Orange crystal aggregates of the rare mineral katoptrite from the type locality in Jakobsberg, Sweden (Jakobsberg Mine, Nordmark, Filipstad, Värmland County, Sweden). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Katoptrite
Katoptrite is an IMA-recognized mineral species in the silicate class, with the chemical formula Mn²⁺₁₃Al₄Sb⁵⁺₂O₂₀(SiO₄)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Katoptrite structural group.
Key properties: Mohs hardness 5.5; specific gravity 4.56; Metallic, brilliant luster; optically biaxial.
IMA symbol Ktt, grandfathered status — known before the IMA’s founding, named in 1917.
Found in metamorphosed limestone (Brattfors mine, Sweden).
It is found associated with Magnetite, garnet, manganosite (Brattfors mine, Sweden).
Its name comes from the Greek for mirror, for the perfectly reflecting cleavage surfaces.