Biotite

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Biotite aggregate - Ochtendung, Eifel, Germany

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂. It is primarily a dark, black or brown mica, but can also vary in color depending on its chemical composition. Biotite is a significant mineral in igneous rocks, particularly granite and diorite, as well as in metamorphic rocks such as schist and gneiss. It forms within these rocks as a result of the cooling and crystallization of magma or through the metamorphic processes of pre-existing rocks.

Characteristics

Biotite is characterized by its sheet-like structure, which is a hallmark of the mica family. This structure allows for perfect cleavage in one direction, resulting in thin, flexible sheets or flakes that can be split easily. The mineral's color ranges from black and brown to greenish or reddish, depending on the amounts of iron (Fe) and magnesium (Mg) in its composition. Biotite has a vitreous to pearly luster and is translucent to opaque.

Formation and Occurrence

Biotite forms in a variety of geological environments. In igneous rocks, it crystallizes from cooling magma, often in the presence of more felsic minerals such as quartz and feldspar. In metamorphic rocks, biotite forms through the alteration of other minerals under high temperature and pressure conditions. It is less common in sedimentary rocks, but when present, it is usually a detrital mineral, transported and deposited by water or wind.

Uses and Economic Importance

Although biotite is not mined for its own sake, it is an important mineral in the context of geology and petrology for understanding the history and conditions of rock formation. It can also be an indicator of the temperature and pressure conditions under which the host rock formed. In the past, large sheets of biotite were used as windows in some types of high-temperature furnaces because of the mineral's ability to withstand high temperatures.

Environmental and Health Concerns

Biotite, like other micas, can contain trace amounts of harmful substances such as asbestos. However, the health risks associated with exposure to biotite itself are minimal. Problems can arise when the mineral is present in dust form, which can be inhaled. Therefore, caution should be exercised during the mining or processing of rocks containing biotite to minimize the inhalation of dust.

See Also

• Mica
• Phyllosilicate
• Igneous rock
• Metamorphic rock