SCHIST

Schist is a medium grade metamorphic rock formed from shales and slates. The mineral crystals in a schist have been flattened into visible thin crystals, resulting in a flaky texture to the rock.

By definition, a schist contains more than 50% of platy and elongated minerals, usually in a quartz and feldspar matrix. Note that even these host minerals may have flattenened, distorted crystal shapes.