Muscovite
Monoclinic
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.
Structure and chemistry
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).
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+ for Al3+ in the octahedral site of the mineral balanced by the substitution of Si4+ for 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.
Properties
Habit: platy, lamellar with pseudo-hexagonal basal face
Hardness: 2.5 – 3
Cleavage: {001} perfect (basal cleavage)
Twinning: {001} composition plane – [310] twin axis
Color: colorless (silver-grey metallic) to light shades of green, red, or brown
Luster: vitreous, micaceous (high reflectivity)
Streak: white
Alteration: clay minerals, vermiculite, illite…
In thin section…
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°
Color: colorless
Pleochroism: relief pleochroism
Birefringence (δ): 0.036-0.049 (high interference colors)
Relief: moderate
Optic sign: –
[Mindat]
[HoM]
Field features
Muscovite has a characteristic platy/lamellar habit that tends to break as ‘sheets’, along its perfect basal cleavage 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 in thin section
Muscovite crystals are commonly platy or lamellar and show the characteristic basal cleavage. 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
PPL
⇔ slider. Group of muscovite crystals surrounded by quartz in a deformed pegmatite. Note the lamellar habit and the high interference colors. Width: 3 mm. Calamita, Island of Elba, Italy.
CPL
PPL
⇔ slider. ‘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
PPL
⇔ slider. ‘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.
Sample: mylonitic metapegmatite
Assemblage: muscovite, quartz, alkali feldspar, tourmaline, andalusite
Locality: Fosso del Pontimento, Calamita, Island of Elba, Italy
Sample courtesy Giovanni Musumeci.
Occurrence
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.
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 Geochemistry, 13(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 Acta, 58(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 Petrology, 96(2), 212-224.
Merino, E., & Ransom, B. (1982). Free energies of formation of illite solid solutions and their compositional dependence. Clays and Clay minerals, 30(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 minerals, 46(5), 586-595.
Vidal, O., & Parra, T. (2000). Exhumation paths of high‐pressure metapelites obtained from local equilibria for chlorite–phengite assemblages. Geological Journal, 35(3‐4), 139-161.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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