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Lustrous Elongated Biotite Crystal
SILICATES; PHYLLOSILICATES; MICA GROUP · MONOCLINIC

Biotite

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

Hardness
2.5–3
Streak
White
Spec. Gravity
2.8–3.4
System
Monoclinic

About

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.

Crystal Forms & 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.

Other ID Marks

Tendency for small pieces or flakes or peel off.

Striking Features

Flaky habit, crystals, sectility, and mode of occurence.

Environment

Varieties

✓ common  · ✗ uncommon

Uses

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.

Noteworthy Localities

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.

Common Mineral Associations

Distinguishing Similar Minerals

Photos

Crystal forms

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Pseudohexagon