mineral
Biotite mica (likely)
Biotite, a mica-group mineral; approximate formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂
AI-generated identification · may be incorrectDark brown to black, vitreous-to-pearly luster, perfect basal cleavage into thin sheets; Mohs hardness 2.5–3, monoclinic crystal structure. The small, irregular specimen limits confident identification from the image alone.
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Physical properties
Dark brown to black, vitreous-to-pearly luster, perfect basal cleavage into thin sheets; Mohs hardness 2.5–3, monoclinic crystal structure. The small, irregular specimen limits confident identification from the image alone.
Formation & geological history
Forms in igneous and metamorphic rocks as magma cools or during metamorphism; found in rocks spanning much of Earth’s geological history.
Uses & applications
Used mainly as a geological indicator and mineral-collection specimen; mica has limited industrial use where its dark color and lower stability are acceptable.
Geological facts
Biotite’s name derives from physicist Jean-Baptiste Biot, who studied mica’s optical properties.
Field identification & locations
The dark, platy appearance and tendency to split into flexible sheets are typical of biotite. It is widespread in granites, schists, and gneisses; a definitive identification may require checking cleavage and hardness.
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Gneiss or Mica Schist with Garnet
Metamorphic
Sandstone (with potential mineral staining/concretions)
Arenite (SiO2 based)
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Tumbled granite
Granite, a coarse-grained felsic intrusive igneous rock
igneous
Epidote
Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)
metamorphic
Pegmatite with Tourmaline and Quartz
Igneous Pegmatite
igneous
Granite
Granite (Phaneritic intrusive igneous rock)
igneous