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
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
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
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 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).
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).
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