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 interactions within the T (Al,Si) and M (Na,Ca) sublattices and the interactions between the T and M sublattices are constrained by the available experimental information on order-disorder transitions in the system. The paper suggests a thermodynamic explanation for the existence of two distinct structural series (high and low) and gives constraints on the thermodynamics of low plagioclases.

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