AND

SUMMARY. Cell dimensions have been measured from correctly indexed powder patterns of thirty natural and two synthetic orthopyroxenes. The natural specimens (mostly metamorphic) ontain exsolved phases largely avoided during microprobe chemical analysis. Linear regression analysis between the cell dimensions and Mg, A1, Ca contents (ignoring minor elements) was quite unsatis-factory until a term in Mg ~ was added. Two separate regression analyses for the ranges Mg o-o'5 and o'5-I using only Mg, AI, and Ca were satisfactory. Olivine yielded satisfactory regressions for the whole range without a term in Mg 2 (Louisnathan and Smith, ~968). The difference between olivine and pyroxene results from absence of site preference by Mg and Fe in olivine compared to strong preference in pyroxene revealed by electron density and M6ssbauer studies (Bancroft, Burns, and Howie, I967; Ghose and Hafner, I967). The data were recalculated by means of a new best-fit procedure developed by Hey in which the errors in the chemical and physical parameters are considered simultaneously. Earlier measurements of a and b for plutonic pyroxenes are consistent within possible xperimental uncertainties with those given here; however aand b data obtained by Kuno and by Hess on volcanic specimens are considerably higher by variable amounts. Although there are uncertainties in the C