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
IMA symbolCbe[1]
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[2][3][4]

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 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]

References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. Mineralienatlas
  3. 1 2 3 Combeite on Mindat.org
  4. 1 2 Combeite data from Webmineral
  5. 1 2 Handbook of Mineralogy


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