Optical and X-ray studies of 15 chemically analyzed feldspars show that the optical properties of potassic feldspars may vary because of (1) compositional differences, such as degree of substitution of Fe3+ for Al and of Na, Ca, Ba, Sr, and Rb for K in the feldspar structure, with corresponding substitution of Al for Si where necessary to balance electrostatic charges, (2) presence on a submicroscopic scale of twinning or intergrowths of materials of different refractivities, such as unmixing of a plagioclase from a potassic host or partial inversion of orthoclase to microcline, and (3) degree of order in the distribution of Si and Al within the tetrahedral sites of the feldspar structure. The α refractive index of homogeneous feldspars provides a measure of composition because it does not vary appreciably with Si/Al ordering. The substitution of Na for K, however, causes such a small index increase in the range Or100Ab0 to Or60Ab40 that its effects normally are masked by the larger increases due to substitution of minor amounts of Ca, Sr, Ba, and Fe. As a consequence refractive indices of alkali feldspars are not useful for determining chemical composition. The decrease in the α index when cryptoperthites are homogenized at 850° C. is a measure of the degree of unmixing. The optic angle 2V and the birefringence “b” — α provide a measure of the degree of Si/Al ordering, provided composition is known. In this paper “b” is defined as the refractive index of the ray vibrating parallel or nearly parallel to the b crystallographic direction. Most existing diagrams relating optical properties to composition of alkali feldspars are constructed for groups of feldspars with the same state of Si/Al ordering and do not emphasize the importance of degree of unmixing in determining refractive indices. These diagrams are valid only if the state of order varies sympathetically with the degree of unmixing. Zoning in some sanidines is attributed to variations in the concentration of divalent alkali ions or, possibly, of Fe3+, and in other sanidines to variations in the degree of Si/Al ordering.