The Berlin blue color of these crystals under crossed polarizers is unique to this variety of chlorite called penninite. The penninite occurs here in a rock consisting of quartz, mica, and smaller chlorite flakes, as well as larger crystals of chloritoid. The combinations of minerals found in metamorphic rocks like this one are useful for estimating the temperature and pressure at which the rock formed. This particular set of minerals occurs only in rocks formed at relatively low temperatures and pressures.

Crossed polarizers