Biotite
Image: Didier Descouens (CC BY-SA 4.0)
Mineral NameBiotite
Chemical FormulaK(Mg,Fe)₃AlSi₃O₁₀(OH)₂
Mohs Hardness2.5
Crystal Systemmonoclinic
Lustervitreous
Streak Colorwhite
Cleavageperfect basal (one direction)
Fractureuneven
Specific Gravity2.7
Colorsblack, dark brown, greenish-black
Transparencytranslucent
Type LocalityLeduc Mine, St-Pierre-de-Wakefield, Wakefield Township, Gatineau Co., Québec, Canada
Mineral GroupSilicates
Usesindustrial, collector, geological indicator
Associated Mineralscalcite, quartz, feldspar, hornblende
Raritycommon
Description
This specimen features a large, dark, pseudohexagonal crystal of biotite prominently embedded within a matrix of light pink, granular calcite. The biotite crystal displays a somewhat mottled, dark surface with subtle brownish undertones.
Geological Context
Biotite is a common rock-forming mineral found in a wide range of igneous rocks such as granite and diorite, as well as metamorphic rocks like schist and gneiss. It typically forms during the cooling of magma or through recrystallization under high heat and pressure.
