Biotite

mica

phlogopite

Biotite refers to dark mica minerals, such as manganophyllite and phlogopite KMg3(AlSi3O10)(OH)2 phyllosilicate (sheet silicate)
In the discontinuous branch of the Bowen reaction series biotite is intermediate between amphibole (higher temperature) and orthoclase (lower temperature, after joining with the continuous branch).
Environments:

Plutonic igneous environments (typical)
Volcanic igneous environments
Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal environments

Solubility: Slightly soluble in sulphuric acid

In metamorphic environments biotite generally the first new product of the recrystallisation; it is formed over a wide range of temperature and pressure conditions, in both regionally metamorphosed and contact metamorphosed rocks.
Typical associations are biotite with chlorite and biotite with muscovite.
Biotite is a primary and secondary mineral.
It is an essential constituent of phyllite.
It is a common constituent of granite, granodiorite, tonalite, diorite, norite, syenite and monzonite.
It may also be found in rhyolite, trachyte, dacite, latite, andesite and basalt.
Less frequently it may be found in schist, gneiss, gabbro, limestone, dolostone and hornfels.
Biotite is characteristic of the greenschist facies and it is also a mineral of the albite-epidote-hornfels, hornblende-hornfels, pyroxene-hornfels and amphibolite facies.
It never occurs in the sanidinite facies

Alteration

Biotite is a significant hydrothermal mineral that commonly replaces ferromagnesian minerals.

biotite and quartz to enstatite- ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O
enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage.

muscovite, biotite and SiO2 to K-feldspar, cordierite and H2O
6KAl2(AlSi3O10)(OH)2 + 2K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 15SiO2 → 8KAlSi3O8 + 3(Fe2+,Mg)3Al4Si5O18 + 8H2O
At the high-grade end of the amphibolite facies biotite is no longer stable and reacts with muscovite according to the above reaction.

muscovite, biotite and SiO2 to K-feldspar, garnet and H2O
KAl2(AlSi3O10)(OH)2 + K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 3SiO2 → 2KAlSi3O8 + (Fe2+,Mg)3Al2(SiO4)3 + 2H2O

muscovite, iron-rich biotite and SiO2 to orthoclase, almandine and H2O
KAl2(AlSi3O10)(OH)2 + KFe2+3(AlSi3O10)(OH)2 + 3SiO2 ⇌ 2KAlSi3O8 + Fe2+3Al2(SiO4)3 + 2H2O
Iron-rich biotite is likely to react at lower PT conditions than iron-poor biotite.

muscovite and garnet to biotite, sillimanite and quartz
KAl2(AlSi3O10)(OH)2 + (Fe2+,Mg)3Al2(SiO4)3 → K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 2Al2SiO5 + SiO2
Muscovite is unstable in combination with garnet.

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