Metamorphic facies
A metamorphic facies is defined by a repeatedly occurring and thus predictable set of stable mineral assemblages in metamorphic rocks. Such metamorphic mineral assemblages crystallize and are stable over a range of externally imposed physical conditions, most crucially lithostatic pressure and temperature, and thus the facies define areas in two-dimensional plots of pressure–temperature space. There is a demonstrable/proven relationship between mineral composition and bulk rock chemical composition and a consistent relationship through geological time.