Thermodynamics of phase transitions in minerals: a macroscopic approach

Abstract

When calculating the stability fields of different mineral assemblages, petrologists have become used to dealing with two broad categories of reactions. In the first category are heterogeneous reactions between unrelated phases, such as jadeite ⇋ albite + quartz, or between phases with the same composition but different structure, such as olivine ⇋ spinel. The second category involves reactions which occur within individual phases and includes solid solution, cation ordering, displacive transitions, magnetic transitions, etc. Because the energy changes for the two types of processes are commonly on a similar scale, internal effects can have a profound influence on how the equilibrium boundaries for the heterogeneous reactions are distributed in PT space. For example, the stability field of cordierite with respect to enstatite + sillimanite + quartz is considerably reduced if the cordierite has a disordered, rather than an ordered distribution of Al and Si between tetrahedral sites (Figure 5.1(a), from Putnis and Holland, 1986. Similarly, the aragonite ⇋ calcite phase boundary shows a marked curvature at least in part due to the orientational disordering of CO3 groups in calcite (Figure l(b), from Redfern et al, 1989. At the same time, there has been an increasing awareness among physicists that structural phase transitions in minerals show characteristic properties worthy of investigation from a purely solid-state-physics point of view. There is a confluence of interests, therefore, which has resulted in a new approach to the thermodynamic analysis of phase transitions in minerals based on Landau theory.