Combeite is a rare silicate mineral with the formula Na2Ca2Si3O9. It has a trigonal crystal system.
Combeite | |
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General | |
Category | Silicate mineral |
Formula (repeating unit) | Na2Ca2Si3O9 |
IMA symbol | Cbe[1] |
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 | [2][3][4] |
It was first described in 1957 for an occurrence in nephelinite lavas and tephra on Mount Nyiragongo, Goma, Kivu, Democratic Republic of Congo (then Zaïre).[3] It has also been reported from the Bellerberg volcano in Ettringen, Germany and the Oldoinyo Lengai volcano, Tanzania.[3] It was named for Arthur Delmar Combe of the Geological Survey of Uganda.[4][5]
It is associated with götzenite at Mount Shaheru, Congo; and with wollastonite, clinopyroxene, nepheline, melilite, titanian garnet and titanian magnetite at Oldoinyo Lengai.[5]