Staurolite grains that are colorless may be confused with kyanite, with which it commonly occurs, but kyanite is optically negative. Notice the inclusions of pleochroic green-blue chloritoid in garnet. They are evidence that the assemblage of the rock when garnet was growing included chloritoid. Continued heating caused reaction of chloritoid with biotite to produce garnet and consumed chloritoid from the matrix. Chloritoid occurs as dark green porphyroblasts that seldom exceed 2mm in length.
Interference color is anomalous purple, similar to chlorite.
Notice the undulatory extinction in the larger quartz grains. Note that in many rocks, andalusite is anhedral, and nearly half of its volume may be inclusions of quartz and biotite, so that individual grains look like a sponge. This habit is typical for cordierite in mica-rich rocks. Rock is from a broad contact aureole developed around the Devonian Skiddaw granite in the Ordovician Skiddaw slates, in the English Lake District. Contact metamorphic assemblages and textures are superimposed on those of low grade regional metamorphism in the chloritoid-biotite zone.