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Osumilite

Cyclosilicate mineral

Field	Value
name	Osumilite
category	Cyclosilicate
image	Osumilite tablets with Mullite - Ochtendung, Eifel, Germany.jpg
imagesize	260px
caption	Osumilite tablets with mullite from Ochtendung, Eifel, Germany.
formula
IMAsymbol	Osm
dana	63.02.01a.06
strunz	9.CM.05
system	Hexagonal
class	Dihexagonal dipyramidal (6mmm)
H-M symbol: (6/m 2/m 2/m)
symmetry	P6/mcc
unit cell	a = 10.15, c = 14.25 [Å]; Z = 2
color	Black, dark blue, dark brown, pink, gray
habit	Crystals tabular to prismatic also anhedral and massive
twinning	Rarely
cleavage	None
fracture	Subconchoidal
mohs	5 - 6
diaphaneity	Translucent
luster	Vitreous
refractive	w=1.545-1.547, e=1.549-1.551
opticalprop	Uniaxial (+) anomalously biaxial
birefringence	0.004
pleochroism	Strong
streak	Blue-gray
gravity	2.62 - 2.64
references

H-M symbol: (6/m 2/m 2/m)

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 . 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 . Normal cyclosilicate have rings composed of six silicate tetrahedrons; . In a double ring structure, two normal rings are linked by sharing six oxygens, one from each tetrahedron in each six membered ring.

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.

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

References

Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
"Osumilite: Mineral information, data and localities.".
Don S. Goldman, George R. Rossman (1978): ''The site distribution of iron and anomalous biaxiality in osumilite'', In: ''American Mineralogist'', 63, S. 490-498 ([http://www.minsocam.org/ammin/AM63/AM55_490.pdf (PDF, 961 kB)]{{dead link. (March 2018)
E. Olsen, T. E. Bunch (1970): ''Compositions Of Natural Osumilites'', In: ''The American Mineralogiste'', 55, S. 875 - 879 ([http://www.minsocam.org/ammin/AM55/AM55_875.pdf (PDF, 328 kB)])
"Osumilite-(Mg) Mineral Data".
http://www.galleries.com/minerals/silicate/osumilit/osumilit.htm ''Mineral Galleries''
http://www.handbookofmineralogy.org/pdfs/osumilite.pdf ''Handbook of Mineralogy''

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magnesium-minerals iron-minerals potassium-minerals aluminium-minerals cyclosilicates hexagonal-minerals minerals-in-space-group-192

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