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Combeite

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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

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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

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  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. ^ a b c Combeite on Mindat.org
  4. ^ a b Combeite data from Webmineral
  5. ^ a b Handbook of Mineralogy