Chemical analyses of nine K-feldspars and thirteen albites, along with their optical, physical, and X-ray data are presented. The obliquity values Δ = 12·5(d₁₃₁-d_13¯1) of the K-feldspars are found to be independent of the contents of silica and volatiles in the rocks. The role of temperature as the primary factor affecting the obliquity values is stressed. The relationship between the (An + Ab) content of the potassic phase of K-feldspars and 2V_α is shown diagrammatically.