Discover Mica Minerals

Biotite

The mica minerals are distinguished by their perfect basal cleavage, which means that they are easily split into thin, often transparent, sheets. Two micas, biotite, and muscovite, are so common that they are considered rock-forming minerals. The rest are relatively uncommon, but phlogopite is the most likely of these to be seen in the field. Rock shops overwhelmingly favor the colorful fuchsite and lepidolite mica minerals.

The general formula for the mica minerals is XY_2-3[(Si,Al)₄O₁₀](OH,F)₂, where X = K,Na,Ca and Y = Mg,Fe,Li,Al. Their molecular makeup consists of double sheets of strongly joined silica units (SiO₄) that sandwich between them a sheet of hydroxyl (OH) plus Y cations. The X cations lie between these sandwiches and bind them loosely.

Along with talc, chlorite, serpentine and the clay minerals, the micas are classified as phyllosilicate minerals, "phyllo-" meaning "leaf." Not only do the micas split into sheets, but the sheets are also flexible.

Biotite or black mica, K(Mg,Fe²⁺)₃(Al,Fe³⁺)Si₃O₁₀(OH,F)₂, is rich in iron and magnesium and typically occurs in mafic igneous rocks. 

Biotite is so common that it's considered a rock-forming mineral. It is named in honor of Jean Baptiste Biot, a French physicist who first described the optical effects in the mica minerals. Biotite actually is a range of black micas; depending on their iron content they range from eastonite through siderophyllite to phlogopite. 

Biotite occurs widely throughout many different rock types, adding glitter to schist, "pepper" in salt-and-pepper granite and darkness to sandstones. Biotite has no commercial uses and rarely occurs in collectible crystals. It is useful, though, in potassium-argon dating.

A rare rock occurs that consists entirely of biotite. By the rules of nomenclature it is called biotite, but it also has the fine name glimmerite.

Celadonite

Celadonite, K(Mg,Fe²⁺)(Al,Fe³⁺)(Si₄O₁₀)(OH)₂, is a dark green mica very similar to glauconite in composition and structure, but the two minerals occur in very different settings. 

Celadonite is best known in the geologic setting shown here: filling openings (vesicles) in basaltic lava, whereas glauconite forms in sediments of the shallow sea. It has a bit more iron (Fe) than glauconite, and its molecular structure is better organized, making a difference in x-ray studies. Its streak tends to be a more bluish green than that of glauconite. Mineralogists consider it part of a series with muscovite, the blend between them being called phengite.

Celadonite is well known to artists as a natural pigment, "green earth," that ranges from bluish green to olive. It is found in ancient wall paintings and is produced today from many different localities, each with its particular color. Its name means "sea-green" in French.

Don't confuse celadonite (SELL-a-donite) with caledonite (KAL-a-DOAN-ite), a rare lead-copper carbonate-sulfate that is also blue-green.

Fuchsite

Fuchsite (FOOK-site), K(Cr,Al)₂Si₃AlO₁₀(OH,F)₂, is a chromium-rich variety of muscovite. This specimen is from the Minas Gerais province of Brazil.

Glauconite

Glauconite is a dark green mica with the formula (K,Na)(Fe³⁺,Al,Mg)₂(Si,Al)₄O₁₀(OH)₂. It forms by alteration of other micas in marine sedimentary rocks and is used by organic gardeners as a slow-release potassium fertilizer. It's very similar to celadonite, which develops in different settings.

Lepidolite

Lepidolite (lep-PIDDLE-ite), K(Li,Fe⁺²)Al₃Si₃AlO₁₀(OH,F)₂, is distinguished by its lilac or violet color, which is to its lithium content. 

This lepidolite specimen consists of tiny lepidolite flakes and a quartz matrix whose neutral color does not obscure the characteristic color of the mica. Lepidolite can also be pink, yellow or gray.

One notable occurrence of lepidolite is in greisens, bodies of granite that are altered by fluorine-bearing vapors. That's what this may be, but it came from a rock shop with no data on its origin. Where it occurs in larger lumps in pegmatite bodies, lepidolite is an ore of lithium, especially in combination with the pyroxene mineral spodumene, the other relatively common lithium mineral.

Margarite

Margarite, CaAl₂(Si₂Al₂O₁₀(OH,F)₂, is also called calcium or lime mica. It is pale pink, green or yellow and is not as flexible as other micas.

Muscovite

Muscovite, KAl₂Si₃AlO₁₀(OH,F)₂, is a high-aluminum mica common in felsic rocks and in metamorphic rocks of the pelitic series, derived from clay. 

Muscovite was once commonly used for windows, and the productive Russian mica mines gave muscovite its name (it was once widely known as "Muscovy glass"). Today mica windows are still used in cast-iron stoves, but the greater use of muscovite is as insulators in electrical equipment.

In any low-grade metamorphic rock, a glittery appearance is very often due to a mica mineral, either the white mica muscovite or the black mica biotite.

Phengite (Mariposite)

Phengite is a mica, K(Mg,Al)₂(OH)₂(Si,Al)₄O₁₀, gradational between muscovite and the celadonite. This variety is mariposite.

Phengite is a catchall name used mostly in microscopic studies for a mica mineral that departs from the ideal attributes of muscovite (specifically, a high α, β and γ and a low 2V). The formula allows considerable iron substituting for the Mg and Al (that is, both Fe⁺² and Fe⁺³). For the record, Deer Howie and Zussman give the formula as K(Al,Fe³⁺)Al_1–x(Mg,Fe²⁺)_x[Al_1–xSi_3+xO₁₀](OH)₂.

Mariposite is a green chromium-bearing variety of phengite, first described in 1868 from the Mother Lode country of California, where it is associated with gold-bearing quartz veins and serpentinite precursors. It generally is massive in habit, with a waxy luster and no visible crystals. Mariposite-bearing quartz rock is a popular landscaping stone, itself often called mariposite. The name comes from Mariposa County. Supposedly the rock was once a candidate for the California state rock, but serpentinite prevailed.

Phlogopite

Phlogopite (FLOG-o-pite), KMg₃AlSi₃O₁₀(OH,F)₂, is biotite without the iron, and the two blend into each other in composition and occurrence. 

Phlogopite is favored in magnesium-rich rocks and in metamorphosed limestones. Where biotite is black or dark green, phlogopite is lighter brown or green or coppery. 

Sericite

Sericite is a name for muscovite with extremely small grains. You'll see it everywhere you see people because it's used in makeup.

Sericite is typically found in low-grade metamorphic rocks like slate and phyllite. The term "sericitic alteration" refers to this kind of metamorphism.

Sericite is also an industrial mineral, commonly used in makeup, plastics and other products to add a silky shine. Makeup artists know it as "mica shimmer powder," used in everything from eye shadow to lip gloss. Craftspeople of all sorts rely on it to add a shimmery or pearly gleam to clay and rubberstamping pigments, among many other uses. Candy makers use it in luster dust.

Stilpnomelane

Stilpnomelane is a black, iron-rich mineral of the phyllosilicate family with the formula K(Fe²⁺,Mg,Fe³⁺)₈(Si,Al)₁₂(O,OH)₃₆∙nH₂O. It forms at high pressures and low temperatures in metamorphic rocks. It's flaky crystals are brittle rather than flexible. Its name means "shining black" in scientific Greek.