Rhönite (Largest Crystal Size: 0.16 mm) Locality: Nickenicher Sattel (Eicher Sattel), Nickenich, Andernach, Eifel, Rhineland-Palatinate, Germany There are lots of other crystals on the specimen, some of which can be seen in the upper left corner, but they are either not as well formed or not as well exposed. — Photo: Modris Baum · Public domain
Rhönite
Rhönite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₄[Mg₈Fe³⁺₂Ti₂]O₄[Si₆Al₆O₃₆].
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Sapphirine structural group.
Key properties: Mohs hardness 5–6; specific gravity 3.4–3.76; Semimetallic luster; optically biaxial.
IMA symbol Rhö, IMA status: approved, named in 1907.
Found in silica-undersaturated, alkalic, mafic to intermediate igneous rocks, as a primary mineral or an alteration product of amphiboles. Also developed at the contact of alkalic basalt and limestone.
It is found associated with Titanian augite, kaersutite, alkalic feldspar, diopside, forsterite, spinel, perovskite, titanian magnetite, magnesioferrite.
The name refers to the RhÄon district, Germany, from where it was first identified.