| Osumilite | |
|---|---|
 Osumilite tablets withmullite from Ochtendung, Eifel, Germany. | |
| General | |
| Category | Cyclosilicate |
| Formula | (K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30 |
| IMA symbol | Osm[1] |
| 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 |
| Diaphaneity | Translucent |
| 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 | [2][3][4] |
Osumilite is a very rare potassium-sodium-iron-magnesium-aluminiumsilicate mineral. Osumilite is part of themilarite group (also known as the milarite-osumilite group) ofcyclosilicates.
Osumilite chemical formula is(K,Na)(Fe,Mg)2(Al,Fe)3(Si,Al)12O30.[5][6] It is translucent and the typical coloring is either blue, black, brown, or gray. It displays nocleavage and has avitreousluster. Osumilite has a hardness between 5-6 on theMohs hardness scale.[4]
Thehexagonalcrystal structure of osumilite is an unusual molecular make-up. The primary unit is a double ring, with a formula ofSi12O30. Normal cyclosilicate have rings composed of six silicatetetrahedrons;Si6O18. In a double ring structure, two normal rings are linked by sharing six oxygens, one from each tetrahedron in each six membered ring.[7]
Osumilite, was first discovered as grains involcanic rocks near Osumi,Japan. It was confused with a similar mineralcordierite because of their similar coloring. It can be found in high-grademetamorphic rocks,xenoliths and in thegroundmass ofrhyolite anddacite.[3]
Osumilite is found in theObsidian Cliffs,Oregon;Sardinia,Italy; Kagoshima and Yamanashi Prefecture, Japan; and theEifel district inGermany.[3] Osumilite pseudomorphs are known from a number of ultrahigh-temperature rocks, including those of southern Madagascar.
