It has been known for a long time that in potash feldspar the optical properties are functions of both the chemical composition and the thermal history of the feldspar. As a first approximation we may speak of low‐temperature optics in adularia and common orthoclase characterized by a large axial angle, and high‐temperature optics in sanidine characterized by a small axial angle. By heating orthoclase it is usually feasible to convert it into sanidine, optically. On cooling the sanidine type is preserved; indeed, no conversion of sanidine into tho low‐temperature type has been reported. One may conclude, therefore, that once the high‐temperature type has been formed its reversal to the low‐temperature type is very slow; observations on natural feldspars indicate that sanidines are (meta) stable at ordinary temperatures for geological tim