Formula: K(Mg,Fe)6(Si,Al)8O20(OH)4 . nH2O
Origin of name: From hydrated biotite
Crystal system: Monoclinic
Crystal class: Unknown
Twinning:
Unit cell: a = 5.2, b = 9, c = 24.5, β = 90°
Colour: Blackish, brownish, yellow, pink
Diaphaneity: Semitransparent, translucent
Luster: Dull, submetallic
Habit: Cleavage plates
Hardness: About 2
Specific gravity: 2.49-2.64
Cleavage: 1; {001} perfect
Tenacity: flexible
Streak: White
Synonyms/varieties: Biotitic vermiculite,
hydromica, jefferisite
Comments: A 1:1 interlayering of biotite and vermiculite
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