Fibrolite is probably the result of a tie-line switching reaction that also produces biotite. Direct textural evidence for the polymorphic inversion from either andalusite or kyanite to sillimanite is rare. Sillimanite occurs both as knots of fibrolite and prismatic porphyroblasts. Crude foliation imparted by alignment of brown biotite and fibrolite. The loss of platy muscovite and the production of alkali feldspar results in the transformation of mica-rich schist into a foliated or massive feldspathic gneiss.
Alkali feldspar is usually untwinned orthoclase.
Prismatic sillimanite has square cross-section with single cleavage parallel to diagonal of square. It is colorless, high relief. Interference colors are 1 yellow to 2 blue on prism sections. Cordierite is colorless with low relief in plane light and 1 gray or whit under crossed polars. Cordierite in sillimanite-orthoclase zone rocks occurs in large anhedral grains that may be mistaken for plagioclase due to the diffuse twinning or mistaken for quartz where twinning is absent.