Thermal expansion of SrZrO3 and BaZrO3 perovskites

High-temperature X-ray diffraction studies of SrZrO₃ and BaZrO₃ perovskites have been carried out to 1200° C. The diffraction patterns are analyzed with Rietveld method so as to refine the unit cell dimensions. The volumetric thermal expansion coefficient are observed to be 2.98*10^-5K^-1 for orthorhombic Pbnm phase, 3.24*10^-5K^-1 for orthorhombic Cmcm phase, 3.75*10^-5K^-1 for tetragonal I4/mcm phase of SrZrO₃ perovskite, and 2.06*10^-5K^-1 for cubic Pm3m phase of BaZrO₃ perovskite, respectively. The linear thermal expansion coefficients of SrZrO₃ perovskite show considerable anisotropy of α _a >α _c >α _b for orthorhombic Pbnm phase, which reflect the decrease of distortion of the perovskite. It is demonstrated that thermal expansion of the centrosymmetrically distorted ABX₃ perovskite can be empirically expressed as a combination of the changes of [B-X] bond length and tilting angle of BX₆ octahedral framework. The octahedral tilting is considered to be the primary order parameter for the ferroelastic type of structural phase transitions in perovskite. Thermodynamically, the tilting induced volume change denotes the “excess volume” and the corresponding thermal expansion represents the “excess thermal expansion” for the lower symmetry phase with respect to its prototype of the cubic perovskite.     

Compression and coordination changes in pyroxenoids: an EXAFS study of MgGeO3 enstatite and CaGeO3 wollastonite

The evolution of the distortion of MgGeO₃ enstatite and CaGeO₃ wollastonite with increasing pressure, has been investigated using X-ray absorption spectroscopy (XAS) in a diamond anvil cell. At room temperature and low pressure (P<7 GPa), the compressibility of the GeO₄ tetrahedron is higher in MgGeO₃ enstatite (K _[GeO4]∼135 GPa) than in CaGeO₃ wollastonite (K _[GeO4]≥ 280 GPa). The compression mechanisms of the two compounds are different: the whole mineral compressibility of Ge-enstatite appears to be very homogeneous, in contrast to that of Ge-wollastonite which exhibits an inhomogeneous tretrahedral compressibility. This result is consistent with the variation of the Debye-Waller factors of the two compounds with increasing pressure. At higher pressures, the coordination of germanium atoms in the two compounds gradually changes from fourfold to sixfold. For CaGeO₃ the coordination change starts at 7 GPa and is complete a 12 GPa, whereas it starts at about 8.5 GPa for MgGeO₃ and is not complete at 31 GPa. The progressive evolution of the measured Ge-O distances as well as the modification in the X-ray absorption near-edge structure indicate two coexisting different sites rather than a progressive site modification. The transformation is found to be partially reversible in CaGeO₃ wollastonite, whereas it is totally reversible in MgGeO₃ enstatite.     

Calorimetric study of perovskite solid solutions in the CaSiO3–CaGeO3 system

 Enthalpies of drop solution (ΔH _drop-sol) of CaGeO₃, Ca(Si_0.1Ge_0.9)O₃, Ca(Si_0.2Ge_0.8)O₃, Ca(Si_0.3Ge_0.7)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     

Raman scattering study of the high-temperature vibrational properties and stability of CaGeO3 perovskite

The vibrational properties of metastable Ca-GeO₃ perovskite are investigated as a function of temperature using Raman scattering. Mode-assignments are derived from polarized spectra of optically oriented single crystals. Neither symmetry-breaking structural transformations nor soft-mode behavior is revealed in the Raman spectra between room temperature and 650° C. Only a small decrease in the local static octahedral tilt angles can be inferred from the Raman data over this temperature range. A Landau extrapolation of the lowest frequency A_g modes suggests that these modes become critical near 2000° C, a temperature that is above the extrapolated zero pressure melting point.     

