Thick Osumilite tabulars, together with Mullite, image width: 1.,5 mm - Locality: Wannenköpfe, Ochtendung, Eifel region, Germany — Photo: Fred Kruijen · CC BY-SA 3.0 nl
Osumilite
A very rare potassium-iron-magnesium ring silicate of the milarite-osumilite cyclosilicate group.
Intro adapted from Wikipedia (CC BY-SA).
Osumilite is an IMA-recognized mineral species in the silicate class, with the chemical formula KFe₂(Al₅Si₁₀)O₃₀.
It crystallizes in the hexagonal crystal system (space group P6/mcc). It is a member of the Milarite structural group.
Key properties: Mohs hardness 5.5–6; specific gravity 2.58–2.68; typically blue or brown or gray or black; white streak; Vitreous luster; refractive index 1.548; optically uniaxial.
IMA symbol Osm, grandfathered status — known before the IMA’s founding, named in 1953.
Found in cavities and the groundmass of rhyolite and dacite, and in high-grade contact metamorphic rocks and xenoliths.
It is found associated with Tridymite, cristobalite, quartz, oligoclase, potassic feldspar, fayalite, \hypersthene, " biotite, magnetite, zircon.
The name commemorates Osumi Province, an old province in Sakkabira, on Kyushu, Japan.
Collections: Collector Favorites, Rare & Remarkable.