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Formula: KAl2(AlSi3O10)(OH)2
Phyllosilicate (sheet silicate),
dioctahedral mica subgroup of the
mica group, forms a continuous series with
celadonite
Varieties
Damourite is a very fine-grained, compact variety of muscovite with a greasy feel
Fuchsite is a greenish, chromium-bearing variety of muscovite
Gieseckite is a name for pseudomorphs of muscovite after
an unknown mineral
Gilbertite ia a compact variety of muscovite
Illite series minerals are K-deficient varieties of muscovite that frequently contain
montmorillonite/beidellite
layers. They occur as constituents of some
shale, soil and recent sediments.
Illite is a mineral of the zeolite and
prehnite-pumpellyite facies. In the
zeolite facies clay
minerals transform to illite, kaolinite and
vermiculite.
Phengite is a variety of muscovite with fairly large amounts of Mg and Fe
Sericite is a a fine-grained white, pale green to oily greenish variety of muscovite, or rarely
paragonite that occurs as fibrous aggregates with a silky lustre.
The development of sericite from feldspar and other minerals, such as
topaz, kyanite,
spodumene and andalusite, is a
common feature of retrograde metamorphism. Sericite
also forms as an alteration of the wall rock of hydrothermal ore veins.
Properties of Muscovite
Crystal System: Monoclinic
Specific gravity: 2.77 to 2.88 measured, 2.83 calculated
Hardness: 2½
Streak: White
Colour: White, yellow
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Common impurities: Cr,Li,Fe,V,Mn,Na,Cs,Rb,Ca,Mg,H2O
Environments (muscovite):
Plutonic igneous environments
Pegmatites
Sedimentary environments
Metamorphic environments
Hydrothermal environments
In the Bowen reaction series muscovite is intermediate
between orthoclase (higher temperature) and
quartz (lower temperature).
Muscovite is a widespread and common rock-forming mineral. It is a
primary mineral typically found in
granite and
granite pegmatites,
where it is
associated with quartz and feldspar.
Muscovite occurs in detrital and authigenic (formed in place) sediments.
Muscovite is also very common in metamorphic rocks, being the chief constituent of some
mica schists. It occurs in
every zone of progressive regional metamorphism. In the
chlorite zone muscovite is a characteristic constituent of
albite-chlorite-sericite
(variety of muscovite)
schist. In the biotite zone
muscovite is less common than in the chlorite zone, because
muscovite reacts with chlorite to form
phlogopite and amesite
(DHZ 3 p23).
Muscovite is a mineral of the
albite-epidote-hornfels,
hornblende-hornfels,
prehnite-pumpellyite,
greenschist,
amphibolite,
blueschist and
eclogite facies.
Muscovite is an essential constituent of
phyllite, and a common constituent of
granite. It also may be found in
gneiss, schist and
amphibolite
(R&M 90.6.539).
Localities
In the Koksha valley, Jurm district, Badakhstan province, Afghanistan,
lazurite
pseudomorphs after muscovite have been found
(KL p264).
At the Urucum mine, Minas Gerais, Brazil, muscovite pseudomorphs
after garnet have been found
(KL p236).
At Heng Fa Chuen, Hong Kong Island, Hong Kong, China, muscovite occurs in a
pegmatite at the Chai Wan MTR (Mass Transit Railway) railway
depot, just
southeast of the Heng Fa Chuen MTR station, adjacent to the eastern entrance of the MTR railway tunnel. The
pegmatite cavity occurs in medium-grained
granite almost completely devoid of
biotite.
About 20 m away the contact of granite with pyroclastic rocks is exposed.
The pegmatite cavity is about 40 cm wide and contains
axinite,
K-feldspar, milky quartz and
muscovite
(Geological Society of Hong Kong newsletter 2.1.9).
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.
Muscovite occurred as rosettes several millimetres in diameter in
beryl-quartz veins
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Needle Hill Mine, Needle Hill, Sha Tin District, New Territories, Hong Kong, China, is a tungsten mine, abandoned
in 1967. The principal ore is wolframite, and the principal
gangue mineral is
quartz. Molybdenum also occurs. The
mineralisation consists of a series of parallel fissure veins that cut through
granite. Wolframite and
quartz are the main minerals, but galena,
sphalerite, pyrite,
molybdenite and fluorite have also
been found here
(Geological Society of Hong Kong Newsletter 9.3.29-40).
