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
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
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 [1][2][3]

Osumilite 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.[4][5] 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.[3]

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.[6]

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.[2]

Osumilite is found in the Obsidian Cliffs, Oregon; Sardinia, Italy; Kagoshima and Yamanashi Prefecture, Japan; and the Eifel district in Germany.[2] Osumulite 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. ↑ Mindat.org
  2. 1 2 3 http://www.handbookofmineralogy.org/pdfs/osumilite.pdf Handbook of Mineralogy
  3. 1 2 Webmineral.com
  4. ↑ 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))
  5. ↑ E. Olsen, T. E. Bunch (1970): Compositions Of Natural Osumilites, In: The American Mineralogiste, 55, S. 875 - 879 ((PDF, 328 kB))
  6. ↑ http://www.galleries.com/minerals/silicate/osumilit/osumilit.htm Mineral Galleries
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