Biotite

megalodon

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Biotite is a common mineral group of phyllosilicate minerals within the mica group. They are black mica minerals commonly found in igneous and metamorphic rocks. Biotite group includes annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. They are all sheet silicate minerals with similar physical properties but have varying chemical composition. The approximate chemical formula for Biotite group is K(Mg,Fe)3(AlSi3O10)(F,OH)2.

Biotite is usually called as black mica whereas muscovite is called as white mica. They both form in the same rocks and often occur together. The term Biotite is generally used for all minerals of the group as they can't be identified without optical, chemical, or x-ray analysis.

Biotite is very easy to identify due to perfect cleavage and vitreous luster. It occurs in igneous and metamorphic rocks as sheets and can break into small flakes. It transforms into clay minerals due to weathering but can also occur in sediments and sandstones.

Biotite is a name given for a group of black mica minerals which can't be distinguished from one another without laboratory analysis. Experienced Geologists can identify phlogopite by its brown color. You can find some of the biotite minerals below with their chemical compositions.

Mineral
Chemical Composition
Annite​
KFe3(AlSi3)O10(OH)2​
Phlogopite​
KMg3(AlSi3)O10(F,OH)2​
Siderophyllite​
KFe2Al(Al2Si2)O10(F,OH)2​
Eastonite​
KMg2Al(Al2Si3)O10(OH)2​
Fluorannite​
KFe3(AlSi3)O10F2​
Fluorophlogopite​
KMg3(AlSi3)O10F2​

Biotite is extensively used in dating igneous rocks by either potassium-argon and argon-argon dating methods to assess minimum ages for rocks. It is also used in assessing temperature histories of metamorphic rocks. Biotite is also used as an additive to drilling muds, as a filler and extender in paints, as rubber mould release agent, and as coating over roll roofings and shingles.

Physical Properties

PropertyValue
CategoryPhyllosilicates
Crystal SystemMonoclinic
ColorYellow, and greenish, dark, blackish shades of brown
StreakWhite
LusterVitreous
CleavageBasal, Perfect
Mohs Scale Hardness2.5 to 3
DiaphaneityTransparent to translucent to opaque
Specific Gravity2.7 to 3.4
Chemical CompositionK(Mg,Fe)3(AlSi3O10)(F,OH)2
Diagnostic PropertiesSheet structure, dark color, and perfect cleavage.
 

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