to the observed crystal structures of these two phases as well as to the measured elastic constants of the ilmenite phase. The resulting potential model is applied to predicting the elastic constants of the perovskite phase. The calculated bulk modulus of the perovskite phase compares favorably with the data obtained from volume-compression experiments as well as the values estimated from empirical elasticity systematics of perovskite type compounds. The predicted shear modulus of the perovskite phase is also in reasonable agreement with the values proposed from similar empirical elasticity systematics. Subsequently, the model is used to simulate the high pressure behaviors of the crystal structures and elastic constants of these two phases. 相似文献
7.
At high pressures, CdGeO3 pyroxenoid transforms to garnet, then to ilmenite, and finally to perovskite. Enthalpies of transition among the four phases were measured by high temperature calorimetry. The entropies of transition and slopes of the boundaries were calculated using the measured enthalpies and free energies calculated from the phase equilibrium data. Pyroxenoid and garnet are very similar energetically. However garnet is a high pressure phase because of its lower entropy and smaller volume. The pyroxenoid-garnet transition has a small positiveP-T slope. Ilmenite is intermediate in enthalpy between garnet and perovskite, but is lower in entropy than both phases. Therefore the garnet-ilmenite transition has a positivedP/dT, while a negativedP/dT is calculated for the ilmenite-perovskite transition. The thermochemical data for the CdGeO3 phases are generally consistent with the observed high pressure phase relations. The high entropy of perovskite relative to ilmenite, observed in several ABO3 comounds including CdGeO3, is related to the structural features of perovskite, in which relatively small divalent cations occupy the large sites of 8–12 fold coordination. The thermochemistry of the CdGeO3 polymorphs shows several similarities to that of the CaGeO3 system. 相似文献
8.
Masaki Akaogi Akira TanakaEiji Ito 《Physics of the Earth and Planetary Interiors》2002,132(4):303-324
Phase relations in the system Mg4Si4O12-Mg3Al2Si3O12 were examined at pressures of 19-27 GPa and relatively low temperatures of 800-1000 °C using a multianvil apparatus to clarify phase transitions of pyroxene-garnet assemblages in the mantle. Both of glass and crystalline starting materials were used for the experiments. At 1000 °C, garnet solid solution (s.s.) transforms to aluminous ilmenite s.s. at 20-26 GPa which is stable in the whole compositional range in the system. In Mg4Si4O12-rich composition, ilmenite s.s. transforms to a single-phase aluminous perovskite s.s., while Mg3Al2Si3O12-rich ilmenite s.s. dissociates into perovskite s.s. and corundum s.s. These newly determined phase relations at 1000 °C supersede preliminary phase relations determined at about 900 °C in the previous study. The phase relations at 1000 °C are quite different from those reported previously at 1600 °C where garnet s.s. transforms directly to perovskite s.s. and ilmenite is stable only very close to Mg4Si4O12. The stability field of Mg3Al2Si3O12 ilmenite was determined at 800-1000 °C and 25-27 GPa by reversed phase boundaries. In ilmenite s.s., the a-axis slightly increases but the c-axis and molar volume decrease substantially with increasing Al2O3 content. Enthalpies of ilmenite s.s. were measured by differential drop-solution calorimetry method using a high-temperature calorimeter. The excess enthalpy of mixing of ilmenite s.s. was almost zero within the errors. The measured enthalpies of garnet-ilmenite and ilmenite-perovskite transitions at 298 K were 105.2±10.4 and 168.6±8.2 kJ/mol, respectively, for Mg4Si4O12, and 150.2±15.9 and 98.7±27.3 kJ/mol, respectively, for Mg3Al2Si3O12. Thermodynamic calculations using these data give rise to phase relations in the system Mg4Si4O12-Mg3Al2Si3O12 at 1000 and 1600 °C that are generally consistent with those determined experimentally, and confirm that the single-phase field of ilmenite expands from Mg4Si4O12 to Mg3Al2Si3O12 with decreasing temperature. The earlier mentioned phase relations in the simplified system as well as those in the Mg2SiO4-Fe2SiO4 system are applied to estimate mineral proportions in pyrolite as a function of depth along two different geotherms: one is a horizontally-averaged temperature distribution in a normal mantle, and the other being 600 °C lower than the former as a possible representative geotherm in subducting slabs. Based on the previously described estimated mineral proportions versus depth along the two geotherms, density and compressional and shear wave velocities are calculated as functions of depth, using available mineral physics data. Along a normal mantle geotherm, jumps of density and velocities at about 660 km corresponding to the post-spinel transition are followed by steep gradients due to the garnet-perovskite transition between 660 and 710 km. In contrast, along a low-temperature geotherm, the first steep gradients of density and velocities are due to the garnet-ilmenite transition between 610 and 690 km. This is followed by abrupt jumps at about 690 km for the post-spinel transition, and steep gradients between 700 and 740 km that correspond to the ilmenite-perovskite transition. In the latter profile along the low-temperature geotherm, density and velocity increases for garnet-ilmenite and ilmenite-perovskite transitions are similar in magnitude to those for the post-spinel transition. The likely presence of ilmenite in cooler regions of subducting slabs is suggested by the fact that the calculated velocity profiles along the low-temperature geotherm are compatible with recent seismic observations indicating three discontinuities or steep velocity gradients at around 600-750 km depth in the regions of subducting slabs. 相似文献
9.
