Biotite Schist with Garnet

Physical Properties

Hardness: 2.5-3 (biotite), 6.5-7.5 (garnet); Color: Black or dark brown with reddish inclusions; Luster: Pearly to submetallic; Crystal Structure: Monoclinic (biotite), Isometric (garnet); Cleavage: Perfect basal.

Formation & Geological History

Formed through the regional metamorphism of clay-rich sedimentary rocks (shales) under high temperature and pressure conditions, typically during mountain-building events.

Uses & Applications

Primarily used as a geological teaching specimen or for mineral collecting; schist is sometimes used in construction for fill or decorative garden stones.

Geological Facts

Schist is characterized by its foliation, meaning its minerals are aligned in layers. The shiny black flakes are biotite mica, which allows the rock to split into thin sheets.

Field Identification & Locations

Identify by looking for flaky, shiny black mica surfaces (schistosity) and small, hard reddish-brown granular crystals (garnets). Found globally in metamorphic belts like the Appalachians or the Alps.

Identified on: 5/1/2026

Mode: Standard