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Biotite refers to dark mica minerals, such as the
manganese-rich variety manganophyllite and
phlogopite. These are all phyllosilicates (sheet silicates)
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
(DHZ 3 p71).
It may also be found in
rhyolite,
trachyte, dacite, latite,
andesite and
basalt
(DHZ 3 p71).
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
Localities
At Kwun Yum Shan, Yuen Long District, New Territories, Hong Kong, China, the deposit is a hydrothermal deposit which lies
along a fault zone withi altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite with
scattered quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
(Geological Society of Hong Kong newsletter 14.1).
At the Lin Ma Hang mine, North District, New Territories, Hong Kong, China, the
lead-zinc deposit is a hydrothermal
deposit which lies along a fault zone within altered acid volcanic rocks, consisting mainly of
chlorite, biotite,
sericite and actinolite, with
scattered quartz.
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
At Devil's Peak, Sai Kung District, New Territories, Hong Kong, China, the mineralisation occurred in
quartz veins in the contact zone between a
granite intrusion and acid volcanic rocks. The mine is now closed,
and inaccessible for collecting.
Biotite occurred as aggregates of lamellar plates up to 3 cm in diameter in veins in
granite
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Biotite is found as aggregates of dark green plates in veins in the
skarn
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
Alteration
Biotite is a significant hydrothermal mineral that commonly replaces ferromagnesian minerals
(AofA).
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
(DHZ 2A p134).
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
(http://www.tulane.edu/~sanelson/eens212/metaminerals.htm).
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
(DHZ 2A 139).
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
(DHZ 3 p73).
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
(DHZ 3 p23)
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
(DHZ 3 p72).
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
(DHZ 3 p24).
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