A high-temperature and high-pressure Raman spectroscopic study of CaGeO3 garnet

 High-pressure and high-temperature Raman spectra of CaGeO₃ tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO₃ majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO₃ garnet. The observed mode shifts of CaGeO₃ garnet were used to determine the isothermal and isobaric mode Grüneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a _i , for CaGeO₃ garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10⁻⁵ K⁻¹ to −1.3 × 10⁻⁵ K⁻¹, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO₃ majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO₃ tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour. Received: 21 April 1999 / Revised, accepted: 11 September 1999     

Oxide perovskites: pressure derivatives of the bulk and shear moduli

The pressure derivatives of elastic moduli (∂M/∂P; M=K_S and G) for a suite of polycrystalline oxide perovskites (2 titanates, 1 stannate and 2 aluminates) have been measured up to 3 GPa using the ultrasonic interferometry method combined with a buffer rod technique. Two empirical systematic relationships (∂G/∂P vs K_S/G and ∂K_S/∂P vs K_S (/ρ)^1/3) have been used to investigate the elasticity systematics of this suite of perovskites and to estimate ∂M/∂P of MgSiO₃ perovskite. The pressure derivatives ∂G/∂P and ∂K_S/∂P for this suite of perovskites scatter between well-defined linear trends for the rutile, rocksalt and spinel structures. The more diffuse trends observed for the perovskites might reflect greater flexibility in the response of its corner-connected octahedral framework structure to changing pressure. The pressure derivatives of the elastic moduli for MgSiO₃ perovskite estimated by the “perovskite bands” are ∂G/∂P=1.6–2.2 and ∂K_S/∂P=3.9–4.2. Received: 13 November 1997 / Revised, accepted: 31 August 1998     

An EXAFS study of Gd,Er and Lu site location in the epidote structure

L _III edge extended X-ray absorption fine structure (EXAFS) spectra of Gd, Er and Lu environments in synthetic epidotes of composition CaLa_0.9 X _0.1Al₂MgSi₃O₁₃H (X=Gd or Er or Lu) were recorded using synchrotron radiation. The Fourier transforms of the Gd-, Er- and Lu-EXAFS are clearly different from one another indicating wholly or partially different site occupancy. Model fitting of the Fourier-filtered partial EXAFS and comparison of pair distribution functions with those calculated for natural epidote leads to the conclusion that three different sites are probably involved in the accommodation of these elements in the epidote structure, and that site preference is a function of the rare earth ionic size. Gd is located in A2-type sites, whereas the local atomic environment of Er is consistent with A1 site occupancy and the Lu environment has been modelled on an M3-type octahedral site.     

Structural relaxation in the MnCO3–CaCO3 solid solution: a Mn K-edge EXAFS study

Mn K-edge EXAFS spectroscopy of solid-solution samples encompassing the complete MnCO₃–CaCO₃ series shows that first-shell Mn–O distances deviate little from the 2.19-Å distance observed in pure MnCO₃. Very slight lengthening is observed only in the limiting case of dilute Mn(II) calcite solid solutions, where the Mn–O distance is 2.21 Å. The observed nearly complete structural relaxation and the composition independence of the Mn–O distance are consistent with the Pauling model behavior of solid solutions, and agree with previous studies showing a high degree of relaxation around hetero-sized substituents in the calcite structure. Strain occurs through bond bending, which is facilitated by the exclusively corner-sharing topology of calcite. Observed distances from Mn to more distant neighbors show significant variation across the solid-solution series that resembles Vegard's law-type behavior but reflects averaging. The high degree of relaxation suggests modest enthalpies of mixing in the solution, consistent with calorimetric studies.     

Contribution to the crystal chemistry of orthorhombic perovskites: MgSiO3 and NaMgF3

The structures of MgSiO₃ and NaMgF₃ are described in terms of the angle ø by which the SiO₆ (MgF₆) octahedra are rotated from the ideal cubic perovskite structure. The expected effects of temperature and pressure on ø (and hence on the atomic coordinates and volume) are discussed. It is predicted that the effect of pressure will be to decrease the coordination of Mg in MgSiO₃.     