The quartz-wolframite veins are of
high-temperature hydrothermal formation,
and grade into wolframite-bearing
pegmatites.
Muscovite is mainly found as small scales mixed with granular quartz in
greisen. Associated minerals include
fluorite, topaz,
pyrite, molybdenite,
wolframite and beryl
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
The Lin Fa Shan deposit, Tsuen Wan District, New Territories, Hong Kong, China, is located in a remote area of the
Tai Mo Shan Country Park, on the steep west facing slope of Lin Fa Shan, just above the abandoned village of Sheung Tong.
The surrounding hillsides are covered with shallow excavations, representing past searches for
wolframite, the natural ore of tungsten. The abandoned workings are extremely
dangerous with unsupported tunnels, open shafts and no maintenance since
their closure in 1957; the workings should not be entered
(http://industrialhistoryhk.org/lin-shan).
Muscovite is mainly found as small scales mixed with granular quartz in
greisen. Associated minerals include
fluorite, topaz,
pyrite, molybdenite,
wolframite and beryl
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council).
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).
At Yaogangxian, Hunan, China, muscovite sometimes occurs coating earlier formed
arsenopyrite and
ferberite
(R&M 80.1.55).
At the Merelani hills, Tanzania, muscovite coloured green by traces of
vanadium
V3+ has been found with inclusions of
graphite and pyrite. Laths of
muscovite also
extend into void spaces within larger crystals. The mineral is associated with
quartz,
graphite, phlogopite and traces
of
fluorapatite
(R&M 95.2.166-167).
At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, the prehnite -
pumpellyite wall-rock alteration haloes that surround the
zeolite - bearing veins in the
quartz-diorite contain minor
muscovite variety sericite, associated with
prehnite, Fe-bearing
pumpellyite,
chlorite, quartz and
analcime, and also minor kaolinite
and
smectite.
(JRS 20.21).
At the Benallt Mine, Rhiw, Aberdaron, Gwynedd, Wales, UK, distinctive pale amethyst-purple muscovite occurs
with hyalophane, albite and
baryte in veinlets in a
barium-feldspar-hematite
matrix.
Muscovite from the Benallt Mine is barium-rich, and it owes its
distinctive pale amethyst-purple colour to a relatively high manganese
content and unusually low iron content.
Evidence shows that the assemblage formed in low-grade metamorphic conditions, a conclusion which is consistent with
the barium content of the
hyalophane
(JRS 22.43-47).
At the Santo Nino mine, Arizona, USA, muscovite encloses rutile.
(R&M 87.2.133).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, muscovite is commonly associated with
fluorapatite, hydroxylherderite
and bertrandite, and sometimes with
columbite group minerals and wodginite.
Overgrowths of lepidolite, secondary muscovite or
cookeite may develop on muscovite crystals. Muscovite is a common
replacement mineral of schorl and almandine.
The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.512).
At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, muscovite occurred as crystals to
1 cm across, and as inclusions in quartz. Sericite occurred as
clay in the pockets and as fine white phantoms in
quartz, named “snowflake phantoms”
(R&M 96.6.494).
At the Chickering Mine, Walpole, Cheshire county, New Hampshire, USA, muscovite is a component of the outer
pegmatite zones, having formed before
the melt became enriched in lithium in the inner zones, forming the
lithium-rich mica
lepidolite. It also occurs much later as a druse lining
moulds where elbaite crystals have dissolved
(R&M 90.5.420).
At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the
pegmatites are beryl-type
rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most
complex zonation and diverse mineralogy of any of the Keyes
pegmatites. Six zones are distinguished, as follows, proceeding
inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase
border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite
wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite
outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite
middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate
zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to
1.2 meters in size that were altered to vuggy
albite-muscovite with
fluorapatite crystals. This unit presumably was the source of the
albite, muscovite,
fluorapatite, quartz and other
crystallised minerals found in pieces of vuggy albite
rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including
tourmaline, graftonite,
triphylite, vivianite,
pyrite, pyrrhotite, and
beryl crystals to 30.5 cm long and 12.7 cm across.
Clusters of freestanding muscovite crystals are among the most desirable specimens from the Keyes mines.
Individual crystals commonly are up to 10 cm, but exceptionally may be as much as 20 cm across. The matrix for most of
the muscovite crystals is massive to coarsely crystallised albite.
Blue-green fluorapatite crystals also occur with the muscovite on
some specimens. An unusual piece has been found with microsized black
tourmaline needles densely covering a large cluster of muscovite
crystals
(R&M 97.4.321-322).
