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
CategorySilicate mineral
Formula
(repeating unit)
Na2Ca2Si3O9
Strunz classification9.CJ.15a
Crystal systemTrigonal
Crystal classTrapezohedral (32)
(same H-M symbol)
Space groupP3121
Unit cella = 10.42 Å, c = 13.14 Å; Z = 6
Identification
ColorColorless
Crystal habitStout prisms
StreakWhite
DiaphaneityTransparent
Specific gravity2.844
Optical propertiesUniaxial (+)
Refractive indexnω = 1.598 nε = 1.598
Birefringenceδ = 0.000
References[1][2][3]

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).[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]

References


This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.