Mechanisms of exsolution in sodic pyroxenes
Mechanisms of exsolution in sodic pyroxenes
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DOI：
10.1007/bf00371671
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发表时间：
1980
影响因子：
3.5
通讯作者：
M. Carpenter
中科院分区：
文献类型：
--
作者：
M. Carpenter
结合简单二元固溶体的自由能曲线与有限的结晶性或原子有序已被预测的相关系和出溶机制的系统，其中有序和出溶都是可能的。有序化过程的性质影响着可以使用哪种脱溶机制。如果有序是第二（或更高）阶的字符，然后连续机制占主导地位，一个“条件spinodal”（外国人和卡恩，1976年）可以描述的有序和无序的端元之间的操作。在一级情况下，有序相只能通过成核和生长析出无序相，绿辉石的透射电镜显微结构包括出溶片晶和反相畴，并利用它们之间的关系解释了地质条件下的出溶机制。看来大多数绿辉石进行阳离子有序化，然后保持均匀或exsolve通过spinodal分解的无序辉石。利用连续机制的优势，提出了硬玉-普通辉石的C2/c→P2/n相变可能是二级（或更高级）相变的特征。它是基于这样的想法，即在低温下，无序端元之间应该有有限的相容性，并且在中间组合物（绿辉石）处的阳离子有序化叠加在溶线上。这足以解释许多观察到的显微结构，并与不纯翡翠和绿辉石之间以及绿辉石和钠质辉石之间广泛的两相场的岩相学证据相吻合。加入尖晶石的效果类似于升温，因此也可以预测翡翠-尖晶石-普通辉石的相关系。
The free energy curves for simple binary solid solutions with limited miscibility or atomic ordering have been combined to predict the phase relations and exsolution mechanisms for a system in which both ordering and exsolution are possible. The nature of the ordering process affects which exsolution mechanisms may be used. If the ordering is second (or higher) order in character then continuous mechanisms predominate and a ‘conditional spinodal’ (Alien and Cahn, 1976) can be described which operates between ordered and disordered end members. For a first order case, the ordered phase can only precipitate a disordered phase by nucleation and growth.Microstructures in omphacites observed by transmission electron microscopy include exsolution lamellae and antiphase domains and the relations between them in selected specimens have been used to interpret the exsolution mechanisms which operated under geological conditions. It appears that most omphacites undergo cation ordering, and then remain homogeneous or exsolve a disordered pyroxene by spinodal decomposition. The predominance of continuous mechanisms has been used to indicate that theC2/c→P2/ntransformation may be second (or higher) order in character.A possible phase diagram for jadeite-augite is presented. It is based on the idea that there should be limited miscibility between the disordered end members at low temperatures and that the cation ordering at intermediate compositions (omphacite) is superimposed on a solvus. It is adequate to explain many of the observed microstructures and fits with petrographic evidence of broad two phase fields between impure jadeite and omphacite and between omphacite and sodic augite. The effect of adding acmite is analogous to increasing temperature so that the phase relations for jadeite-acmite-augite can also be predicted.