Electrostatic energy for ion clustering in intermediate plagioclase feldspar

Electrostatic calculations estimated the energy of hypothetical ion clustering in intermediate plagioclase feldspars. The calculated energy appears too great to allow segregation of Ca and Na atoms into domains upon annealing without a corresponding movement of A1 and Si from an original disordered structure. The calculated energy may be sufficiently small that Ca and Na atoms can disorder partially without movement of A1 and Si upon heating a low-plagioclase to high temperatures.