The Distribution of Fe⁺⁺and Mg⁺⁺in Coexisting Olivines and Pyroxenes

Abstract

The Mg/(Mg + Fe) ratios of coexisting olivines and orthopyroxenes from different types of rocks have been determined by means of indices-of-refraction measurements. Some data from literature have also been added. The distribution function of Mg and Fe in olivine-pyroxene pairs is discussed. It is shown that this function may be used as a geologic thermometer when some additional thermochemical data are obtained. The distribution function also indicates that orthopyroxene is a nonideal mixed crystal, the intermediate members of the series being more stable than one would expect. This nonideal character of the orthopyroxene series is indicated by deviation from linearity of the indices of refraction and the unit-cell dimensions when these quantities are plotted against the mol fraction of enstatite in the pyroxenes. The a and b parameters of the unit cell of different orthopyroxenes are measured. The aluminum content in orthopyroxenes is discussed briefly.