Dutchess County, New York
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.