Combeite is a rare silicate mineral with the formula Na2Ca2Si3O9. It is a trigonal-trapezohedral colorless mineral containing calcium, oxygen, silicon, and sodium. It has a trigonal crystal system. It was first described in 1957 for an occurrence in nephelinite lavas and tephra on Mt. Nyiragongo, Goma, Kivu, the Democratic Republic of Congo (then Zaïre).
General Information
- Category: Silicate mineral
- Formula: Na2Ca2Si3O9
- Crystal system: Trigonal
- Crystal class: Trapezohedral (32) (same H-M symbol)
Properties
- Color: Colorless
- Crystal habit: Stout prisms
- Streak: White
- Diaphaneity: Transparent
- Specific gravity: 2.844
- Optical properties: Uniaxial (+)
Occurrence
In nephelinite (Mt. Shaheru, Congo); in nephelinite and ash ejecta (Oldoinyo Lengai volcano, Tanzania). It has also been reported from the Bellerberg volcano in Ettringen, Germany and the Oldoinyo Lengai volcano, Tanzania. It was named for Arthur Delmar Combe of the Geological Survey of Uganda. It is associated with götzenite at Mt. Shaheru, Congo; and with wollastonite, clinopyroxene, nepheline, melilite, titanian garnet and titanian magnetite at Oldoinyo Lengai.
Association: GÄotzenite (Mt. Shaheru, Congo); wollastonite, clinopyroxene, nepheline, melilite, titanian garnet, titanian magnetite (Oldoinyo Lengai, Tanzania).
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