MUSCOVITE

Muscovite – KAl3Si3O10(OH)2 – is a very common phyllosilicate of the Mica Group that occurs in many types of magmatic, metamorphic and sedimentary rocks. Like other micas, it has a number of important industrial uses.               

As muscovite also forms in pegmatites, the crystals can be enormous: the record is a crystal measuring 4.5 meters in diameter and weighing 77 tons. Muscovite has several polytypes and can occur interstratified with vermiculite, paragonite and montmorillonite. May contain Fe, Mn, Mg, V, Ba, Ca, Na, H2O, Cr, Li, Cs and Rb.              

It has 35 varieties. Among the most important is sericite, which is a muscovite (sometimes paragonite) that forms very fine grains and often is found in altered feldspars and low-grade metamorphic rocks. “Fengite” is an intermediate mica between muscovite and celadonite, which has a 2V angle of 0-30º and is almost impossible to distinguish from muscovite by optical means. The variety “fuchsite” is macroscopically green (rich in Cr), occurs in metamorphic rocks and constitutes a green ornamental rock (quartzites with fuchsite).

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: White, gray, silvery, brown, green, pink, yellow.

Habit: Massive, micaceous, tabular, columnar, scaly, granular, pseudohexagonal or diamond-shaped.

Cleavage: {001} perfect, as other micas.

Tenacity: Flexible and elastic.

Twinning: Plane at {001}, axis at [310], in 6-pointed stars.

Fracture: Micaceous.

Mohs Hardness: 2 – 2.5 // a {001}; 4 perpendicular to {001}.

Parting: on {110} and {010}.

Streak: White.

Lustre: Vitreous, silky, pearly.

Diaphaneity: Transparent.

Density (g/cm³): 2.76 – 3.

 

2. Geology and Deposits

Muscovite is an abundant mica, the most common of the white micas. It occurs in many types of igneous rocks (aluminous granites, granodiorites, aplites, pegmatites and other felsic rocks) and metamorphic rocks with a pelitic composition and low to medium metamorphic grade (slate, phyllite, shale, gneiss, cornubianite and quartzite).

Sericite occurs in very low-grade metamorphic rocks. Muscovite also is found in some immature sedimentary rocks (sandstones, siltstones, etc.) as detrital mineral. May be autogenic in sedimentary rocks.

Never occurs in volcanic rocks!

 

3. Mineral Associations

In magmatic and metamorphic rocks, muscovite is associated with quartz, plagioclase, microcline, biotite, zircon, fluorapatite and many others. In pegmatites, it occurs with quartz, K-feldspars (orthoclase, microcline), albite, beryl (aquamarine), fluorite, garnet (spessartite), tourmaline and scheelite, among others.

In low-grade metamorphic rocks it is associated with albite; with the advancement of the metamorphic grade it is associated with biotite, chloritoid and potassic feldspar.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.552 – 1.576    nβ: 1.582 – 1.615     nγ: 1.587 – 1.618

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless. It rarely presents shades of pale green, pale yellow or pink, with slight pleochroism. The variety fuchsite shows a discrete pleochroism between colorless and bluish-green, better observed in portions of the thin section with a thickness a little above 30 microns.

Relief: Moderate. It is a well-defined relief, far superior to the relief of quartz and feldspars. It is easier to notice closing the diaphragm by more than 50%.

Cleavage: (001) perfect, can be partial or just indicated. Controls the orientation of the fragments: the cleavage is parallel to the elongation of the lamellae.

Habits: Lamellar, reeds, plates, flakes, columnar, tabular. They may be deformed, curved or bent in a zigzag pattern. May be intergrown with biotite.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.037 – 0.041, with 2nd and 3rd order interference colors: strong, intense, colorful colors. Sections parallel to the cleavage show very low 1st order interference colors.

Extinction: Parallel and mottled in the sections perpendicular to the cleavage; it may be oblique from 1-3º.

Mottled (“birds-eye”) extinction is a type of extinction that is not complete, generating a texture with a backlight. This mottled extinction is diagnostic for biotite, muscovite, talc, pyrophyllite and brucite!

Elongation sign: ES(+) by cleavage, like all micas.

Twins: May show simple twins, but they are very rare. 

Zoning: No.

CONVERGENT LIGHT

Character: B(-), difficult to see due to the strong interference colors.

2V angle: 30–47o or 35–50o; the literature indicates different angles.

Alterations: muscovite is a mineral that is quite resistant to weathering and is generally unaltered. Very fine-grained aggregates can alter to hydromuscovite and illite.

