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	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]

Discovery and occurrence

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]