Osumilite

Osumilite
Osumilite tablets with mullite from Ochtendung, Eifel, Germany.
General
Category	Cyclosilicate
Formula (repeating unit)	(K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30
Strunz classification	9.CM.05
Dana classification	63.02.01a.06
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6mmm) H-M symbol: (6/m 2/m 2/m)
Space group	P6/mcc
Unit cell	a = 10.15, c = 14.25 [Å]; Z = 2
Identification
Color	Black, dark blue, dark brown, pink, gray
Crystal habit	Crystals tabular to prismatic also anhedral and massive
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

Osumilite 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.

Osumilite chemical formula is (K,Na)(Fe,Mg)₂(Al,Fe)₃(Si,Al)₁₂O₃₀. 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 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.

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.