受约束条件下矿物相图构型转变中的一种现象——镜像对称构型的稳定性互换及其热力学解释

A phenomenon in the configurational change of phase diagrams under constraints:Stability interchange of Mirror Symmetrical Configuration and its thermodynamic explanation

实验或计算相图及野外观察表明,同一体系在不同但却相近的物理化学条件下常常会呈现出许多截然不同的相组合或经历截然不同的反应途径。为了分析这一现象的原因,本文作者计算了不同温度、压力条件下CaO-FeO-SiO_(2)-O_(2)体系(铁橄榄石、钙铁橄榄石、钙铁辉石、磁铁矿、钙铁榴石、赤铁矿、硅灰石)的氧逸度-硅活度[log f(O_(2))-log a(SiO_(2))]相图。结果表明,随着约束条件的变化,该体系所包含的两个不同一级多体系(由一个无变度组合加上一相所形成的子体系)的相图先后经历了构型转变;在每次转变中,同一个一级多体系中每条单变度线上的两个无变度点都会同时经历彼此靠近、重合于一点、再沿原来方向彼此远离的过程。因此,原来相对稳定的无变度点在新的条件下全部同时变为介稳,而原来介稳的无变度点在新的条件下全部同时变为相对稳定的点,其中每一构型中稳定部分与介稳部分的关系是互补的。因此,整个一级多体系的拓扑构型(包含稳定和介稳部分)呈现镜像对称。对于受约束条件下的多相、多组分体系,这种构型转变现象比较常见。它伴随着一系列相组合和反应的突变,因而可以影响相关地质过程的演化途径;而构型转变的条件可用来确定相组合突变所需要的条件变量的范围,其计算只涉及一级多体系(而不是整个体系)中的相。需要注意的是,在一级多体系中,某个“相对”稳定的无变度点在其它一级多体系中可能会变为介稳点。因此,一级多体系中“相对”稳定的无变度点在高级多体系中可能只有一部分依旧能够保持稳定。另外,受约束条件的变化常会导致新相的产生或已有相的消失,从而使得原有的一级多体系可能还未来得及发生相图构型转变就已经转变为一个新的体系。这种情况会大幅度地减少原有的一级多体系相图构型发生转变的机会。本文利用相图的拓扑分析理论对上述现象做了很好的热力学分析和解释。

Experimental or calculated phase diagrams and field observations demonstrate that the same system under different but similar physicochemical conditions often exhibits many distinct phase assemblages or underwent distinct reaction paths.In order to analyze the reason for this phenomenon,the oxygen fugacity-silica activity[log f(O_(2))-log a(SiO_(2))]phase diagrams of the CaO-FeO-SiO_(2)-O_(2)system(fayalite,kirschsteinite,hedenbergite,magnetite,andradite,hematite and wollastonite)are calculated under different temperatures(T)and pressures(P).The results discover such a phenomenon that during the variation of constraint conditions,the phase diagrams of two one-grade multisystem(i.e.,the subsystems consisting of an invariant assemblage plus an additional phase)undergo change in topological configuration in sequence;in either change,the two invariant points on each univariant line of the same one-grade multisystem approach each other first,then coincide at one point,and then move away from each other in their original directions.Consequently,all relatively stable invariant points under the original conditions become metastable as a whole under new conditions,and all metastable invariant points under the original conditions simultaneously become relatively stable as a whole under new conditions,where the stable part and metastable part in each configuration are complementary in phase relations.As a result,the topological configurations of the whole one-grade multisystem phase diagram(including both stable and metastable parts)exhibit mirror symmetry.Such changes,particularly their conditions,should be common for the phase diagrams of multiphase,multicomponent systems under specific constraints.It is accompanied by abrupt change of a series of phase assemblages and reactions,and thus can influence the evolutional paths of relevant geological processes,while the conditions for one or multiple configurational changes can be used to determine the range of the constrained variables for the change of phase assemblages,and their calculation only involves the phases in a one-grade multisystem(rather than the whole system).Note that a“relatively”stable invariant point in a one-grade multisystem may become metastable in another one-grade multisystem.Therefore,it is possible that only partial“relatively”stable invariant points in a one-grade multisystem are still stable in a higher-grade multisystem.In addition,the change of constrained variable often leads to the occurring of new phase(s)or the disappearance of the original phase(s),and this change may make the original one-grade multisystem evolve into a new system before its phase diagram configuration changes,which significantly reduces the chances for the original one-grade multisystem phase diagram to change its configuration.In this article,the phenomena mentioned above are well analyzed and explained by the topological analysis theory of phase diagrams.