Biotite

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Other names: Black mica
Mineral class: Phyllosilicate
Composition: Aluminum silicon hyrdoxides with potassium, magnesium or iron. K(Mg, Fe)₃AlSi₃O₁₀(OH)₁₀
Hardness: 2.5 - 3
Luster: Vitreous
Color: Black, Brown, Green
Streak: White or grey
Cleavage: Perfect, 1 plane; splits into thin sheets
Specific Gravity: 2.8 - 3.2
Crystal form: Monoclinic. Irregular foliated masses.
Comments: Dark color due to iron. Fuses with difficulty. Soluble only in hot concentrated sulfuric acid. Often visible in granite, gneiss and other igneous rocks. Crystals up to 70 square meters have been found in the Ilmen mountains of Russia. Beautiful crystals have been found in geodes of Vesuvius. [phyllosilicates.jpg]

Other names:	Black mica
Mineral class:	Phyllosilicate
Composition:	Aluminum silicon hyrdoxides with potassium, magnesium or iron. K(Mg, Fe)₃AlSi₃O₁₀(OH)₁₀
Hardness:	2.5 - 3
Luster:	Vitreous
Color:	Black, Brown, Green
Streak:	White or grey
Cleavage:	Perfect, 1 plane; splits into thin sheets
Specific Gravity:	2.8 - 3.2
Crystal form:	Monoclinic. Irregular foliated masses.
Comments:	Dark color due to iron. Fuses with difficulty. Soluble only in hot concentrated sulfuric acid. Often visible in granite, gneiss and other igneous rocks. Crystals up to 70 square meters have been found in the Ilmen mountains of Russia. Beautiful crystals have been found in geodes of Vesuvius.