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The biotite Mineral Group
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Biotite is a very common form of mica. It is named in honor Jean Baptiste Biot (1774 - 1862), a French physicist, mathematician, and astronomer who researched the mica minerals for their optical properties. Because of Biotite's
abundance, its presence is usually lacking in collections except for it
being an accessory mineral to other minerals. Biotite can come in
enormous crystal sheets that can weigh several hundred pounds. Thin
sheets can be peeled off as layers, and the thinner a layer is peeled
the greater its transparency becomes. In 1998, the IMA removed the status of Biotite as an individual mineral species, and instead declared it as a group name for the following individual members: Phlogopite, Annite, Siderophyllite, and Eastonite. However, mineral collectors still refer to Biotite by its traditional name and rarely make a distinction among its members except for Phlogopite. Biotite is very hard to clean because if washed
it will absorb water internally and start to break apart. The best way
to wash Biotite and other Micas is with a dry electric toothbrush. |
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Chemical Formula |
The classic formula for Biotite is:
K(Mg,Fe2+3)(Al,Fe3+)Si3O10(OH,F)2
The group formula including all member minerals:
K(Mg,Fe2+)3(Al,Fe3+)[(Al,Si)3O10](OH,F)2
Individual members are:
Phlogopite: KMg3AlSi3O10)(F,OH)2
Siderophyllite: KFe2+2Al(AlSi2O10)(OH)2
Eastonite: KMg2Al(AlSi2O10)(OH)2
Annite: KFe2+3AlSi3O10(OH)2
Fluorannite: KFe2+3AlSi3O10)F2
Tetraferriannite: K(Fe2+3Mg)(Fe3+,Al)Si3O10)(OH)2
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Composition |
Basic fluoro potassium, magnesium, iron aluminum silicate
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Color |
Black, dark brown, dark green, reddish black. Individual group member minerals such as Phlogopite and Eastonite can be in lighter colors. |
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Streak |
White |
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Hardness |
2.5 - 3 |
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Crystal System |
Monoclinic |
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Crystal Forms
and Aggregates |
Crystals are in thick flakes, micaceous masses and groupings, and in
tabular, foliated, flaky, and scaly forms. Crystals may
also be elongated with one dimension flat, or stubby triangular or hexagonally shaped crystals. Also forms in prismatic barrel-shaped or tapered pyramid-shaped crystals composed of dense parallel plates, and as rounded nodules of dense crystals.
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Transparency |
Translucent to opaque. Thin flakes will always be translucent if held up to the light. |
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Specific Gravity |
2.8 - 3.4 |
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Luster |
Pearly |
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Cleavage |
1,1 |
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Fracture |
Uneven |
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Tenacity |
Sectile, elastic |
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Other ID Marks |
Tendency for small pieces or flakes or peel off. |
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In Group |
Silicates; Phyllosilicates; Mica Group |
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Striking Features |
Flaky habit, crystals, sectility, and mode of occurence. |
| Environment |
Biotite is a common rock-forming mineral, and is especially noted in metamorphic rocks such as schist and gneiss. It is also found in igneous rock such as granites and rhyolites. Biotite is also the primary mica in rare earth pegmatites.
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Igneous, Metamorphic |
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Popularity (1-4)
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2 |
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Prevalence (1-3)
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1 |
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Demand (1-3) |
2 |
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- Manganese-rich variety of Biotite.
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Biotite, like Muscovite, is a very poor conductor of heat and electricity, and is thus
used as an insulator for various electrical products and semiconductors.
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Biotite is found in abundance worldwide, but collectors specimens are not common and come from few localities. Noteworthy localities include Mabubas, Bengo Province, Angola; Mendig, Eifel Mts, Germany; and Serra da Freita, Arouca, Portugal, where it comes in rounded nodules.
In Canada huge "books" and well formed "barrels" come from Bancroft, Hastings Co, Ontario and from Cardiff and Wilberforce, Haliburton Co., Ontario; and smaller crystals from Mont Saint Hilaire, Quebec. In the U.S., important localities include the Strickland Quarry, Portland, Middlesex Co., Connecticut; Mt. Apatite, Auburn, Androscoggin Co., Maine; and the old mines of the Hudson Highland and Ramapo Mountains of Rockland and Orange County, New York. The manganese-rich variety Manganophyllite is common at Franklin, Sussex Co., New Jersey.
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Muscovite, Phlogopite - Usually lighter in color, otherwise
very difficult to distinguish.
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Have a question about Biotite? Visit our Q&A Community and ask the experts!
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