Biotite Schist
Biotite mica in Schistose rock matrix · Mineral Specimen / Metamorphic Rock
Crystal System
Monoclinic; commonly occurs in tabular or platy habits, forming foliated or micaceous masses with a schistose texture.
Mineral Group
Silicates; Phyllosilicates (Mica Group)
Chemical Formula
K(Mg,Fe)3(AlSi3O10)(F,OH)2
Formation Process
Metamorphic; forms through regional metamorphism of clay-rich sedimentary rocks (pelites) under medium to high-grade temperature and pressure conditions.
About This Crystal
A dark, foliated metamorphic rock dominated by biotite mica. It displays a characteristic 'schistosity' or layering with a vitreous to pearly luster on cleavage faces. Appearance is dark brown to black with flaky, plate-like grains that overlap.
Physical Characteristics
Color: Dark brown to black; Streak: White to gray; Cleavage: Perfect basal cleavage in one direction; Fracture: Uneven/Micaceous; Specific Gravity: 2.7-3.3; Fluorescence: None.
Optical Properties
Biaxial negative; Refractive Index: 1.565-1.696; Strong pleochroism (pale yellow to reddish-brown); high birefringence but often obscured by dark body color.
Hardness & Durability
Origin Region
Worldwide distribution; notable in Canada, Norway, Russia, and the Northeastern United States in metamorphic terrains.
Hardness & Durability
Mohs Hardness: 2.5 to 3.0; very low durability. It is brittle and flaky, easily scratched with a copper coin or fingernail. Not recommended for jewelry.
Care & Maintenance
Avoid ultrasonic cleaners or immersion in water as liquids can penetrate between mica layers; clean with a dry, soft brush; store away from harder minerals that can cause mechanical flaking.
Rarity & Value
Common; high geological abundance. Value is low for raw specimens, usually sold as educational samples or mineralogical curiosities for a few dollars per kilogram.
Special Characteristics
Exceptional basal cleavage allows the mineral to be split into 'books' or thin, flexible sheets; creates a sparkling 'shimmer' when light hits the foliated plates.
Lore & History
Historically used in ground form as a filler in various industrial materials. In metaphysical traditions, it is often called the 'Stone of Life' and is believed by some to assist in rational thought and seeing through illusions.