Dark iron- and magnesium-rich mica. One of the most common rock-forming minerals, giving many rocks their dark sparkly appearance. Color: Black, dark brown, dark green. Luster: Vitreous to submetallic. Streak: White to gray. Cleavage: Perfect basal cleavage. Hardness: 2.5-3. Transparency: Translucent to opaque. Crystal system: Monoclinic. Chemical formula: K Mg,Fe ₃ AlSi₃O₁₀ OH ₂. Specific gravity: 2.7-3.4. Formation: Crystallizes from a wide range of magmas and forms during regional and contact metamorphism. Common geological settings: Worldwide; essential mineral in granites, gneisses, schists, and many igneous rocks. Common look-alikes and related entries: Phlogopite, Muscovite, Chlorite.
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These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.
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GeoMiner
Last updated
2026-03-30T14:36:45.10956+00:00
Review status
Published; not independently peer-reviewed
IMA–CNMNC List of Mineral Names — International Mineralogical Association. Official mineral-name, nomenclature, and classification records.
Mineral ID Key — Mineralogical Society of America. Physical-property identification methods, repeat-testing guidance, and limitations.
Dark iron- and magnesium-rich mica. One of the most common rock-forming minerals, giving many rocks their dark sparkly appearance.
How can you identify Biotite?
Compare several properties rather than relying on colour alone. This entry records colour: Black, dark brown, dark green; lustre: Vitreous to submetallic; streak: White to gray; Mohs hardness: 2.5-3; cleavage: Perfect basal cleavage; crystal system: Monoclinic; transparency: Translucent to opaque. Confirm an important identification with additional field tests or qualified laboratory work.
How does Biotite form?
Crystallizes from a wide range of magmas and forms during regional and contact metamorphism.
Where is Biotite commonly found?
Worldwide; essential mineral in granites, gneisses, schists, and many igneous rocks.
What is Biotite commonly confused with?
Biotite may be confused with Phlogopite, Muscovite, Chlorite. Compare multiple diagnostic properties because colour and surface appearance can overlap.
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