metamorphic
Smoky Quartz and Biotite Schist
Quartz-Biotite Schist (SiO2 + K(Mg,Fe)3AlSi3O10(F,OH)2)
Hardness: 7 (Quartz) and 2.5-3 (Biotite). Color: Smoky gray, brown, black, and tan. Luster: Vitreous to sub-metallic. Crystal structure: Hexagonal (quartz) and Monoclinic (mica). Cleavage: Perfect in one direction for mica, none for quartz.
- Hardness
- 7 (Quartz) and 2
- Color
- Smoky gray, brown, black, and tan
- Luster
- Vitreous to sub-metallic
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Physical properties
Hardness: 7 (Quartz) and 2.5-3 (Biotite). Color: Smoky gray, brown, black, and tan. Luster: Vitreous to sub-metallic. Crystal structure: Hexagonal (quartz) and Monoclinic (mica). Cleavage: Perfect in one direction for mica, none for quartz.
Formation & geological history
Formed through the regional metamorphism of clay-rich sedimentary rocks (like shale) under high pressure and temperature. The dark inclusions are likely biotite or iron oxides within a quartz matrix.
Uses & applications
Primary interest for rock collectors and students of geology. Historically, quartz and mica have been used for industrial insulation and glass manufacturing.
Geological facts
Biotite is often called 'black mica.' Schist rocks are characterized by having more than 50% platy and elongated minerals, making them look layered or sparkly when split.
Field identification & locations
Identify in the field by the 'schistose' texture—a sparkly, layered appearance and the presence of visible mica flakes and crystalline quartz veins. Common in mountain ranges like the Appalachians or the Alps.
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