Chlorite Schist

Chlorite schist is a green, foliated metamorphic rock dominated by chlorite, a soft, platy, water-bearing mineral that gives the rock both its color and its greasy sheen. The aligned chlorite flakes produce a well-developed schistosity along which the rock readily splits.

It is a low-grade metamorphic rock, forming at relatively modest temperatures and pressures, and is closely related to greenschist. The green color comes from the iron- and magnesium-rich chlorite, sometimes deepened by accessory minerals like actinolite or epidote.

Chlorite schist is common in altered volcanic and oceanic crust and is a familiar sight in the lower-grade parts of mountain belts.

Chlorite schist forms by low-grade regional metamorphism, in the greenschist facies, of mafic igneous rocks such as basalt and gabbro, or of iron- and magnesium-rich sediments and volcanic ash. At temperatures around 300-450 degrees C, original minerals alter to chlorite along with minerals like epidote and actinolite.

Directed pressure aligns the platy chlorite into the foliation that defines the schist. Much chlorite schist derives from the hydrothermal alteration and metamorphism of seafloor basalts, and it commonly occurs in subduction-related and collisional belts.

It is widespread in ancient greenstone belts and in the marginal zones of mountain chains, including the Alps and parts of the Appalachians.

Look for a green, soft, foliated rock with a slightly greasy or pearly sheen that splits into thin flakes and can be scratched easily with a knife or even a fingernail (hardness about 2-3). The green color and softness together are strong clues.

The foliation surfaces feel smooth and slippery, and the rock may stain the fingers. Accessory garnet or magnetite crystals sometimes dot the surface.

Look-alikes: mica schist is silvery or brown and not green; serpentinite is also green and soft but lacks the well-developed schistose foliation and is more massive and waxy; talc schist is even softer and feels soapy. Color plus foliation distinguishes chlorite schist.

Chlorite schist has limited commercial use because it is soft and splits easily. It is occasionally used as a decorative or landscaping stone for its green color, and crushed for low-grade fill.

It can host useful accessory minerals; some chlorite schists carry magnetite, talc, or even gold in associated veins, making them of interest in mineral exploration. Garnet and other porphyroblasts in chlorite schist are collected as specimens.

Geologically it is an important marker of low-grade metamorphism. In metaphysical lore chlorite is linked to cleansing and growth, but these are traditional beliefs, not established science.