Schist

Physical Properties

Hardness (Mohs scale): 3.5–4; Color: Gray, silver, brownish-black; Luster: Pearly to sub-metallic due to mica; Crystal structure: Foliated (schistosity); Cleavage: Perfect basal cleavage in individual mica grains; Specific gravity: 2.7–2.8

Formation & Geological History

Formed through the regional metamorphism of shale or mudstone under medium-grade temperatures and pressures. Most schistose rocks are Precambrian to Paleozoic in age and form in mountain-building environments (orogenic belts).

Uses & Applications

Historically used as dimension stone for building or paving. Mica recovered from schist is used as an insulator in electrical equipment, in joint compounds, and as a filler in paints and plastics.

Geological Facts

The word 'schist' is derived from the Greek word 'schizein', meaning 'to split', referring to how easily the rock can be split along its foliated planes. Large garnets are often found as porphyroblasts embedded within schist matrices.

Field Identification & Locations

Identify in the field by its distinct foliated texture where individual mineral grains (like mica) are large enough to be seen with the naked eye, giving the rock a characteristic 'glittery' appearance. Common in the Appalachian Mountains and the Alps.

Identified on: 5/5/2026

Mode: Standard