SHORT COMMUNICATIONS 745 References

Optical properties and composition in the orthopyroxene series THE general relationship between optical properties and composition in the orthopyroxene s ries has been well established for some years. Deer et al. (1963) have given a recent summary of the previous work and presented new diagrams relating refractive indices, birefringence, and optic axial angle with mg = Mg2+/(Mg2+§247 2+) for 37 selected orthopyroxenes. The refractive indices, specific gravity, and cell sizes of natural En0Fsl00, EnsoFss0 and Enl00Fs0 have been calculated by regression both for pure, A1 and Ca-free compositions and for composi-tions including added A1 and added A1 with Ca (Winchell and Leake, 1965) using over 200 analysed orthopyroxenes. None of the previous diagrams relating composition and optical properties have used more than a small selection of the available data and many do not give the data upon which the correlations are base