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Category Silicate mineral
(repeating unit)
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
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]

Combeite is a rare silicate mineral with the formula Na2Ca2Si3O9. It has a trigonal crystal system.

Discovery and occurrence[edit]

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]