gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1970.037.289.08 Optical Data on Some Authigenic Feldspars from Western Australia Glover J. E. Department of Geology, University of Western Australia, Nedlands, Western Australia Hosemann P. West Australian Petroleum Pty. Ltd., Box C1580, G.P.O., Perth, Western Australia 3 1970 37 289 588 592 9 12 1968 24 6 1969 Copyright © 1970, The Mineralogical Society 1970 Summary

Authigenic envelopes up to 0·18 mm thick on clastic potassium feldspar grains from arkoses and subgreywackes of the Devonian Nannyarra Greywacke have been examined with the four-axis universal stage. The plane of the optic axes is parallel to (010) in some segments of individual envelopes and normal to (010) in others, and the range of 2Vα over all envelopes is from close to 0° to 65°. The optics are ascribed to modifications of potassium feldspar corresponding to high-and low-sanidine and orthoclase.