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Geology is the Way

Muscovite

Monoclino

K(Al, Mg, Fe)2[Si3AlO10](OH,F)2

Muscovite is the most common white mica in igneous and metamorphic rocks and also occurs in sedimentary rocks as a detrital mineral. The name derives from the Elizabethan term ‘Muscovy-glass’, since in medieval Russia muscovite (which breaks in transparent sheets) was used as an alternative to glass in windows. 

Struttura e chimica
Muscovite is a dioctahedral sheet silicate or phyllosilicate. Its structure consists of a pile of ‘sheets’ of (Si,Al)O4 tetrahedrons interconnected as six-sided rings extending infinitely in the 2D plane, and dioctahedral ‘gibbsite’ layers, where each anion (O2- or OH) is surrounded by 2 Al3+ cations. Each trioctahedral sheet (O) is sandwiched between 2 tetrahedral layers (T) and this structure repeats indefinitely perpendicular to the sheet. The T-O-T ‘sandwiches’ are separated by large cation sites containing K+ (interlayer sites).

muscovite crystal structure

Sketch of the crystal structure of muscovite, seen on a section perpendicular to the sheets (parallel to the c-axis), including a plain view of the tetrahedral sheets (right). Based on Deer et al. (1992).

The term ‘muscovite’ is commonly used to refer to potassic white mica in general but, strictly speaking, muscovite is the aluminium-rich end-member of the solid solution between muscovite [KAl2Si3AlO10(OH,F)2] and celadonite [K(Mg,Fe)(Al,Fe3+)Si4O10(OH,F)2], both belonging to the mica group. This solid solution is characterized by the substitution of (Mg,Fe)2+ per Al3+ in the octahedral site of the mineral balanced by the substitution of Si4+ per Al3+ in the tetrahedral site (Tschermak substitution). Potassic white micas that contain a proportion of muscovite and celadonite are commonly known as phengite, a term that has no strict definition but that is useful to denote muscovites with high-Si content. Some K can be replaced by an empty site or vacancy (□), due to the contemporaneous substitution of Al3+ by Si4+. The resulting K-poor white micas are known as illite (K = 0.6 – 0.85 per formula unit on 11 oxygen basis)

Another important substitution in muscovite is the pyrophyllitic substitution, the replacement of K by an empty site or vacancy and the coeval substitution of  which produces a K-deficient white mica known as illite . Other substitutions in muscovite are the ferromagnesian substitution (Fe → Mg), the di/trioctahedral substitution (Mg,Fe → Al, □), the substitution of octahedral Al3+ with Fe3+, and a partial solid-solution with paragonite (K → Na). Rb, Cs, Ca, and Ba can also substitute in part K and Mn, Li, Cr, Ti, V can enter the octahedral site. Cr-rich muscovites are known as fuchsites. (OH) can be replaced by F.


Muscovite on white orthoclase feldspar. Sapucaia pegmatite, Minas Gerais, Brazil. Photo © Parent Géry.

Proprietà
Abito: platy, lamellar with pseudo-hexagonal basal face
Durezza: 2.5 – 3
Sfaldatura: {001} perfect (basal cleavage)
Geminazione: {001} composition plane –  [310] twin axis
Colore: colorless (silver-grey metallic) to light shades of green, red, or brown
Lucentezza: vitreous, micaceous (high reflectivity)
Struscio: bianco
Alterazione: clay minerals, vermiculite, illite…
In sezione sottile...
α(α^c = 0-5°): 1.552-1.576
β(β^a = 1-3°): 1.582-1.615
γ(γ//b): 1.587-1.618
2Vα: 28-47°
Colore: incolore
Pleocroismo: relief pleochroism
Birifrangenza (δ): 0.036-0.049 (high interference colors)
Rilievo: moderate
Segno ottico:
[Mindat]

white mica mineral chemistry

Ternary diagram showing the most common solid-solutions of potassic white micas. Based on Vidal & Parra (2000) and references therein.

Caratteristiche di terreno

muscovite crystal habit

Muscovite crystal sketch. Based on Deer et al. (1992).

Muscovite has a characteristic platy/lamellar habit that tends to break as ‘sheets’, along its perfect basal sfaldatura planes. Even tiny grains tend to show a strong micaceous luster: for this reason many muscovite-rich schistose rocks (phyllites and schists) commonly show a lustrous sheen. Euhedral crystals of muscovite show basal faces with a characteristic pseudo-hexagonal shape. Basal faces are easily recognizable because they are highly reflective and show the characteristic micaceous, metallic luster. Basal faces lacks cleavage traces (visible on prismatic faces) and this allows to distinguish muscovite from other metallic minerals (e.g. pyroxene). The color tend to be colorless and metallic (silver grey), sometimes with shades of green, red or brown, differently from biotite which is generally dark to black. Muscovite has very low hardness (2.5 – 3.0) and can be split even by hand along its basal cleavage planes.

muscovite and biotite

Micas typically break as thin sheets. In photo biotite or black mica (left) and muscovite or white mica (right). Photo by Siim Sepp (sandatlas.org).

mica-rich sand

Micas are strongly reflective thanks to their perfect basal cleavage and metallic color. They are easily identifiable even when they are very tiny. This sand contains black biotite and white mica (muscovite). Width: 20 mm. Photo by Siim Sepp (Sandatlas.org).

muscovite biotite schist

Schist with muscovite (silver-grey, metallic) and biotite (black, metallic) coexisting with quartz (grey, transparent) and feldspars (white). Manhattan Schist, Manhattan, New York City, USA. Size: 4.6 cm wide. Photo © James St. John.

phyllite

The lustrous sheen of many low-grade schists and phyllites is related to the presence of tiny, aligned grains of muscovite. Microfolds in this structure define tiny crenulations. Phyllite from Godrevy, Cornwall, England. Photo © Margaret W. Carruthers.

