Biotite
Monoclinic
K2(Mg,Fe2+)6-4(Fe3+,Al,Ti)0-2[Si6-5Al2-3O20](OH,F)4
Biotite, also known, as ‘black mica’ is a rock-forming femic phyllosilicate that occurs in several igneous and metamorphic rocks. The mineral was named in honor of the French scientist Jean-Baptiste Biot, who devoted his life to study the optical properties of the micas.
Structure and chemistry
Biotites are a group of trioctahedral phyllosilicates (sheet silicate). They are characterized by a pile of ‘sheets’ of (Si,Al)O4 tetrahedrons interconnected as six-sided rings extending infinitely in 2 dimensions, and trioctahedral ‘brucite’ layers, where each anion (O2- or OH–) is surrounded by 3 octahedral (Mg, Fe) cations. Each trioctahedral sheet (O) is sandwiched between 2 tetrahedral layers (T) and this structure repeats indefinitely perpendicular to the sheets. The T-O-T ‘sandwiches’ are separated by large cation sites that contain K+ (interlayer cations).
Biotites constitute a solid-solution between the annite – phlogopite and siderophyllite – eastonite end-members. This solid-solution is controlled by two main substitutions: the ferromagnesian substitution (Fe → Mg) and the so-called Tschermak substitution, i.e. the contemporaneous substitution of Al → (Fe,Mg) in the trioctahedral site balanced by Al → Si in the tetrahedral site. Biotites may also contain vacancies, i.e. empty octahedral sites, and accommodate many other cations like Ti4+, Fe3+, Mn2+, and Li+ in octahedral sites. Ti4+ may also enter the tetrahedral sites, while K+ in the interlayer sites may be substituted by other large ion elements (Na, Ca, Ba, Rb, Cs…). Halogen elements – F– and Cl– in particular – can substitute (OH)– in the structure.
Biotite crystals with pseudo-hexagonal, lamellar habit and well-developed basal cleavage planes on orthoclase. Size: 7.8 x 6.4 x 3.2 cm. Erongo Mountains, Namibia. Ex Charlie Jey Collection. Photo © Robert M. Lavinsky.
Properties
Habit: platy, lamellar with pseudo-hexagonal basal face
Hardness: 2 – 3
Cleavage: {001} perfect (basal cleavage)
Twinning: {001} composition plane – [310] twin axis
Color: black, dark brown, metallic brown, yellow
Luster: vitreous, micaceous (high reflectivity)
Streak: white
Alteration: chlorite
In thin section…
α(α^c low): 1.530-1.625
β(//b): 1.557-1.696
γ(γ^a = 0-9°): 1.558-1.696
2Vα: 0-25°
Color: colorless to yellow, brown, green, reddish brown
Pleochroism: strong, pale to dark brown/yellow/green/reddish brown (α < β = γ)
Birefringence (δ): 0.028-0.080 (high interference colors commonly masked by the brownish color)
Relief: moderate, higher than white mica
Optic sign: –
[Mindat]
[HoM]
Field features
Biotite in the field is easily identifiable due to its distinctive habit, cleavage, and luster. Even small grains of biotite tend to show a lamellar or platy habit, reflecting light very well (micaceous luster). Its color is often dark (brown-black), ranging to yellowish for Mg-rich varieties (phlogopite). Biotite has a perfect basal cleavage, which can be observed as traces on prismatic sections and that determines the strong reflectivity of basal sections. The absence of cleavage traces on the basal planes allow to distinguish biotite from pyroxene and amphibole, when the elongated prismatic habit of the latter two is not evident. Biotite can be distinguished from white mica, based on its much darker color. The hardness of biotite is very low (2.5 – 3.0) and can be split even by hand along its basal cleavage planes.