Phase transition in CaGeO3 perovskite: Evidence from X-ray powder diffraction,thermal expansion and heat capacity

High-temperature x-ray powder diffraction study by the full pattern Rietveld method of orthorhombic CaGeO₃ (Pbnm at ambient condition) perovskite confirms the previously observed phase transition at Tc=520 K. The measured volumetric thermal expansion coefficients are 3.1 x 10^-5 (K^-1) below Tc and 3.5x 10^-5 (K^-1) above Tc. The space group at T>Tc has been tentatively identified as Cmcm. Such a transition involves the disappearance of one of the two octahedral rotations in the (001) plane, and the doubling of the unit cell volume, with c axis unchanged. Although this transition should be of first order from symmetry considerations, the distortion of the Pbnm phase decreases continuously as the temperate approaches Tc and there is no observable volume discontinuity at Tc. The measured heat capacity places an upper limit on the enthalpy of transition of 50 J/mol, which is quite reasonable in terms of the crystallographic nature of this phase transition.A National Science Foundation Science and Technology Center     

高温高压菱镁矿电导率实验研究

在俯冲带中,菱镁矿相比于其他碳酸盐矿物会在更高的温度压力环境下稳定存在,常被认为是地球深部碳循环的主要载体,其电学性质的研究对俯冲带及地球深部的电性结构具有重要的意义。本文在YJ-3000t高温高压设备上利用Solartron-1260阻抗/增益-相位分析仪,在1～3 GPa和773～1173 K条件下,对天然菱镁矿的电导率进行了原位测量。实验结果表明,在实验条件下,天然菱镁矿的电导率在10^-2～10^-7 S/m范围内,菱镁矿的电导率对温度有很强的依赖性,随着温度的升高,电导率增大,电导率与压力的关系呈正相关,随着压力的升高而增强。结合前人的研究成果及样品电导率随温度、压力的效应和活化能,可推断天然菱镁矿导电机制为大极化子导电。     

Elasticity of CaTiO3, SrTiO3 and BaTiO3 perovskites up to 3.0 Gpa: the effect of crystallographic structure

Elasticity of CaTiO₃, SrTiO₃ and BaTiO₃ perovskites has been experimentally investigated as a function of pressure up to 3.0 GPa in a liquid-medium piston cylinder apparatus using a high precision ultrasonic interferometric technique. Specimens used are hot-pressed fine-grained (3–10 μm) polycrystalline aggregates with low porosity (<1.5%). Compressional and shear wave velocities and their pressure derivatives have been measured. The results are compared with previous studies on other perovskites and the role of structural transitions is examined. We find that the role of Ti-O₆ polyhedral tilting (such as observed in CaTiO₃) is small in the sense that a single well-defined general trend exists in perovskites with a wide range of tilting angles, although there is suggestion that cubic perovskites have slightly higher bulk modulus than orthorhombic perovskites. In contrast, cation-anion displacement that changes crystal symmetry from cubic to tetragonal in BaTiO₃ has very large effects on elasticity. This distortion significantly reduces the bulk modulus (but not much the shear modulus) and results in an unusually large pressure derivative of bulk modulus (dK/dP～10). A large change in elasticity in BaTiO₃ associated with the structural transition (without a significant volume change) is a clear example of the breakdown of the Birch's law between densities and elastic wave velocities.     

A quantum-mechanical study of the relative stability under pressure of MgSiO3-ilmenite,MgSiO3-perovskite,and MgO-periclase+SiO2-stishovite assemblage

The relative stability of MgSiO₃-ilmenite, MgSiO₃-perovskite and (periclase+stishovite) assemblage phases as a function of the pressure is investigated with the periodic quantum mechanical ab initio HartreeFock program CRYSTAL. For the first time, the structure of MgSiO₃-ilmenite is fully optimized. Basis set effects are explored. It turns out that relatively small basis sets reproduce correctly experimental geometries. However, larger basis sets (triple zeta quality, plus polarization d functions) are needed to yield significant thermochemical results. All contributions to the 0 K enthalpy are discussed. On the basis of the present highest level calculations, it appears that in the explored range of pressure (0P< 60=" gpa)=" the=" mineralogical=" assemblage=" periclase+stishovite=" has=" higher=" enthalpy=" than=">₃-ilmenite or perovskite, and that ilmenite transforms to orthorhombic perovskite around to 29.4 GPa in good agreement with experimental data extrapolated down to 0 K.     

