metamorphic
Schist with Biotite Inclusion
Biotite-bearing Mica Schist (K(Mg,Fe)3AlSi3O10(F,OH)2)
Hardness: 2.5–3 on Mohs scale (for the dark biotite); Color: Silvery-gray matrix with dark brown to black crystalline flakes; Luster: Pearly to submetallic; Structure: Foliated with distinct layering; Specific Gravity: 2.7–3.3
- Hardness
- 2
- Luster
- Pearly to submetallic
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 2.5–3 on Mohs scale (for the dark biotite); Color: Silvery-gray matrix with dark brown to black crystalline flakes; Luster: Pearly to submetallic; Structure: Foliated with distinct layering; Specific Gravity: 2.7–3.3
Formation & geological history
Formed through high-grade regional metamorphism of clay-rich sedimentary rocks (like shale) under intense heat and pressure. Most specimens range from Proterozoic to Paleozoic in age, formed during mountain-building events.
Uses & applications
Primarily used as a decorative stone, in construction as flagstone, or as a source for mica in industrial lubricants and electronics. Collector interest is focused on the crystalline inclusions.
Geological facts
The word 'schist' is derived from the Greek 'schistos', meaning 'split', referring to the ease with which the rock can be layered or cleaved. The dark inclusion is a 'porphyroblast'—a large crystal that grew within the finer-grained matrix during metamorphism.
Field identification & locations
Identify in the field by its 'sparkly' appearance and ability to peel off thin, dark flakes with a fingernail. Found globally in metamorphic belts, specifically in New England (USA), the Alps, and the Himalayas.
More like this
