Optical properties of the analbite-high sanidine solid solution series

For homogeneous, high-temperature alkali feldspars, precise spindle stage measurements of the refractive indices na, nb and nc -- those for sodium light vibrating most nearly paralle1 to the a, b, and c crystallographic axes, respectively -- indicate that they vary linearly with mol % Or (R2 > 0.976). The lines for nb and nc cross at ~72% Or (for 2t1 ~ 0.56). Thus 2Vx is 0° for that composition and varies sigmoidally (~t linearly as previously assumed) from -68° for analbite and ~52° for high sanidine. Concomitantly, the optic axial plane (OAP) changes from approximately perpendicular to (010) to parallel to (010). Then na (or α ) refractive index seems to have the best correlation with composition for high temperature alkali feldspars because it is the parameter least sensitive to chemical and structural variations. Corrections for these differences in minor substituents and structural state need to be employed before meaningful correlations are determined between all optical properties and composition.