Plenty of black microcrystals of the rare Mn arsenite mineral armangite in a white contrasting matrix from the type and only known locality worldwide: Långban, Filipstad, Värmland County, Sweden. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Armangite
Armangite is an IMA-recognized mineral species in the carbonate class, with the chemical formula Mn²⁺₂₆[As³⁺₆(OH)₄O₁₄][As³⁺₆O₁₈]₂(CO₃).
It crystallizes in the hexagonal crystal system (space group P-3).
Key properties: Mohs hardness 4; specific gravity 4.43; Brown streak; optically uniaxial.
IMA symbol Amg, grandfathered status — known before the IMA’s founding, named in 1920.
On a museum specimen collected from a metamorphosed Fe–Mn deposit.
It is found associated with Calcite, dolomite, barite, hematite, fluorite, manganarsite, hausmannite.
The name refers to ARsenic and MANGanese in the composition.