In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics
In situ X-ray observations of the coesite-stishovite transition: reversed phase boundary and kinetics
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DOI：
10.1007/bf00202987
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发表时间：
1996-02
影响因子：
1.4
通讯作者：
Jianzhong Zhang;Baosheng Li;W. Utsumi;R. Liebermann
中科院分区：
文献类型：
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作者：
Jianzhong Zhang;Baosheng Li;W. Utsumi;R. Liebermann
利用DIA型立方顶锤高压装置(SAM-85)，结合X-射线衍射仪，以合成粉末为原料，研究了高达12 Gpa和1530°C的柯石英和辉钛矿的相关系。柯石英和辉石之间的相变是通过观察一个尚不存在的相的首次出现或通过两种图案的相对强度的变化来识别的。在大多数实验中，样品的衍射图是在达到压力和温度条件后的10分钟内收集的。在这一时间尺度上，确定了两个与柯石英-斯石英转变有关的相界：(1)在950-1530℃的温度范围内观察到了柯石英向柯石英的转变；(2)在500-1300°C的温度范围内观察到了柯石英-斯石英的转变。这些观察结果表明，存在一个约1000℃的临界温度来约束柯石英-斯石英的平衡相界面。在此温度以上，两个边界都是线性的，具有正的dp/dt斜率，并且位于0.4 Gpa的压力区间内。在此温度以下，辉钛矿-柯石英相界的dp/dt斜率显著增大，柯石英-柯石英相界的dp/dt斜率由正变为负。因此，平衡相边界只能从1000°C以上的结果确定，并可用线性方程P(Gpa)=6.1(4)+0.0026(2)T(°C)来描述。这一dp/dt斜率与张等人的结果符合得很好。(1993)，但比八木和秋本(1976)的两倍还多。在相变动力学方面，获得了1160℃和1430℃时，辉钛矿向柯石英转变的初步速率数据，符合Avrami和Cahn的简单几何转变模型。
Using a DIA-type, cubic-anvil, high-pressure apparatus (SAM-85) in conjunction within situX-ray diffraction, we have investigated phase relations between coesite and stishovite up to 12 GPa and 1530 °C using synthetic powders of the two phases as the starting materials. The phase transition between coesite and stishovite was identified by observing the first appearance of a phase that did not already exist or by a change in the relative intensity of the two patterns. In most experiments, the diffraction patterns on samples were collected within 10 minutes after reaching a pressure and temperature condition. On this time scale, two phase boundaries associated with the coesite-stishovite transition have been determined: (1) for the stishovite-to-coesite transition, observations were made in the temperature range of 950–1530 °C, and (2) for the coesite-to-stishovite transition from 500 to 1300 °C. These observations reveal that there exists a critical temperature of about 1000 °C to constrain the coesite-stishovite equilibrium phase boundary. Above this temperature, both boundaries are linear, have positive dP/dT slopes, and lie within a pressure interval of 0.4 GPa. Below this temperature, the dP/dT slope for the stishovite-to-coesite phase boundary becomes significantly larger and that for the coesite-tostishovite phase boundary changes from positive to negative. As a result, an equilibrium phase boundary can only be determined from the results above 1000 °C and is described by a linear equation P (GPa)=6.1 (4)+ 0.0026 (2) T (°C). This dP/dT slope is in good agreement with that of Zhang et al. (1993) but more than twice that of Yagi and Akimoto (1976). For the kinetics of the phase transition, preliminary rate data were obtained for the stishovite-to-coesite transition at 1160 and 1430 °C and are in agreement with the simple geometric transformation model of Avrami and Cahn.