基于假三元模型的透辉石-硬玉固溶体热力学性质计算

Calculation of thermodynamic properties of diopside-jadeite solid solution based on the fictive ternary model

自然界中矿物多以固溶体形式存在,据其晶体化学特征计算热力学性质是开展矿物成因理论研究的基础。本文引入描述二元矿物固溶体热力学性质的假三元模型,计算得到了透辉石-硬玉固溶体系列的热力学性质。该模型通过构造一种高度有序的中间相,同时考虑长程和短程有序效应,基于热力学平衡态矿物固溶体自由能最低的规律,可以计算特定组分下矿物的平衡自由能、焓和熵等热力学参数。本文针对透辉石-硬玉固溶体体系,取绿辉石为其中间有序态,计算了其活度-成分关系和温度-组分相图等,发现绿辉石随温度升高的有序无序相变为一级相变,相变温度为1 148±25 K,与实验研究结果一致。本文获得的透辉石-绿辉石-硬玉体系的热力学参数可用于视剖面图方法研究MORB成分的岩石的榴辉岩相变质作用过程。

Most minerals in the nature exist as solid solutions. Calculating the thermodynamic properties based on their crystal chemistry characteristics is the basis for the theoretical study of the formation of minerals. In this paper, a fictive ternary model for the simulation of thermodynamic properties of binary mineral solid solutions is introduced. The thermodynamic properties of diopside-jadeite solid solution were calculated. By constructing one ordered intermediate state mineral and considering both long-range order and short-range order effects, the model could calculate equilibrium thermodynamic parameters such as free energy, enthalpy, and entropy for a specific composition based on the rule that the lowest free energy corresponds to thermodynamical equilibrium mineral solid solution. The activity-composition relationship and temperature-composition phase diagram of diopside-jadeite solid solution system were calculated with omphacite as ordered intermediate state. It is found that the order/disorder transition of omphacite with increasing temperature is a first-order phase transition with a transition temperature of1 148 ± 25 K, which is in agreement with the experimental results. The thermodynamic parameters of the diopsideomphacite-jadeite system obtained in this paper can be applied to the study of the eclogitefacies metamorphism of rocks with the MORB bulk composition by means of pseudosection thermobarometry method.