Outstanding prismatic black crystal of the very rare borate mineral orthopinakiolite from the type and only known locality in Langban (Långban, Filipstad, Värmland County, Sweden). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Orthopinakiolite
Orthopinakiolite is an IMA-recognized mineral species in the borate class, with the chemical formula Mg₂Mn³⁺O₂(BO₃).
It crystallizes in the orthorhombic crystal system (space group Pnnm).
Key properties: Mohs hardness 6; specific gravity 3.94–4.03; Metallic luster.
IMA symbol Opki, IMA status: approved, named in 1960.
A very rare species formed in veinlets in granular dolomite in a metamorphosed Fe–Mn orebody.
It is found associated with Hausmannite, manganophyllite, dolomite, calcite.
As an ORTHOrhombic mineral related to pinakiolite.