Intracrystalline Cation Distribution in Natural Clinopyroxenes of Tholeiitic, Transitional, and Alkaline Basaltic Rocks

Abstract

Thermodynamic studies of Mueller (1962) and Thompson (1969) and the crystallographic work of Ghose (1965) gave new impetus to a rapidly developing branch of mineralogy. Orthopyroxenes were studied by Virgo and Hafner (1969) and Saxena and Ghose (1971) using Mossbauer technique. Site-occupancy data have been used in many mineralogical and petrological problems, e.g., to estimate the thermodynamic solution properties of pyroxenes and to understand the cooling history of rocks. A parallel development in the mineralogy of clinopyroxene did not take place, particularly because the Mossbauer technique proved unsatisfactory for calcic pyroxenes. Recognizing this, we undertook a detailed X-ray crystallographic study of the clinopyroxenes and succeeded to determine crystal-structural parameters for Fe-Mg site occupancies of the nonequivalent Ml and M2 sites. Until now there are no published site-occupancy data on a series of clinopyroxene of intermediate composition. This paper attempts to fill this important gap in the data on clinopyroxenes. Besides our concern for the crystal-chemical and thermodynamic aspects of such a study, we were also motivated by the possible use of pyroxene crystallographic parameters in the classification and characterization of magmatic rocks. With the advent of automation in X-ray crystallography, it is possible to study a number of crystals from one or more rock samples in a relatively short time and provide petrologically useful information.