Combeite

Combeite
General
Category Silicate mineral
Formula
(repeating unit)
Na2Ca2Si3O9
Strunz classification 09.CJ.15a
Crystal system Trigonal
Space group Trigonal trapezohedral
H-M symbol: (3 2)
Space group: P 3221,P 3121
Unit cell a = 10.42 Å, c = 13.14 Å
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]

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

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).[1] It has also been reported from the Bellerberg volcano in Ettringen, Germany and the Oldoinyo Lengai volcano, Tanzania.[1] It was named for Arthur Delmar Combe of the Geological Survey of Uganda.[2][3]

It is associated with götzenite at Mt. Shaheru, Congo; and with wollastonite, clinopyroxene, nepheline, melilite, titanian garnet and titanian magnetite at Oldoinyo Lengai.[3]

References


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