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Biotite

A dark-brown or dark-green to black mica, K(Mg,Fe)₃(Al,Fe)Si₃O₁₀(OH)₂, found in igneous and metamorphic rocks. A ferro-magnesian mineral. Black, shiny, basal cleavage (one cleavage resulting in thin sheets). In this specimen we see face up the basal cleavage face, very flat, very smooth. Up on end view the crystal will look like a stack of paper with all the sheets laying together.
In felsic and intermediate igneous rocks biotite may be confused with amphibole but biotite has flat, slick, shiny faces not broken by the numerous cleavages typical of amphibole.

Biotites are found commonly in igneous and metamorphic rocks. They are the common ferromagnesian phase in most granitic rocks, and are also found in some siliceous and intermediate volcanic rocks. In basaltic rocks biotite sometimes occurs in the crystalline groundmass, and is a common late interstitial phase in gabbroic rocks. It has been recognized in samples of the Earth's mantle found as inclusions in volcanic rocks. Biotites are not stable at the surface of the Earth, as they decompose by both hydrolysis and oxidation when exposed to the Earth's atmosphere. They alter to vermiculite, chlorite, and iron oxides, and thus are uncommon in sedimentary rocks.

Biotites are important constituents of metamorphic rocks such as schist and gneiss, and the first appearance of biotite is an important marker in metamorphism. Biotite persists to very high grades of metamorphism, where it reacts with quartz to form granulites made up of potassium feldspar and orthopyroxene, garnet, or cordierite, in addition to quartz and plagioclase. Prismatic or tabular (short prismatic) crystals. Crystals may appear rhombohedral and are called pseudorhombohedral. Normally biotite occurs as foliated masses or as scaly aggregates.

Under conditions of ultrametamorphism, biotite reacts with quartz, plagioclase, and alkali feldspar to form siliceous melts. Biotite is also a common gangue mineral in ore deposits. The mineral has been used as an indicator of H₂O, HF, O₂, and S₂ activities in both rock- and ore-forming processes.

Material Notes:

One cleavage direction. Weathering end product(s): Loss of Fe, Mg. Occurrence: Igneous, sediments, metamorphic. Rusty discoloration by loss of Fe. It is common in a wide range of igneous and metamorphic rocks and may be an important detrital mineral in sediments. Biotite is a widespread mineral found in igneous rocks, especially granite and syenite. It is also found in large sheets in pegmatites and in felsic lavas as phenocrysts. Biotite mica is dark green to black in color because it contains iron and magnesium.

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