Colorless gemmy single biterminated crystal of the very rare mineral sulphate borate mineral heidornite, from one of the only two known localities worldwide. From Kohnstein Quarry, Niedersachswerfen, Nordhausen District, Thuringia, Germany. Ex Dirk Rheinländer collection. Ex. Erich Laskowski collection. — Photo: David Hospital · CC BY-SA 4.0
Heidornite
Heidornite is an IMA-recognized mineral species in the borate class, with the chemical formula Na₂Ca₃B₅O₈(SO₄)₂(OH)₂Cl.
It crystallizes in the monoclinic crystal system (space group C2/c).
Key properties: Mohs hardness 4–5; specific gravity 2.753; [Vitreous.] luster; optically biaxial.
IMA symbol Hdn, grandfathered status — known before the IMA’s founding, named in 1956.
Very rare, in a cavity in a marine evaporite deposit.
It is found associated with Glauberite, anhydrite.
The mineral was named to honor Dr. F. Heidorn, Bentheim, Germany, for his scientific study of the Zechstein deposits.