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. — Photo: David Hospital · CC BY-SA 4.0
Combeite
A rare trigonal sodium-calcium silicate found in nepheline syenites.
Intro adapted from Wikipedia (CC BY-SA).
Combeite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na₄.₅Ca₃.₅Si₆O₁₇.₅(OH)₀.₅.
It crystallizes in the hexagonal crystal system (space group R-3m). It is a member of the Lovozerite-Zirsinalite–Lovozerite structural group.
Key properties: specific gravity 2.844; refractive index 1.598; optically uniaxial.
IMA symbol Cbe, grandfathered status — known before the IMA’s founding, named in 1957.
Found in nephelinite (Mt. Shaheru, Congo); in nephelinite and ash ejecta (Oldoinyo Lengai volcano, Tanzania).
It is found associated with GÄotzenite (Mt. Shaheru, Congo); wollastonite, clinopyroxene, nepheline, melilite, titanian garnet, titanian magnetite (Oldoinyo Lengai, Tanzania).
The mineral was named to honor Arthur Delmar Combe, Geological Survey of Uganda.