Formation
(1) Magmatic, formed by crystallization from silica-rich plutonic melts (granites, pegmatites) under moderate to high pressures.
(2) Metamorphic, formed by the transformation of clay minerals or feldspars in the green schist to amphibolite facies (mica schists, gneisses).
(3) Hydrothermal, formed by the alteration of aluminum silicates in residual magmatic solutions (greis formation).
