mineral
Biotite
K(Mg,Fe)3AlSi3O10(OH)2
AI-generated identification · may be incorrectMohs hardness 2.5-3; dark brown to black color; pearly to submetallic luster; monoclinic crystal system with perfect basal cleavage yielding flexible sheets.
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Physical properties
Mohs hardness 2.5-3; dark brown to black color; pearly to submetallic luster; monoclinic crystal system with perfect basal cleavage yielding flexible sheets.
Formation & geological history
Forms in igneous and metamorphic rocks, particularly granites, pegmatites, and schists, through crystallization from magma or high-grade regional metamorphism.
Uses & applications
Used primarily in industrial fillers, paints, lubricants, and as a geochronological marker in radiometric dating.
Geological facts
Commonly called black mica, biotite flakes are easily mistaken for gold by novices, earning it the nickname fool's gold.
Field identification & locations
Identify by its dark color and ability to be peeled into thin, elastic sheets using a knife tip; found worldwide in granitic terrains.
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Other mineral specimens
Gneiss or Schist
Gneiss or Mica Schist with Garnet
Metamorphic
Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
sedimentary
Tumbled granite
Granite, a coarse-grained felsic intrusive igneous rock
igneous
Pegmatite with Tourmaline and Quartz
Igneous Pegmatite
igneous
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Granite
Granite (Phaneritic intrusive igneous rock)
igneous