Order-modulated structures and the thermodynamics of cordierite reactions

Abstract

The determination of palaeo pressures and temperatures (P, T) in metamorphic belts is of fundamental importance to our understanding of large-scale physical processes in the Earth. Such determinations are made by comparing observed mineral assemblages with experimental data on their P, T stability fields1, thus assuming that the experiments are an adequate model for the natural reaction. The characterization of the structural state of natural and experimental materials is particularly important in this context. This note demonstrates the effect of the state of cation order on the P, T slopes of metamorphic reactions involving cordierite, (Mg, Fe)2Al4Si5O18, an indicator phase in metamorphism. Particular emphasis will be placed on the mechanism of Al, Si ordering in cordierite in so far as it relates to the characterization of the degree of cation order and hence the configurational entropy contribution to the thermodynamics of cordierite reactions.