Armangite

Formula:  Mn₂₆As₁₈O₅₀(OH)₄(CO₃)

Origin of name:  From arsenic and manganese in the composition

Crystal system:  Hexagonal

Crystal class:  -3

Twinning: on {11-20}

Unit cell: a = 13.491 Å, c = 8.855 Å

Colour:  Black to pale brown

Diaphaneity: Opaque, transparent in splinters

Luster:

Habit:  Crystals short prismatic; lamellar

Hardness:  about 4

Specific gravity:  4.43

Cleavage: 1; {0001}fair to poor

Tenacity:

Streak:  Brown

Synonyms/varieties:

Comments: Found on a museum specimen collected from a metamorphosed Fe-Mn deposit (Långban, Sweden)