Biotite - K(Mg,Fe²⁺)₃[AlSi₃O₁₀(OH,F)₂

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂. Biotite is a solid solution between the end members Phlogopite KMg₃AlSi₃O₁₀(OH)₂ and Annite KFe₃AlSi₃O₁₀(OH)₂, although pure Annite does not occur in nature. In addition, small amounts of Na, Rb, Cs, and Ba may substitute for K, and like in other minerals, F can substitute for OH and increase the stability of Biotite to higher temperatures and pressures.

Biotite was named by J.F.L. Hausmann in 1847 in honour of the French physicist Jean-Baptiste Biot, who, in 1816, researched the optical properties of mica, discovering many unique properties. Biotite is a sheet silicate; iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" (muscovite). Biotite is the most ubiquitous ferromagnesian mineral occurring in most igneous and metamorphic rocks. Nearly pure phlogopite is found in hydrous ultrabasic rocks like kimberlite, and is also found in metamorphosed dolomites. Biotite, with more Fe-rich compositions is common in dacitic, rhyolitic, and trachytic volcanic rocks, granitic plutonic rocks, and a wide variety of metamorphic rocks.

Optical Properties:
• Form: pseudoexagonal basal section (001), rectangular (110) section.
• Relief: Moderate.
• Color: brown to black.
• pleochroism: yellow to brown to green colors.
• Interference colors: third-order colors.
• cleavage: perfect {001} micaceous cleavage.
• extinction: Bird's eye extinction, or bird's eye maple, is a specific type of extinction exhibited by minerals of the mica group under cross polarized light of the petrographic microscope. It gives the mineral a pebbly appearance as it passes into extinction (Fig.1-2).

birdeye2020(1).jpg

Fig.1: Biotite with bird's eye extinction in a granite.

Fig.2: Biotite with bird's eye extinction in a granite.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)
Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. XPL image, 2x (Field of view = 7mm)
Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. PPL image, 2x (Field of view = 7mm)
Biotite crystals in a granite. PPL image, 10x (Field of view = 2mm)	Biotite crystal in a granite. XPL image, 10x (Field of view = 2mm)	Biotite crystals in a granite. PPL image, 10x (Field of view = 2mm)
Biotite crystals in a granite. XPL image, 10x (Field of view = 2mm)	Biotite crystals in a granite. PPL image, 10x (Field of view = 2mm)	Biotite crystals in a granite. XPL image, 10x (Field of view = 2mm)
Biotite crystals in a granite. XPL image, 2x (Field of view = 7mm)	Biotite crystals in a granite. XPL image, 2x (Field of view = 7mm)