What is Biotite?
Biotite is a traditional group name for dark mica compositions dominated by iron and magnesium. Modern mineral classification distinguishes species such as annite and phlogopite, but biotite remains useful in petrology and collecting.
Mineralogy and structure
Biotite micas have a layered silicate structure and are generally monoclinic. A simplified formula is K(Mg,Fe)3AlSi3O10(F,OH)2. Perfect basal cleavage allows thin flexible sheets to separate from a crystal.
Colour and appearance
Biotite is commonly black, brown, bronze or dark greenish brown. It forms books of platy crystals, flakes and foliated masses with pearly to vitreous lustre. Alteration can turn the surface golden or earthy.
Formation and localities
It is widespread in granites, pegmatites, gneisses, schists and some volcanic rocks. Important collector material comes from Russia, Pakistan, Brazil, Madagascar, Norway, Canada and the United States.
Collecting and care
Biotite is soft and easily split, so store it away from harder specimens. In the guide it is linked with Phlogopite and the mica group; the exact species should be stated when confirmed.
Biotite and zodiac signs
Popular crystal lists sometimes connect Biotite with Scorpio or Capricorn because of its dark colour and protective-looking sheets. This is symbolic folklore, not a physical property.
Symbolic and esoteric associations
Esoteric traditions may describe dark mica as a symbol of reflection, boundaries and layered understanding. These associations are personal or cultural and have no established therapeutic effect.