M. Akaogi M. Haraguchi K. Nakanishi H. Ajiro H. Kojitani 《Earth and Planetary Science Letters》2010,289(3-4):503-508
High-pressure phase relations in the system NaAl3Si3O11–CaAl4Si2O11 were examined at 13–23 GPa and 1600–1900 °C, using a multianvil apparatus. A Ca-aluminosilicate with CaAl4Si2O11 composition, designated CAS phase, is stable above about 13 GPa at 1600 °C. In the system NaAl3Si3O11–CaAl4Si2O11, the CAS phase dissolving NaAl3Si3O11 component coexists with jadeite, corundum and stishovite below 22 GPa, above which the CAS phase coexists with Na-rich calcium ferrite, corundum and stishovite. At 1600 °C, the solubility of NaAl3Si3O11 component in the CAS solid solution increases with increasing pressure up to about 50 mol% at about 22 GPa, above which the solubility decreases with pressure. The maximum solubility of NaAl3Si3O11 component in the CAS phase increases with temperature up to around 70 mol% at 1900 °C at 22 GPa. The dissociation of NaAlSi2O6 jadeite to NaAlSiO4 calcium ferrite plus stishovite occurs at about 22 GPa. Lattice parameters of the CAS phase with the hexagonal Ba-ferrite structure change with increase of the NaAl3Si3O11 component: a-axis decreases and c-axis slightly increases, resulting in decrease of molar volume. Enthalpies of the CAS solid solutions were measured by high-temperature drop-solution calorimetry techniques. The results show that enthalpy of hypothetical NaAl3Si3O11 CAS phase is much higher than the mixture of NaAlSi2O6 jadeite, corundum and stishovite and is close to that of the mixture of NaAlSiO4 calcium ferrite, corundum and stishovite. When we adopt the Na:Ca ratio of 75:25 of the natural Na-rich CAS phase in a shocked Martian meteorite, Zagami, the phase relations determined above suggest that the natural CAS phase crystallized from melt at pressure around 22 GPa and temperature close to or higher than 2000–2200 °C. The inferred P, T conditions are consistent with those estimated using other high-pressure minerals in the shocked meteorite. 相似文献
10.
Phase relations in the system CaTiO3-CaSiO3 were experimentally examined at 5.3–14.7 GPa and 1200–1600 °C with a 6–8 type multianvil apparatus. As pressure increases, stability field of perovskite solid solution extends from CaTiO3 to CaSiO3, and the perovskite becomes stable for the entire composition range above about 12.3 GPa. The stability field of Ca(Ti1?X, SiX)2O5 (0.78<x≦1) titanite solid solution +Ca2SiO4 larnite exists in the CaSiO3-rich composition range at 9.3–12.3 GPa and 1200 °C. Perovskite solid solutions containing CaSiO3 component of 0 to 66 mol% could be quenched to 1 atm. The composition-molar volume relationship of perovskite solid solution showed that molar volume of perovskite solid solution linearly reduces from the value of CaTiO3 to that of CaSiO3. 相似文献