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, the crystalline forms of silica (silicon dio ...

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

cyclosilicate 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, the crystalline forms of silica (silicon dio ...

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 Cleavage may refer to: Science * Cleavage (crystal), the way in which a crystal or mineral tends to split * Cleavage (embryo), the division of cells in an early embryo * Cleavage (geology), foliation of rock perpendicular to stress, a result of ...

and has a vitreous luster. Osumilite has a hardness between 5-6 on the Mohs hardness scale. The

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

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 (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...

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 Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and met ...

near Osumi,

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

. 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, caus ...

, 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 (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

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 rhyolite. ...

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

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

;

Sardinia Sardinia ( ; ; ) is the Mediterranean islands#By area, second-largest island in the Mediterranean Sea, after Sicily, and one of the Regions of Italy, twenty regions of Italy. It is located west of the Italian Peninsula, north of Tunisia an ...

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

; Kagoshima and Yamanashi Prefecture, Japan; and the

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

district in

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

. 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