Combeite
Combeite is a rare silicate mineral with the formula Na2Ca2Si3O9. It has a trigonal crystal system.
| Combeite | |
|---|---|
 Pale pink crystal aggregates of the very rare silicate mineral combeite from one of the only 4 localities known worldwide: Mount Oldoinyo Lengai, Arusha Region, Tanzania. | |
| General | |
| Category | Silicate mineral |
| Formula (repeating unit) | Na2Ca2Si3O9 |
| Strunz classification | 9.CJ.15a |
| Crystal system | Trigonal |
| Crystal class | Trapezohedral (32) (same H-M symbol) |
| Space group | P3121 |
| Unit cell | a = 10.42 Å, c = 13.14 Å; Z = 6 |
| Identification | |
| Color | Colorless |
| Crystal habit | Stout prisms |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 2.844 |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.598 nε = 1.598 |
| Birefringence | δ = 0.000 |
| References | [1][2][3] |
Discovery and occurrence
It was first described in 1957 for an occurrence in nephelinite lavas and tephra on Mt. Nyiragongo, Goma, Kivu, Democratic Republic of Congo (then Zaïre).[2] It has also been reported from the Bellerberg volcano in Ettringen, Germany and the Oldoinyo Lengai volcano, Tanzania.[2] It was named for Arthur Delmar Combe of the Geological Survey of Uganda.[3][4]
It is associated with götzenite at Mt. Shaheru, Congo; and with wollastonite, clinopyroxene, nepheline, melilite, titanian garnet and titanian magnetite at Oldoinyo Lengai.[4]
References
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