Dark brown prismatic crystals of the extremely rare Pb arsenite chloride mineral finnemanite from the TL in Langban (Långban, Filipstad, Värmland County, Sweden). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Finnemanite
Finnemanite is an IMA-recognized mineral species in the halide class, with the chemical formula Pb₅(As³⁺O₃)₃Cl.
It crystallizes in the hexagonal crystal system (space group P6_3/m). It is a member of the Apatite structural group.
Key properties: Mohs hardness 2.5; specific gravity 7.265; Subadamantine luster; optically uniaxial.
IMA symbol Fnn, grandfathered status — known before the IMA’s founding, named in 1923.
Found in crevices in hematite in a metamorphosed Fe–Mn orebody (Långban, Sweden).
It is found associated with Hematite, rouseite, trigonite (Långban, Sweden).
The name refers to K.J. Finneman, of Långban, Sweden, who found the first specimen.