Osumilite is a very rare potassium-sodium-iron-magnesium-aluminium

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consi ...

. Osumilite is part of the milarite group (also known as the milarite-osumilite group) of cyclosilicates.

Characteristics

Osumilite chemical formula is .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)
E. Olsen, T. E. Bunch (1970): ''Compositions Of Natural Osumilites'', In: ''The American Mineralogiste'', 55, S. 875 - 879
(PDF, 328 kB)
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 In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...

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

tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...

s; . In a double ring structure, two normal rings are linked by sharing six oxygens, one from each tetrahedron in each six membered ring.http://www.galleries.com/minerals/silicate/osumilit/osumilit.htm ''Mineral Galleries''

Occurrence

Osumilite, was first discovered as grains in volcanic rocks near Osumi, Japan. It was confused with a similar mineral

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-tempera ...

because of their similar coloring. It can be found in high-grade

metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, cau ...

, xenoliths and in the groundmass of

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The min ...

and

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained ( aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhy ...

.http://www.handbookofmineralogy.org/pdfs/osumilite.pdf ''Handbook of Mineralogy'' Osumilite is found in the Obsidian Cliffs,

Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idah ...

;

Sardinia Sardinia ( ; it, Sardegna, label= Italian, Corsican and Tabarchino ; sc, Sardigna , sdc, Sardhigna; french: Sardaigne; sdn, Saldigna; ca, Sardenya, label= Algherese and Catalan) is the second-largest island in the Mediterranean Sea, af ...

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; Kagoshima and Yamanashi Prefecture, Japan; and the

Eifel The Eifel (; lb, Äifel, ) is a low mountain range in western Germany and eastern Belgium. It occupies parts of southwestern North Rhine-Westphalia, northwestern Rhineland-Palatinate and the southern area of the German-speaking Community of ...

district in

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. Osumilite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar.

References

{{reflist Magnesium minerals Iron minerals Potassium minerals Aluminium minerals Cyclosilicates Hexagonal minerals Minerals in space group 192

Characteristics

Occurrence

See also

References