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Osumilite tablets with Mullite - Ochtendung, Eifel, Germany.jpg
Osumilite tablets with mullite from Ochtendung, Eifel, Germany.
Category Cyclosilicate
(repeating unit)
Strunz classification 09.CM.05
Dana classification 63.02.01a.06
Crystal symmetry Hexagonal 6/m 2/m 2/m
Unit cell a = 10.15 Å, c = 14.25 Å; Z = 2
Color Black, dark blue, dark brown, pink, gray
Crystal habit Crystals tabular to prismatic also anhedral and massive
Crystal system Hexagonal; 6/m 2/m 2/m.
Twinning Rarely
Cleavage None
Fracture Subconchoidal
Mohs scale hardness 5 - 6
Luster Vitreous
Streak Blue-gray
Specific gravity 2.62 - 2.64
Optical properties Uniaxial (+) anomalously biaxial
Refractive index w=1.545-1.547, e=1.549-1.551
Birefringence 0.004
Pleochroism Strong
References [1][2][3]

Osumilite a very rare hydrate potassium-sodium-iron-magnesium-aluminium silicate mineral. Osumilite is part of the milarite group (also known as the milarite-osumilite group) of cyclosilicates.


Osumilite chemical formula is (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30·H2O. 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.[4]


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] Osumulite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar

See also[edit]