Ternary feldspar experiments and thermodynamic models

This paper reports the results of 20 experiments in which mixes of two or three feldspars were reacted to produce coexisting plagioclase feldspar (PF) and alkali feldspar (AF). Starting materials with similar bulk compositions were prepared using different combinations of two and three minerals, and experiments were designed to produce similar AF and PF minerals in the experimental products from different starting binary and ternary compositions. The coexisting AF and PF compositions produced as products define compositional fields that are elongate parallel to the ternary solvus. In 11 experiments reaction was sufficient to product fields of coexisting AF and PF, or AF, PF, and melt with a bulk composition close to that of the starting mixture. In six experiments significant reaction occurred in the form of reaction rim overgrowths on seeds of the starting materials. Three experiments produced AF, PF, and melt from a natural granite starting material. A two-feldspar thermometer is presented in which temperature is constrained by equilibria among all three components - Albite, Orthoclase, and Anorthite - in coexisting ternary feldspars. -from Authors