Structural discontinuities in plagioclase and constraints on mixing properties of the low series: a computational study

Subsolidus phase relations in the plagioclase system are investigated using an Ising-type model. The model treats ordering of Al,Si and Na,Ca cations in three dimensions using a version of the cluster variation method. The in teractions within the T (Al,Si) and M (Na,Ca) sublattices and the interacti ons between the T and M sublattices are constrained by the available experi mental information on order-disorder transitions in the system. The paper s uggests a thermodynamic explanation for the existence of two distinct struc tural series thigh and low) and gives constraints on the thermodynamics of low plagioclases.