metamorphic
Biotite Schist
Biotite-rich Schist (K(Mg,Fe)3AlSi3O10(OH,F)2)
Hardness: 2.5-3 (on the Mohs scale for the mica component); Color: Dark brown, black, to dark green with bronze highlights; Luster: Pearly to submetallic; Crystal Structure: Monoclinic (foliated/tabular); Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3.
- Hardness
- 2
- Color
- Dark brown, black, to dark green with bronze highlights
- Luster
- Pearly to submetallic
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Physical properties
Hardness: 2.5-3 (on the Mohs scale for the mica component); Color: Dark brown, black, to dark green with bronze highlights; Luster: Pearly to submetallic; Crystal Structure: Monoclinic (foliated/tabular); Cleavage: Perfect basal (peels into thin sheets); Specific Gravity: 2.7-3.3.
Formation & geological history
Formed through the regional metamorphism of clay-rich sedimentary rocks (like shale) or felsic igneous rocks under intermediate temperatures and pressures. This creates a medium-to-coarse grained texture where mica flakes align in parallel layers.
Uses & applications
Primarily used as an indicator mineral in geological mapping to determine metamorphic grade. Commercially, biotite is used in the manufacture of drywall, paint fillers, and as a soil conditioner due to its potassium content.
Geological facts
The name 'Biotite' honors French physicist Jean-Baptiste Biot, who researched the optical properties of mica. It is common in the Earth's crust and is often nicknamed 'black mica'.
Field identification & locations
Identify in the field by its 'schistose' texture (distinct layering/foliation) and shiny, flakey appearance. It is commonly found in mountain ranges like the Appalachians, Alps, and Himalayas. Collectors should look for 'glassy' sheet-like reflections when tilting the rock under light.
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