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31.
Enthalpies of drop solution (ΔH
drop-sol) of CaGeO3, Ca(Si0.1Ge0.9)O3, Ca(Si0.2Ge0.8)O3, Ca(Si0.3Ge0.7)O3 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 ΔH
drop-sol of CaSiO3 perovskite of 0.2 ± 4.4 kJ mol−1. The difference in ΔH
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 A2+B4+O3 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 β-Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where ΔG(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.
Received: 20 September 2000 / Accepted: 17 January 2001 相似文献
32.
The crystal structure of MgFe2O4 was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite
undergoes a phase transformation at about 25 GPa, which leads to a CaMn2O4-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe2O4 form on the high-pressure phase transformations in the (MgSi)0.75(FeIII)0.5O3 system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a
spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe2O4 and Mg2SiO4. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure.
We thus conclude for the formation of Mg3Fe2Si3O12 perovskite.
Received: 27 March 2000 / Accepted: 1 October 2000 相似文献
33.
重点介绍了近年来国际地球物理学界在钙钛矿高温塑性研究领域内取得的进展。依据高温高压实验岩石学和矿物物理测试结果，科学家们提出硅酸盐钙钛矿是下地幔最主要的矿物相。采用相似材料理论和高温高压实验技术，在过去几年里对钙钛矿的高温塑性变形进行了深入细致的实验研究，就钙钛矿的高温塑性强度，流变机制和塑性各向异性等进行了探讨，这些开创性的研究特别强调了钙钛矿晶体结构相变和高温塑性的关系，并对应用矿物的晶体结构－－塑性系统相关性外推未知矿物流变强度的可能性作了探讨。作为一个直接的应用，这些研究成果被广泛的用来讨论有关的下地幔地球动力学问题，特别是下地幔的流动强度，流动机制和流动弱化。 相似文献
34.
M. Akaogi H. Kojitani T. Morita H. Kawaji T. Atake 《Physics and Chemistry of Minerals》2008,35(5):287-297
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. 相似文献
35.
Hitoshi Yusa Masaki Akaogi Nagayoshi Sata Hiroshi Kojitani Ryo Yamamoto Yasuo Ohishi 《Physics and Chemistry of Minerals》2006,33(3):217-226
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. 相似文献