Class : Silicates
    Subclass : Cyclosilicates
    Crystal system : Hexagonal
    Chemistry : K(Fe,Mg)₂(Al,Fe)₃(Si,Al)₁₂O₃₀
    Rarity : Rare

Osumilite is above all a complex silicate of acidic to intermediate volcanic rocks (rhyolites, dacites, etc...) where it enters the composition of the matrix and occupies the last voids, often associated with tridymite. It is also encountered in haloes generated by particularly intense contact metamorphism, and in very metamorphic xenoliths (sanidinite facies) of certain lavas. Its name comes from its locality of discovery : Osumi, the old name of the province of Sakkabira (Japan). Osumilite is usually dark blue to black, sometimes dark brown, exceptionally pink (osumilite-Mg). The crystals are tabular to prismatic and do not exceed one centimeter, osumilite occurs mainly in masses. It is a fragile mineral, unstable in humid environments at low temperatures where it transforms into a mixture of cordierite, mica and quartz.

Main photo : Osumilite from Obsidian Cliffs, Oregon, USA © Douglas Merson

Twins are rare and perpendicular to {0001}.

No fakes listed for this mineral species.