Biotite in thin section
The lamellar habit of biotite is easily recognizable in thin section: basal sections (001) appear six-sided or pseudohexagonal, prismatic sections as rectangles or very thin sheets. These sections can show the characteristic basal cleavage of biotite. Biotite is strongly colored (brown/green) and show higher refractive indices (hence relief) with respect to other micas. The distinctive strong pleochroism of biotite (from pale brown/yellow to dark brown/green/reddish brown) is visible at PPL on the elongated, prismatic sections. The pleochroic colors are maximum when the long axis of biotite is oriented parallel to the polarizer (normally horizontal or E-W). Tourmaline can show a similar pleochroism but its colors are darker when its long axis is oriented perpendicular to the polarizer/vertical (hence at 90° with respect to biotite). Stilpnomelane show similar pleochroism and habit compared to biotite. Luckily, stilpnomelane shows a less perfect basal cleavage and may show a second, imperfect prismatic cleavage, perpendicular to the basal one. Phlogopite (the Mg-rich biotite) is also pleochroic, but shows yellowish to pale yellow colors. At CPL, the high interference colors of biotite are generally masked by its strong brownish color. Biotite commonly alters to chlorite. Biotite may also show needle-like inclusions of rutile, which forms due to unmixing of TiO2 from biotite and generally follow hexagon-like patters. Biotite also commonly includes zircons, which are surrounded by dark pleochroic haloes that form as biotite is ‘bombarded’ by the particles produced by the radioactive decay of U, Th, and other heavy elements contained in zircon.
CPL
PPL
⇔ slider. Biotite grains (brown, pleochroic at PPL) defining the foliation in high-grade schist. Note the well-developed basal cleavage. The other minerals visible are quartz and alkali feldspar (transparent at PPL, first-order grey at CPL). Calamita Schists (Isle of Elba, Italy). Field of view: 1.2 mm.
CPL
PPL
⇔ slider. Platy biotite grains (brownish at PPL, high interference colors at CPL) associated with altered cordierite (yellowish). Calamita Schists (Isle of Elba, Italy). Field of view: 1.2 mm.
CPL
PPL
⇔ slider. Biotite grain altered to chlorite. Some brownish lamellae of biotite are still visible, surrounded by greenish/grayish chlorite. Posada Valley, Sardinia (Italy). Field of view: 1 mm.
Occurrence
Biotite is a common femic minerals in a wide range of rocks. In igneous rocks, it is typical of granitoids but it can be found also in pegmatites, granodiorites, tonalites, and diorites. Phlogopitic varieties may occur in feldspathoid-bearing rocks and kimberlites. In some cases, biotite has been reported also in mafic rocks like norites. Biotite is less common in volcanic rocks, as it tends to destabilize and being replaced by other minerals at low pressures, but it may be found as resorbed relics.
In metamorphic rocks, biotite is an important K- and (Fe, Mg)-bearing mineral that occurs from low- to high-grade conditions. At upper greenschist-facies, biotite appears in metapelitic/metasedimentary rocks from muscovite-, chlorite-, and K-feldspar-consuming reactions. Once formed, biotite may coexist with other ferromagnesian minerals like cordierite, staurolite, chloritoid, or garnet. In metabasic rocks, biotite may occur at amphibolite-facies, forming from reactions between muscovite and amphibole. At high-grade, biotite destabilizes at upper amphibolite- to granulite-facies where it breaks down to orthopyroxene and K-feldspar, often in the presence of melt. Biotite is not stable at high pressure and, at blueschist-facies, it is replaced by muscovite and chlorite.
Biotite may occur in sedimentary rocks as a detrital mineral.
Bailey, S. W. (2018). 1. Classification and structures of the MICAS. Micas, 1-12.
David, R. W., & Hans, P. E. (1965). Stability of biotite: experiment, theory, and application. American Mineralogist: Journal of Earth and Planetary Materials, 50(9), 1228-1272.
Eggleton, R. A., & Banfield, J. F. (1985). The alteration of granitic biotite to chlorite. American Mineralogist, 70(9-10), 902-910.
Guidotti, C. V. (1984). Micas in metamorphic rocks. Reviews in Mineralogy and Geochemistry, 13(1), 357-467.
Resources
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
Traduzione in corso!
Le pagine in Italiano dovrebbero essere disponibili nuovamente nel giro di qualche mese.
English 
Italiano