承压破碎煤体空隙结构演化及渗流特征

垮落带破碎煤岩体空隙结构演变直接影响垮落带气体渗透特性,进而影响废弃矿井采空区煤层气运移富集规律。利用自主设计研发的第二代破碎煤岩体压实−渗流−CT扫描试验系统,开展5种粒径破碎无烟煤在不同气体压力、轴向应力及空隙率条件下162组渗透实验。结果表明：(1) 气体在破碎无烟煤中运移时气体压力、流量随时间一直处于动态变化直至达到吸附平衡,流量、压力恒定。(2) 低雷诺数下气体在破碎无烟煤中流动需要拟启动压力梯度,其值处于158.89~1 408.64 Pa/m,并随着轴压增大、空隙率减小而增大。(3) 破碎无烟煤渗透率处于10⁻¹²~10⁻¹⁰ m²,且渗透率随气体压力、空隙率增加分别呈现对数、指数函数式增长趋势。(4) 颗粒粒径越大则相同装料空间内破碎煤样初始空隙越大,更易于气体流动,因此相同空隙率时破碎无烟煤渗透率随粒径的增大而增大;不同粒径颗粒渗透率平均变化幅度均随空隙率减小而减小;然而颗粒粒径越大,减小相同空隙率时破碎无烟煤渗透率变化幅度减小越明显。废弃矿井采空区煤层气地面抽采时,地面钻井应优先布设在垮落带破碎煤岩体连同纵向“高位环形裂隙体”共同构成的“U”型高渗煤层气富集区内。

     

矿物反应与变形关系研究——以糜棱岩高温高压实验为例

矿物反应和变形局部化在中下地壳普遍存在,两者相互影响和促进。实验研究表明,矿物反应与变形关系非常复杂。本文在糜棱岩高温高压流变实验的基础上,分析了实验变形样品中的矿物反应分布特征以及矿物反应引起的化学成分变化,讨论了矿物反应与变形的相互影响。微观结构分析表明,实验变形后的糜棱岩样品在温度800~890℃时,角闪石和黑云母出现脱水反应,生成微晶角闪石和黑云母,并伴有局部熔融。受应变局部化控制,脱水反应产物主要出现在黑云母、角闪石条带边缘。微晶和熔体的成分分析表明,不仅脱水反应形成的微晶与熔体的SiO 2含量非常低,而且黑云母周围的反应产物和熔体主要来自于黑云母的脱水,角闪石边缘的反应产物和熔体主要来自于角闪石脱水,石英、钾长石和斜长石没有参与反应与熔融。本研究中的脱水反应产物中,没有发现辉石和石榴石,这种脱水反应与文献中报道的无局部化的均匀样品在静高压和高熔融比例条件的脱水反应产物和熔体的成分有很大差别。黑云母和角闪石的局部化分布和脱水程度低,可能是造成脱水反应产物有差别的巨大原因。在本实验结果中,脱水反应对变形的影响主要体现为,脱水反应产生了细粒混合矿物相,使得在局部化的剪切带内变形机制从位错蠕变转变为扩散蠕变,导致样品出现应变弱化。另外脱水反应还引起了局部脆性破裂。变形引起晶体塑性变形,增加了位错密度和矿物细粒化,促进了晶体内部成核和黑云母与角闪石的脱水分解;差应力作用增加了局部的正应力和平均应力,增加了黑云母和角闪石能够稳定存在的压力范围,这可能是反应产物以微晶黑云母和角闪石为主,而没有转化为辉石的原因。     

High-pressure behavior of MnGeO3 and CdGeO3 perovskites and the post-perovskite phase transition

The stability and high-pressure behavior of perovskite structure in MnGeO₃ and CdGeO₃ were examined on the basis of in situ synchrotron X-ray diffraction measurements at high pressure and temperature in a laser-heated diamond-anvil cell. Results demonstrate that the structural distortion of orthorhombic MnGeO₃ perovskite is enhanced with increasing pressure and it undergoes phase transition to a CaIrO₃-type post-perovskite structure above 60 GPa at 1,800 K. A molar volume of the post-perovskite phase is smaller by 1.6% than that of perovskite at equivalent pressure. In contrast, the structure of CdGeO₃ perovskite becomes less distorted from the ideal cubic perovskite structure with increasing pressure, and it is stable even at 110 GPa and 2,000 K. These results suggest that the phase transition to post-perovskite is induced by a large distortion of perovskite structure with increasing pressure.     