Muscovite in thin section
Muscovite crystals are commonly platy or lamellar and show the characteristic basal sfaldatura. At PPL, muscovite is colorless and shows moderate relief. At CPL, muscovite displays moderate to high interference colors. Differently from biotite, muscovite lacks color and pleochroism. Muscovite can be confused with other phyllosilicates that show similar habit and interference colors, like paragonite, talc, or pyrophyllite. Talc generally (but not always) does not coexist with muscovite, as it occurs largely in Mg-rich rocks like serpentinites or mafic or ultramafic metamorphic rocks. Paragonite and pyrophyllite can coexist with muscovite in schistose metamorphic rocks and their identification requires special analytical techniques (e.g. scanning electron microscope).
Sericite, occurring in many igneous and metamorphic rocks, is a high birefringence mixture of very fine-grained white mica and other phyllosilicates that forms as a result of alteration of silicates. Muscovite alters to smectite, illite, pyrophyllite, and kaolinite, losing K in the process. Detrital muscovite in sedimentary rocks often contains some of these phyllosilicates as alteration products.

CPL
CPL
NX
PPL
N//

Group of muscovite crystals surrounded by quarzo in a deformed pegmatite. Note the lamellar habit and the high interference colors. Width: 3 mm. Calamita, Island of Elba, Italy.

CPL
CPL
NX
PPL
N//

‘Pancake’. The muscovite crystal at the center has been cut close to the basal section and, therefore, shows low interference colors and the cleavage is not visible, in sharp contrast with the neighboring grain (top-right), which is cut along the prismatic section (note the high interference colors and the perfect basal cleavage). Width: 3 mm. Deformed pegmatite. Calamita, Island of Elba, Italy.

CPL
CPL
NX
PPL
N//

‘Ukrainian flag’. The muscovite crystal above is twinned along its basal plane ({001}). Since the twinning law of muscovite involves a rotation, the two twins show different interference colors along the thin section plane. Width: 3 mm. Deformed pegmatite. Calamita, Island of Elba, Italy.

Examples of muscovite-bearing rocks

Deformed muscovite pegmatite
Sample of a pegmatitic leucogranite that intruded a shear zone and was deformed at high temperature conditions.
Campione: mylonitic metapegmatite
Associazione mineralogica: muscovite, quartz, alkali feldspar, tourmaline, andalusite
Località: Fosso del Pontimento, Calamita, Island of Elba, Italy
Sample courtesy Giovanni Musumeci.

Ritrovamento
Muscovite is a common mineral in metamorphic rocks. It is stable in many metapelites, metamarls, metasandstones, and metagranites from greenschist to upper amphibolite facies. It forms from the recrystallization of clay minerals at the low metamorphic grade and remains stable to the high-grade, when it breaks down to K-feldspar and Al-silicates, sometimes in the presence of melt. Across this range, muscovite may coexist with several silicates like chlorite, biotite, chloritoid, staurolite, garnet, cordierite, and Al-silicates. At high pressure (greenschist to blueschist facies), muscovite is commonly rich in celadonite (i.e. phengite) and may occur together with paragonite, chloritoid, glaucophane, lawsonite, garnet, and omphacite.
Muscovite is rarer in igneous rocks, but it is an important component of Al-rich granitoids, biotite-muscovite granites, and pegmatites. It is even rarer in volcanic rocks but has been reported in some rhyolites. Muscovite can occur also in some metasomatic rocks and veins.
In sedimentary rocks, muscovite occurs as a detrital mineral, as it is relatively resistant to weathering, but it is commonly altered to mixtures of phyllosilicates.

Bibliografia
Bailey, S. W. (2018). 1. Classification and structures of the MICAS. Micas, 1-12.
Guidotti, C. V. (1984). Micas in metamorphic rocks. Reviews in Mineralogy and Geochemistry13(1), 357-467.
Guidotti, C. V., Sassi, F. P., Blencoe, J. G., & Selverstone, J. (1994). The paragonite-muscovite solvus: I. PTX limits derived from the Na-K compositions of natural, quasibinary paragonite-muscovite pairs. Geochimica et Cosmochimica Acta58(10), 2269-2275.
Massonne, H. J., & Schreyer, W. (1987). Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite, and quartz. Contributions to Mineralogy and Petrology96(2), 212-224.
Merino, E., & Ransom, B. (1982). Free energies of formation of illite solid solutions and their compositional dependence. Clays and Clay minerals30(1), 29-39.
Rieder, M., Cavazzini, G., D’yakonov, Y. S., Frank-Kamenetskii, V. A., Gottardi, G., Guggenheim, S., … & Wones, D. R. (1998). Nomenclature of the micas. Clays and clay minerals46(5), 586-595.
Vidal, O., & Parra, T. (2000). Exhumation paths of high‐pressure metapelites obtained from local equilibria for chlorite–phengite assemblages. Geological Journal35(3‐4), 139-161.

           

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