Outstanding black prismatic crystals of the very rare borate mineral takeuchiite from the TL and only known locality worldwide (Långban, Filipstad, Värmland County, Sweden). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Takéuchiite
Takéuchiite 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 3.93; Brown streak; Metallic luster.
IMA symbol Tké, IMA status: approved, named in 1980.
Very rare in museum specimens from a metamorphosed Fe–Mn orebody.
It is found associated with Dolomite, calcite.
The mineral was named to honor Professor Yoshio Takéuchi (1924–), University of Tokyo, Tokyo, Japan, who predicted the existence of the species and its crystal structure.