Osumilite

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.

Osumilite
Osumilite tablets with mullite from Ochtendung, Eifel, Germany.
General
CategoryCyclosilicate
Formula
(repeating unit)		(K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30
IMA symbolOsm[1]
Strunz classification9.CM.05
Dana classification63.02.01a.06
Crystal systemHexagonal
Crystal classDihexagonal dipyramidal (6mmm)
H-M symbol: (6/m 2/m 2/m)
Space groupP6/mcc
Unit cella = 10.15, c = 14.25 [Å]; Z = 2
Identification
ColorBlack, dark blue, dark brown, pink, gray
Crystal habitCrystals tabular to prismatic also anhedral and massive
TwinningRarely
CleavageNone
FractureSubconchoidal
Mohs scale hardness5 - 6
LusterVitreous
StreakBlue-gray
DiaphaneityTranslucent
Specific gravity2.62 - 2.64
Optical propertiesUniaxial (+) anomalously biaxial
Refractive indexw=1.545-1.547, e=1.549-1.551
Birefringence0.004
PleochroismStrong
References[2][3][4]

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
Wikimedia Commons has media related to Osumilite.

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))
  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
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