Phyllosilicate (sheet silicate), mica group. Muscovite forms a continuous series with celadonite.
Damourite is a very fine-grained, compact variety of muscovite with a greasy feel
Fuchsite is a greenish, chromium-bearing variety of muscovite
Gilbertite ia a compact variety of muscovite
Illite series minerals are K-deficient varieties of muscovite
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
Specific gravity: 2.77 to 2.88
Colour: White, yellow
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Plutonic igneous 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).
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 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 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 Chickering mine, 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).
The illite series minerals are 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.
Sericite is a variety of muscovite that occurs as fibrous aggregates with a silky lustre.
Environments for sericite:
The development of sericite from
feldspar and other minerals, such as
and andalusite, is a common feature of
retrograde metamorphism. Sericite also forms as an
alteration of the wall
rock of hydrothermal ore veins.
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
muscovite to corundum, K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 + K(AlSi3O8) + H2O
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)
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).
Common impurities: Cr,Li,Fe,V,Mn,Na,Cs,Rb,Ca,Mg,H2O
Back to Minerals