ORDER-DISORDER PHASE TRANSITIONS

Abstract

Cation ordering is often one of the most efficient ways a mineral can adapt to changing temperature or chemical composition. Disorder of distinct species across different crystallographic sites at high temperature provides significant entropic stabilisation of mineral phases relative to low-temperature ordered structures. For example, the calcite-aragonite phase boundary shows a significant curvature at high temperature due to disorder of CO3 groups within the calcite structure, associated with an orientation order-disorder phase transition (Redfern et al. 1989a). This leads to an increased stability of calcite with respect to aragonite over that predicted by a simple Clausius-Clapeyron extrapolation of the low pressure-temperature thermochemical data (Fig. 1⇓). Similarly, the pressure-temperature boundary of the reaction albite ↔ jadeite + quartz curves significantly at high temperature due to the entropic stabilisation of albite related to the high-low albite Al/Si order-disorder process. The energy changes associated with cation order-disorder phase transitions in a number of materials have been observed to be as great as the associated melting transitions (see Parsonage and Staveley 1978 for an earlier review). It is unsurprising, therefore, that there has been much interest in recent years in examining and modelling the processes of cation order-disorder in minerals. Computational studies of cation order-disorder have advanced together with experimental investigations and theoretical explanatory frameworks, and the three are increasingly being combined to provide interpretative descriptions of this process. Figure 1. The calcite-aragonite phase boundary in P/T space is curved, due to the temperature-dependence of the entropy of calcite. The entropic stabilisation of calcite at high- T arises from the order-disorder phase transition from orientationally ordered R 3̅ c calcite at low temperatures to the orientationally disordered R 3̅ c structure, stable above 1260 K. Figure after Redfern et al. (1989a). An order-disorder phase transition occurs when the low-temperature phase of a system …