There are two special (very rare!) situations of alteration:

(1) Occasionally, muscovite changes to kaolinite, this alteration may generate perfect pseudomorphosis.

(2) Under conditions of more intense metamorphism, muscovite becomes unstable and is replaced by an aggregate of small grains of potassic feldspar.

May be confused with: other micas.

Lepidolite, talc, pyrophyllite, margarite and paragonite are very difficult or impossible to distinguish from muscovite under the petrographic microscope.

Chlorite, when faintly colored, may be similar, but it has lesser birefringence and often shows anomalous interference colors.

Kaolinite has less birefringence.

Sericite can only be identified by X-Ray Diffractometry.

Any colorless mica can be considered a muscovite; many petrologists use the term “white mica” instead of naming species. Distinguishing muscovite from other white micas and brittle micas may be impossible.

 

Muscovite 01: in PPL, mica with its typical habit, with a good to perfect cleavage and moderate relief. Pleochroism generally is absent, the mineral is colorless.
Muscovite 01: in XPL, the same crystal as in the previous figure. Muscovite shows intense colors and a straight and birds-eye extinction (in a position where the cleavage is horizontal or vertical.
Muscovite 02: colorless muscovites, with much higher relief than the quartz grains that also occur in this schist. Image taken with somewhat closed diaphragm, to enhance the relief of muscovite.
Muscovite 02: the same section as of the previous image, now in XPL. Muscovites exhibit intense interference colors; the muscovite lamella in the vertical position is in extinction position (parallel and mottled). Quartz is gray to white.
In XPL, at right in the image, muscovite in extinction position showing “mottled” (“birds-eye”) and parallel extinction. At left, in yellow, a non-extinguished muscovite with its strong interference colors. On far left, quartz and feldspars (gray).
In XPL, muscovites in a metamorphic rock with a “mica-fish” texture, with strong red colors. Around it, quartz (gray-white) and biotite (brownish-yellowish).
Twinned muscovite. The twin plane is the straight line that divides the crystal into two portions; each portion is a muscovite crystal. XPL.
Muscovite in XPL. Unaltered: green. A bit altered to kaolinite: yellow. Completely altered to kaolinite: gray (center of image). Left and right, in shades of gray and white: quartz.
In XPL, muscovite in a mica-schist, forming parallel bands of aligned crystals. In white/gray, quartz. Some biotites and opaque minerals are also there.
In another mica-schist, the aligned muscovite crystals are bend, forming a texture that resembles valleys and ridges. In white/gray, quartz. In black, opaque minerals (magnetite?). XPL.
Muscovite 03: In PPL, fuchsite (a chromiferous variety of muscovite) showing very faint colorless to bluish-green pleochroism. In thicker thin section (>30 microns) pleochroism is very evident. This is the first pleochroism color, the next image shows the other one.
Muscovite 03: the same crystal as of the previous image, now rotated 90 degrees to show the second pleochroism color. PPL.
Plagioclase phenocrystal in volcanic rock in XPL, altered to clay minerals (brown colors) and sericite (small bright spots). Sericite is a very common alteration product in feldspars.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of muscovite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with muscovite.

Sample preparation: the polish of muscovite is not of good quality due to the excellent cleavage and due to the different orientations of the cleavage in relation to the plane of the polished section. Furthermore, the hardness is low and, in addition, hardness varies greatly in the various possible sections of the mineral. So the polish will always be heterogeneous, but generally bad.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, slightly lighter than feldspar and quartz, much lighter than amphibole, pyroxene and biotite.

Pleochroism: No.

Reflectivity: Very low (4%?)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Anisotropy is not visible.

Internal reflections: Generalized from light gray to colorless.

May be confused with: other micas, who are very similar. Furthermore, it can be confused with other minerals with the same physical characteristics: transparent, of low hardness and with good to very good cleavage, with a tabular or lamellar habit.

General Characteristics:

Cleavage is often quite visible.

Polishing pits, despite the cleavage, do not occur or are not conspicuous.

Polishing scratches, due to the low hardness, are almost unavoidable. 

Muscovite 01: Muscovite bundles along the edge of an orthoclase (bottom) and with associated epidote (top), in PPL. The lamellar form of muscovite and its good cleavage is very evident.
Muscovite 01: now in XPL, the same section of the previous image. The correct internal reflections of muscovite are colorless to gray. Here, the green colors are due to the associated epidotes.
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