Enter our Virtual Gallery of Lepidolite Mica Mineral Specimens

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Mica: Mineral Information Page

Including the Mica varieties: Biotite, Fuchsite, Lepidolite, Muscovite and Zinnwaldite

Links To Mica Information Topics On This Page:
Mica Varieties And Items in our VIRTUAL GALLERY
Physical Properties
Background Info
Occurance and Diagnostic Features
History and Uses
Metaphysical Properties

MICA PHYSICAL PROPERTIES

• Chemical composition of some of the mica varieties:
  • Biotite: K₂(Mg,Fe)_6-5Al_0-1(Si_6-5Al_2-3)O₂₀(OH,F)₄
  • Fuchsite: K₂(Al,Cr)₄Si₆Al₂O₂₀(OH,F)₄
  • Lepidolite: K₂(Li_4-3Al_2-3)Si_8-6Al_0-2O₂₀(OH,F)₄
  • Muscovite: K₂Al₄Si₆Al₂O₂₀(OH,F)₄
  • Zinnwaldite: K₂Fe_1-2Li_2-3Al₂Si_6-7Al_2-1O₂₀(OH,F)₄
• Class: phyllosilicates
• Crystal system: monoclinic; 2m
• Crystal habit: platy, micaceous, also compact massive. Foliated sheets can be (rarely) aggregated into globular or botryoidal form, also cryptocrystalline. Distinct crystals are uncommon and are typically tabular with a book-like form with basal planes, showing a hexagonal or diamond-shaped outline. Prism faces are striated horizontally and frequently taper.
• Twinning: occaisional twinning on {110}, a twin axis in the {001} composition plane forms the complex mica twin.
• Specific gravity: 2.2 - 3.3 (Biotite 2.7-3.3, lepidolite 2.2-3.3, muscovite 2.77-2.88, zinnwaldite 3.0)
• Index of refraction: 1.53-1.70 (Biotite 1.56-1.70, lepidolite 1.52-1.59, muscovite 1.55-1.62, zinnwaldite 1.53-1.59)
• Birefringence: .018-.070 (Biotite .04-.07, lepidolite .018-.038, muscovite .036-.049, zinnwaldite .035)
• Pleochroism: Biotite is strongly pleochroic in shades of brown or green, fuchsite in shades of green, lepidolite pale violet or green, muscoviteis colorless, zinnwaldite in pale yellow and brown to reddish-brown
• Hardness: 2.0 -4.0, hardness is greater perpendicular to the cleavage (Biotite 2..5-3.0, Lepidolite 2.5-4.0, muscovite 2.0-2.5, zinnwaldite 3.0)
• Color: Biotite is dark green, brown, or black (sometimes yellow), fuchsite is an emerald green, lepidolite is pink to lilac to light gray, muscovite is colorless to light shades of yellow, brown, red or green, zinnwaldite is yellow, brown or gray.
• Luster: vitreous, pearly, metallic
• Transparency: translucent, transparent in thin sheets
• Cleavage: perfect basal cleavage
• Streak: white

MICA BACKGROUND INFORMATION

The micas are complex hydrous potassium-aluminum silicate minerals. There are more than 20 chemically distinct mica species with considerable variance in geologic occurance, but all have essentially the same crystal structure. The micas crystallize with a sheet structure, the sheets being held together by relatively weak bonds resulting in the perfect basal cleavage of the micas.

Biotite is the most common of the micas, containing iron and/or magnesium impurities substituting for octahedral aluminum. Muscovite is the pure potassium mica, containing no impurities. Fuchsite mica is a chromium rich variety of muscovite where chromium cations substitute for some of the octahedral aluminum in the muscovite crystal structure. The chromium impurities in fuchsite are responsible for its emerald green color. Lepidolite mica is a lithian mica where an occaisional lithium cation substitutes for some of the octahedral and tetrahedral aluminum in the mica crystal structure. Zinnwaldite is a lithian ferrous mica, where lithium and iron cations substitute for some of the octahedral and tetrahedral aluminum.

OCCURRENCE AND DIAGNOSTIC FEATURES

The micas have a very wide range of geological occurances. The many varieties of mica are distinguished by their chemical composition, some varieties occuring only in very specific geological settings while others have more widespread occurances.

Biotite mica is the most common of the micas, in fact it is the most common ferromagnesian mineral! It occurs in most igneous and metamorphic rocks and is commonly found in detrital sediments.

Muscovite mica also is a very widespread and common rock forming mineral. In igneous rocks it is characteristic of granites and granite pegmatites, and in metamorphic rocks it is very common especially in some mica schists where it is the most predominant mineral. In some mica schists and quartzites, muscovite crystallizes with chromium impurities resulting in the emerald green colored variety fuchsite. When microscopic flakes of fuchsite crystallize inside of quartzite giving it a green color, the rock is known as green aventurine quartz which is a popular lapidary material. Muscovite also commonly occurs as sericite, a fine-grained alteration product of other minerals.

Lepidolite mica occurs mainly in granitic pegmatites associated with quartz, micrcoline, albite, amblygonite and occaisionally the gem varieties of tourmaline and spodumene.
Zinnwaldite mica occurs mainly in granite pegmatites and high temperature hydrothermal veins.

The micas are usually recognized by their perfect basal cleavage. Some mica varieties can be distinguished with reasonable certainty by their color.

HISTORICAL INFORMATION AND USES

The name mica was derived from the Latin micare which meant to shine. Biotite was named after the French physicist J. B. Biot. The name Muscovite was derived from the term muscovy-glass which it was previously known by because of its widespread use as a glass substitute in the old Russian state of Muscovy in the 14th century.

Mica (muscovite in particular) has a wide variety of uses as an insulating material in electrical equipment due to its strongly dielectric properties and to its good electrical resistivity and capacitance stability. Mica also has a low coefficient of expansion, and it has been used extensively because of its heat resisting properties. Mica is used extensively in paints to increase weatherability and to reduce running, it is also used in the manufacture of wallpapers to give them a silky or shiny luster. Mica is also used in the manufacture of lubricants and in dry-powder fire extinguishers.

The astrological sign of biotite is Scorpio, and the sign of muscovite is Aquarius.

METAPHYSICAL PROPERTIES

Biotite is said to enhance one's perception, allowing one to more clearly understand what is really happening in any situation.
Fuchsite is said to enhance the body's ability to heal itself, and to increase the effectiveness of other minerals that are being used in healing.
Lepidolite is said to be very soothing and to have a calming effect, relieving anxiety and tension.
Lepidolite with rubellite tourmaline is said to bring happiness, joy and increased vitality, and to inspire love.
Muscovite is said to increase one's understanding and perception, and to enhance one's reflective abilities.

For more in-depth metaphysical information, see our Metaphysical Books section.

This is the end of our Mica Factsheet and Information page.