Dark brown prismatic crystals of the very rare Mg-Mn mineral fredrikssonite from Langban (Sweden), the type and only one of two known localities worldwide. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Fredrikssonite
Fredrikssonite 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 Pbam). It is a member of the Ludwigite structural group.
Key properties: Mohs hardness 6; specific gravity 3.84; Pale brown to yellow streak; Vitreous to dull luster; optically biaxial.
IMA symbol Fdk, IMA status: approved, named in 1983.
From a metamorphosed Fe–Mn orebody.
It is found associated with Hausmannite, manganoan calcite, brucite, adelite, dolomite–kutnohorite, clinohumite, jacobsite.
The mineral was named to honor Dr. Kurt A. Fredriksson (1926–), Swedish–American geochemist and meteoriticist, Smithsonian Institution, Washington, D.C., USA.