Biotite

Hand Specimen identification: Biotite is distinguished by its association, micaceous nature, and dark color. It may be confused with other micas, especially chlorite. Composition is variable and cannot be determined without detailed ana-lytical work.

Chemical Composition : K(Mg,Fe)3(AlSi 3O10)(OH)2

Hardness : 21/2 to 3

Specific Gravity2.9 to 3.1

Cleavage/Fracture : perfect basal {001}/ragged

Luster/Transparency : vitreous/transparentto opaque

Color : black, greenish black,brown-black

Streak :

Optical Properties : Brown, red, or green in thin section, biotite exhibits strong pleochroism, perfect cleavage, bird’s-eye ex-tinction, and parallel or near-parallel extinction. Biaxial 1- 2, a = 1.57, b = 1.60, g = 1.61, d = 0.04, 2V = 0° to 32°. Plates 5.3, 5.4, 5.7, and 5.9 show biotite in thin section. Stilpnomelane, a rarer related sheet silicate, looks much like biotite but lacks “bird’s eye” extinction.

Crystallography : Monoclinic, pseudohexagonal, a = 5.33, b = 9.31, c = 10.16, b = 99.3°, Z = 2; space group C2/m; point group 2/m.

Habit : Rare, foliated books of pseudohexagonal crystals are distinctive but uncommon (Plate 6.6). More fre-quently, biotite is disseminated as irregular grains or flakes or foliated masses.

Structure and composition : The basic biotite structure is identical to that of phlogopite: Two tetrahedral layers surround an octahedral layer. The three-layer sandwiches are linked by K+ ns occupying large sites between them. Fe mixes freely with Mg in octahedral sites, leading to complete solid solution between the two principal biotite endmembers: annite, KFe 1AlSi O 21OH2 , and phlogo-1 2 13 2 3 10 2ite, KMg3 AlSi3 O10 OH 2. Coupled substitution of Al for both Mg and Si creates limited solid solutions with siderophilite, K1Fe,Al2 1Si,Al2 O 1OH2 . Minor3 41 10 21 2solid solution with muscovite, KAl2 AlSi3O10 OH 2, is also possible: Two Al replace three (Fe,Mg), leaving every third octahedral site vacant. Mn, Ti, and a num-ber of alkalis and alkaline earths may also be present, and F may replace some OH.

Occurrence and associations : Biotite is common in a wide variety of igneous and metamorphic rocks and in immature sediments. Associated minerals include other micas, amphiboles, quartz, and feldspars.

Varieties : Annite, phlogopite, and siderophilite are names given to end member Fe-, Mg-, and Al-biotite.

Related Minerals : Several biotite polymorphs differ in the ways the tetrahedral and octahedral sheets are stacked. Biotite is isostructural or isotypical with other micas, and similar in many ways to other sheet silicates.

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Phlogopite

Hand Specimen identification: Phlogopite, an end member biotite, is identified by cleavage, color, and association. …

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