Osumilite is a very rare potassium-sodium-iron-magnesium-aluminium silicate mineral. Osumilite is part of the milarite group (also known as the milarite-osumilite group) of cyclosilicates.
Characteristics
Osumilite chemical formula is (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30.[4][5] It is translucent and the typical coloring is either blue, black, brown, or gray. It displays no cleavage and has a vitreous luster. Osumilite has a hardness between 5-6 on the Mohs hardness scale.[3]
The hexagonal crystal structure of osumilite is an unusual molecular make-up. The primary unit is a double ring, with a formula of Si12O30. Normal cyclosilicate have rings composed of six silicate tetrahedrons; Si6O18. In a double ring structure, two normal rings are linked by sharing six oxygens, one from each tetrahedron in each six membered ring.[6]
Occurrence
Osumilite, was first discovered as grains in volcanic rocks near Osumi, Japan. It was confused with a similar mineral cordierite because of their similar coloring. It can be found in high-grade metamorphic rocks, xenoliths and in the groundmass of rhyolite and dacite.[2]
Osumilite is found in the Obsidian Cliffs, Oregon; Sardinia, Italy; Kagoshima and Yamanashi Prefecture, Japan; and the Eifel district in Germany.[2] Osumilite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar.
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