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
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.

See also

• List of minerals

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. "Osumilite: Mineral information, data and localities". www.mindat.org.
3. http://www.handbookofmineralogy.org/pdfs/osumilite.pdf Handbook of Mineralogy
4. "Osumilite-(Mg) Mineral Data". webmineral.com.
5. Don S. Goldman, George R. Rossman (1978): The site distribution of iron and anomalous biaxiality in osumilite, In: American Mineralogist, 63, S. 490-498 ((PDF, 961 kB)^{[permanent dead link‍]})
6. E. Olsen, T. E. Bunch (1970): Compositions Of Natural Osumilites, In: The American Mineralogiste, 55, S. 875 - 879 ((PDF, 328 kB))
7. http://www.galleries.com/minerals/silicate/osumilit/osumilit.htm Mineral Galleries