Arsenoclasite
A red to orange-brown manganese arsenate named for its arsenic content and its perfect cleavage.
Intro adapted from Wikipedia (CC BY-SA).
Arsenoclasite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Mn²⁺₅(AsO₄)₂(OH)₄.
It crystallizes in the orthorhombic crystal system (space group P2_12_12_1). It is a member of the Arsenoclasite structural group.
Key properties: Mohs hardness 5–6; specific gravity 4.16; refractive index 1.804; optically biaxial.
IMA symbol Asc, grandfathered status — known before the IMA’s founding, named in 1931.
A rare fissure mineral in a metamorphosed Fe–Mn orebody (Långban, Sweden); in a sedimentary Fe–Mn deposit (Iron Monarch quarry, Australia).
It is found associated with Sarkinite, adelite, allactite, calcite, dolomite, hausmannite (Långban, Sweden); gatehouseite, shigaite, hematite, hausmannite, barite.
The name comes from arsenic and the Greek for breaking, for its excellent cleavage.