Optic axial angle, a precise measure of Al, Si ordering in the T ₁ tetrahedral sites of K-rich alkali feldspars

These equations represent two segments of a line derived by least-squares regression performed on literature data for 52 specimens (19 with structural refinements) for which Σt₁ values were calculated from cell parameters using the [110],[110] method of Kroll (1980). The standard error of estimate is 0.02; r² = 0.962. Thus by use of a spindle stage or universal stage, one can rapidly and precisely determine the “structural state” for individual grains or, in thin sections, from point to point in a single grain. For the feldspars used in the regression, no account was taken of their symmetry, of the albite content (up to 20 mol %), of minor amounts of Ca, Ba, Sr, or Fe, or of the possible effects of strain or submicroscopic twinning or exsolution, indicating that these factors may be disregarded in using these equations.

The changes in optic orientation and 2V_X as Al,Si order increases from high sanidine [OAP = (010)] to orthoclase [OAP ⊥ (010)] to low microcline [OAP ~ ⊥ (010)] are explained by assuming a simple linear variation in principal refractive indices with structural state.