Tetra-ferri-annite

Formula: KFe₃FeSi₃O₁₀(OH)₂

Origin of name: Named for the composition and relationship to annite. Renamed to tetra-ferri-annite.

Crystal system: Monoclinic

Crystal class: 2/m

Twinning: 

Unit cell: a = 5.402, b = 9.237, c = 10.306, β = 99.267°

Colour: Brown, brownish black

Diaphaneity: Transparent to opaque

Luster: Vitreous-adamantine

Habit: Scaly, massive, lamellar

Hardness: 2.5-3

Specific gravity: 2.7-3.4

Cleavage: 1, [001] perfect

Tenacity: 

Streak: Light brown

Synonyms/varieties: Ferri-annite   Ferriannite   Ferribiotite   Monrepite

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