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
IMA symbol	Osm[1]
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
Diaphaneity	Translucent
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	[2][3][4]

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)₁₂O₃₀.^[5][6] 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.^[4]

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.^[7]

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.^[3]

Osumilite is found in the Obsidian Cliffs, Oregon; Sardinia, Italy; Kagoshima and Yamanashi Prefecture, Japan; and the Eifel district in Germany.^[3] Osumilite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar.