Osumilite
# Osumilite

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 $(K,Na)(Fe,Mg)_2(Al,Fe)_3(Si,Al)_{12}O_{30}.$ 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.

The hexagonal crystal structure of osumilite is an unusual molecular make-up. The primary unit is a double ring, with a formula of $Si_{12}O_{30}.$ Normal cyclosilicate have rings composed of six silicate tetrahedrons; $Si_6O_{18}.$ In a double ring structure, two normal rings are linked by sharing six oxygens, one from each tetrahedron in each six membered ring.

## 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.

Osumilite is found in the Obsidian Cliffs, Oregon; Sardinia, Italy; Kagoshima and Yamanashi Prefecture, Japan; and the Eifel district in Germany. Osumilite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar.

