Formula: K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2
Origin of name: For Jean Biot, French physicist
Crystal system: Monoclinic or triclinic (-6A)
Crystal class: 2/m
Twinning: On {001} about [310] axis
Unit cell: a = 5.30 Ǻ, b = 9.21 Ǻ, c = 10.16 Ǻ, β = 99.1°
Colour: Black, dark brown, reddish brown, green, rarely white
Diaphaneity:
Luster: Splendent, submetallic, vitreous
Habit: Crystals tabular or short prismatic, pseudohexagonal outline; disseminated, massive aggregates
Hardness: 2.5 to 3
Specific gravity: 2.7 to 3.4
Cleavage: 1; {001} perfect
Tenacity: Thin laminae flexible to brittle
Streak: Colourless
Synonyms/varieties: Iron mica, lepidomelane (Fe3+ -rich variety), manganophyllite (Mn -rich variety)
Comments: Polytypes 1M, 2M1, 4M3, 5M and 6A. Series with phlogopite
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