File:Staurolite schist (Keivy Schist; Mt. Ploskaya, Kola Peninsula, Russia) (16300555144).jpg

Staurolite schist from the Kola Peninsula of Russia. (3.8 centimeters across at its widest)

Metamorphic rocks result from intense alteration of any previously existing rocks by heat and/or pressure and/or chemical change. This can happen as a result of regional metamorphism (large-scale tectonic events, such as continental collision or subduction), burial metamorphism (super-deep burial), contact metamorphism (by the heat & chemicals from nearby magma or lava), hydrothermal metamorphism (by superheated groundwater), shear metamorphism (in or near a fault zone), or shock metamorphism (by an impact event). Other categories include thermal metamorphism, kinetic metamorphism, and nuclear metamorphism. Many metamorphic rocks have a foliated texture, but some are crystalline or glassy.

Schist is an intermediate- to high-grade, foliated metamorphic rock. It is highly variable in appearance, depending on the mineral content, which is a function of the precursor rock and specific temperature-pressure conditions. Garden-variety schists form by metamorphism of phyllites. Schists typically have medium- to large-sized crystals, unlike the microcrystalline nature of slate & phyllite.

Schist's foliated texture (= crystals aligned into bands or stripes or layers) is often only seen when specimens are viewed on edge.

Staurolite schist has prominent dark brown staurolite crystals in a light-colored muscovite schist matrix. It is an intermediate- to high-grade metamorphic rock. The mineral staurolite (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2 - iron magnesium zinc hydroxy-aluminosilicate) often forms cruciform twins - two intergrown crystals forming a cross or X-shaped form. The sample shown above comes from the Keivy Schist at Mt. Ploskaya in the Kola Peninsula of northwestern Russia. The original, pre-metamorphism sedimentary rock was late Neoarchean in age (~2.5 to 2.7 billion years).