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Osumilite

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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
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 ; 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

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. 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.

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.

See also

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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