What is Osumilite?
Osumilite is a rare ring silicate of volcanic and high-grade metamorphic rocks, recognised by its unusual hexagonal framework and strong pleochroism.
Mineralogy and crystal structure
Reference formula: KFe2Al5Si10O30. Osumilite is a complex, compositionally variable ring silicate. The COD model represents an iron-bearing member, while magnesium, vacancies and other substitutions vary in natural crystals. Osumilite is classified with silicates and has a hexagonal crystal system. Typical hardness is 5–6 on the Mohs scale and specific gravity is approximately 2.6–2.8.
Colour, lustre and habit
Typical colours include dark blue, black, brown, pink, grey. The usual hexagonal, prismatic, granular habit and vitreous lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Osumilite forms in or is associated with high-temperature volcanic rocks, granulite-facies metamorphism and potassium-rich silica-alumina systems. Important documented localities include Japan, Norway, Italy, Germany, Antarctica and selected granulite terrains worldwide.
A rare ring silicate from extreme rocks
Osumilite is structurally open yet geologically demanding: its double-ring framework forms where temperature, pressure and composition align in unusual volcanic and metamorphic environments.
Collecting and care
Protect from impact and strong heat; document the host rock because isolated grains can be difficult to identify by eye.