高静水压力压实作用下疏松砂岩渗透特性演化及其机制

以江汉盆地水热型地热田疏松砂岩为研究对象,施加静水压力至实测地层应力(12.5MPa),待试样变形稳定后,在恒定渗透流量下研究了疏松砂岩在高静水压力压实作用下渗透特性演化及其机制,为水热型地热田现场尾水回灌过程设备参数的选择提供一些建议。研究结果表明:在高静水压力压实作用下渗透流量(0.5～3.0 mL/min)影响试样达到稳定渗透率的时间,但对最终的稳定渗透率不产生影响,趋于常数4.0×10-3mm²;试样两端压差呈非线性增加,且非线性程度随渗透流速的增加而逐渐增大,但最终趋于稳定。此外,疏松砂岩试样沿渗流方向形成管状潜蚀通道,延伸至试样的约2/3处;基于颗粒运移停止时间及管状潜蚀通道在渗流方向的扩展、延伸长度,定义了不同渗透流速下颗粒平均运移速度,发现颗粒运移速度随渗透流速的增加呈指数形式增大,而且单位时间内通过管状潜蚀通道运移的可移动微小颗粒(小于0.075mm)量随渗透流速的增加而逐渐增多;试样两端压差超过静水压力约1/2时,试样发生潜蚀破坏,上游出现径缩现象。     

花岗岩剪切变形与矿物成分变化的高温高压实验

在构造变形过程中,流体控制成矿作用的机制是目前世界矿床界共同面临的问题之一。通过研究剪切变形过程中的力学-化学作用,理解剪切构造应力和流体在构造成岩成矿过程中的行为与作用是解决这一科学问题的关键环节。在花岗岩高温高压剪切变形实验的基础上,分析了实验变形的矿物反应特征及矿物反应引起的化学成分变化,讨论了矿物反应与变形的相互影响。实验结果表明,变形样品中斜长石、钾长石和辉石以脆性-塑性变形为主,石英和云母以塑性变形为主。同时流体与岩石的相互作用引起了矿物间的反应,其中钾长石、辉石发生水解作用最典型。实验变形整体受应变局部化控制,随着剪切变形加强,脆性破裂逐渐形成和发展为裂隙,应力不断释放,有金属元素沿裂隙充填,这些微观特征在实验样品中普遍存在,类似野外中的矿脉。本次实验为韧性剪切带的流变行为、化学行为和剪切作用过程提供了实验数据。     

Fractionation of pyroxene-phyric MORB at low pressure: An experimental study

Contributions to Mineralogy and Petrology - One-atmosphere melting experiments are used to assess the role of clinopyroxene in producing the compositional variations observed in mid-ocean-ridge...     

石英高压相变研究进展

文中总结了前人有关石英高温高压相变的实验结果。根据以前的实验,在静水压条件下,石英-柯石英-斯石英-CaCl2结构超斯石英相-α-PbO2结构超斯石英相之间的相变方程分别是:p(GPa)=(2.11±0.03)+(9.8×10-4±1.2×10-4)×T(℃),p(GPa)=(8.0±0.2)+(1.1×10-3±3×10-4)×T(℃),p(GPa)=(51±2)+(0.012±0.005)×T(K),p(GPa)=98+(0.0095±0.0016)×T(K)。文中还初步探讨了非静水压状态对石英相变的影响。实验结果表明,差应力的出现降低了石英相变所需要的围压,即相变边界向低压方向偏移,在周永胜等人实验数据的基础上,笔者尝试将二维的相图扩展到三维相图以考虑差应力的影响。最后讨论了石英相变在地学研究中的作用,对比不同的观点分析了前人对超高压变质作用过程的解释,希望可以为以后解释地质资料提供较为广泛的可能性,促进我们对地球内部动力学过程的了解。