Abstract
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). Experiments were carried out over the range 700 to 900 °C and 1 to 3 kbar under water-saturated conditions. The compositions of experimental products were determined using the electron microprobe. 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 produce 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, but a tie line connecting AF and PF products lies on the Ab-rich side of the starting bulk composition. In these six experiments some of the An-rich plagioclase starting material was removed from reaction by overgrowth of reaction rims. Three experiments produced AF, PF, and melt from a natural granite starting material. The experiments probably approached stable or metastable exchange equilibrium with respect to Na-K-Ca, but an equilibrium degree of Al-Si order in the rims was most likely not achieved. A thermodynamic model following the approach developed by Ghiorso (1984) and Fuhrman and Lindsley (1988) is applied to these new experimental results. 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.
