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 2V_α 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.