Osumilite tablets with mullite from Ochtendung, Eifel, Germany.
|Crystal symmetry||Hexagonal 6/m 2/m 2/m|
|Unit cell||a = 10.15 Å, c = 14.25 Å; Z = 2|
|Color||Black, dark blue, dark brown, pink, gray|
|Crystal habit||Crystals tabular to prismatic also anhedral and massive|
|Crystal system||Hexagonal; 6/m 2/m 2/m.|
|Mohs scale hardness||5 - 6|
|Specific gravity||2.62 - 2.64|
|Optical properties||Uniaxial (+) anomalously biaxial|
|Refractive index||w=1.545-1.547, e=1.549-1.551|
Osumilite chemical formula is (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30·H2O. 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.
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.
|Wikimedia Commons has media related to Osumilite.|
- http://www.handbookofmineralogy.org/pdfs/osumilite.pdf Handbook of Mineralogy
- http://www.galleries.com/minerals/silicate/osumilit/osumilit.htm Mineral Galleries