The Purple Diopside Mound, Rose Road, Pitcairn, St. Lawrence county, New York, USA, is situated in
marble. The development of veins of large crystals probably occurred as
a result of fluid penetration from a concurrent intrusion. Many of the minerals of interest to collectors formed during
this primary event, with additional species resulting from the
subsequent alteration of scapolite. There seems to be little, if any,
secondary, late-stage mineralisation present.
Muscovite variety gieseckite occurs as pseudomorphs of fine-grained
sericitic muscovite after an unidentified mineral. The gieseckite occurs as pale tan to white, or occasionally
pink, crystals to 8 cm associated with graphite clumps and purple
diopside
(R&M 96.6.550).
Alteration
The fine-grained "pinite", which is mainly composed of muscovite and clay minerals, occurs as an alteration
product of
cordierite
(R&M 88.6.554-562).
antigorite and muscovite to
phlogopite, amesite, SiO2 and H2O
5Mg3Si2O5(OH)4 +
3KAl2(AlSi3O10)(OH)2 →
3KMg3(AlSi3O10)(OH)2 + 3Mg2Al(AlSiO5)(OH)4
+ 7SiO2 + 4H2O
(DHZ 3 p23)
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).
dolomite and muscovite to
phlogopite,
calcite, CO2 and Al2O3
3CaMg(CO3)2 + KAl2(AlSi3O10)(OH)2
→ KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 3CO2
+ Al2O3
The excess alumina may be used to form spinel
(DHZ 3 p51).
K-feldspar and H+ to muscovite,
silica and K+
3KaAlSi3O8 + 2H+ ⇌
KAl2(AlSi3O100(OH)2 + 6SiO2 (aqueous) + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction
(KB p99).
montmorillonite and K-feldspar
to illite, aqueous SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAlSi3O8
⇌
KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O
(JVW p328)
muscovite to corundum,
K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 +
K(AlSi3O8) + H2O
(JVW p102)
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure
(hornblende-hornfels facies) to about
720oC at pressure above 4 kbar (amphibolite facies)
(MOM p517).
muscovite, H+ and H2O to kaolinite and K+
2KAl2(AlSi3O10)(OH)2 + 2H+ + 3H2O ⇌
3Al2Si2O5(OH)4 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction
(KB p99).
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).
muscovite and
quartz to sillimanite,
K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 + SiO2 ⇌
Al2SiO5 + KAlSi3O8 + H2O
At 5 kbar pressure the equilibrium temperature is about 690oC
(amphibolite facies)
(SERC).
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat),
hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down
(KB p17).
Although the muscovite-quartz assemblage is stable over a large part of the
PT range of regional metamorphism,
at temperatures around 600 to 650oC it is replaced by
sillimanite and
K-feldspar
(DHZ 3 p24).
phlogopite, muscovite and
SiO2 to orthoclase,
pyrope and H2O
KMg3(AlSi3O10)(OH)2 +
KAl2(AlSi3O10)(OH)2 + 3SiO2
⇌ 2K(AlSi3O8) + Mg3Al2(SiO4)3 +
2H2O
(DHZ 3 p72)
sillimanite, annite and H2O
to
staurolite, muscovite,
SiO2 and O2
31Al2SiO5 + 4KFe2+3(AlSi3O10)(OH)2 +
6H2O → 34Fe2+2Al9si4O23(OH) +
KAl2 (AlSi3O10)(OH)2 + 7 SiO2 + 1.5O2
Staurolite may occur as a product of retrograde metamorphism
according to the above reaction
(DHZ 1A p859).
spodumene, K+ and H+ to muscovite,
quartz and Li+
3LiAlSi2O6 + K+ + 2H+ →
KAl3Si3O10(OH)2 + 3SiO2 + 3Li+
The direct conversion of spodumene to muscovite liberates silica, but
quartz is not usually present in
pseudomorphs
of muscovite after spodumene, and this requires an explanation
(AM 67: 97-113).
staurolite, annite and O2 to
hercynite, magnetite,
muscovite,corundum,
SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3
(AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4
+ Fe2+Fe3+2O4 + KAl2
(AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O
(DHZ 1A p860)
Other reactions:
The fine-grained variety sericite replaces
beryl, topaz and
tourmaline in
pegmatites.
Muscovite itself alters to illite and then to
montmorillonite
(R&M 88.6.554-562).
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