Mineral description : Biotite

Biotite book from Umeå.

Mineral class Silicates : Phyllosilicates : Mica group.
Chemical formula K(Mg,Fe)3(OH,F)2(AlSi3O10)
Crystal system Monoclinic.
Habitus Rarely as small hexagonal crystals. Often as books of mica.
Cleavage Basally perfect.
Hardness 2.5-3
Density 2.8-3.2
Color Black, dark brown, reddish brown, green and rarely white.
Streak Colorless.
Luster Vitreous.
Occurance Common in both igneous and metamorphic rocks.
Notes Soluble in concentrated sulfuric acid.
Locations A very common mineral with big crystal books at many places, for instance :
  • Kolsva, Västmanland.
  • Derome, Halland. Up to 1m crystal books.
  • Brännland quarry, Umeå, Västerbotten. As up to 40cm crystal books in a pegmatite.

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Last changed : 1998-08-26