Carbokentbrooksite

Carbokentbrooksite is a very rare mineral of the eudialyte group,[2] with formula (Na,□)12(Na,Ce)3Ca6Mn3Zr3NbSiO(Si9O27)2(Si3O9)2(OH)3(CO3).H2O.[3] The original formula was extended to show the presence of cyclic silicate groups and silicon at the M4 site, according to the nomenclature of eudialyte group.[4] Carbokenbrooksite characterizes in being carbonate-rich (the other eudialyte-group species with essential carbonate are zirsilite-(Ce), golyshevite, and mogovidite). It is also sodium rich, being sodium equivalent of zirsilite-(Ce),[2] with which it is intimately associated.[3]

Carbokentbrooksite
General
CategorySilicate mineral, Cyclosilicate
Formula
(repeating unit)		(Na,□)12(Na,Ce)3Ca6Mn3Zr3Nb(Si25O73)(OH)3(CO3)•H2O (original form)
IMA symbolCktb[1]
Strunz classification9.CO.10 (10 ed)
8/E.25-32 (8 ed)
Dana classification64.1.2.3
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
(same H-M symbol)
Space groupR3m
Unit cella = 14.24, c = 30.04 [Å]; Z = 3
Identification
ColorYellow, yellow-orange
Crystal habitrhombohedra (cores of zoned crystals)
CleavageNone
FractureConchoidal
TenacityBrittle
Mohs scale hardness5
LusterVitreous
StreakWhite
DiaphaneityTransparent
Density3.14 (measured)
Optical propertiesUniaxial (-)
Refractive indexnω=1.65, nε=1.64 (approximated)
PleochroismNone
Ultraviolet fluorescenceNo
References[2][3]

Occurrence and association

Carbokentbrooksite and zirsilite-(Ce) are found as replacements of grains and crystals of eudialyte.[2] They occur in pegmatites of Darai-Pioz alkaline massif, Tajikistan - a locality known for many rare minerals.[5] The minerals are associated with aegirine, ekanite, microcline, polylithionite, quartz, stillwellite-(Ce) (silicates), pyrochlore-group mineral, fluorite, calcite, and galena.[3]

Notes on chemistry

Beside the elements given in the formula, carbokentbrooksite contains admixtures of lanthanum, strontium, neodymium, iron, yttrium, titanium, potassium, chlorine, and praseodymium. Carbokentbrooksite and zirsilite-(Ce) are chemically similar.[3]

Notes on structure

Carbokentbrooksite is isostructural with kentbrooksite.[3]

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. Mindat, Carbokentbrooksite, http://www.mindat.org/min-25674.html
  3. Khomyakov, A.P., Dusmatov, V.D., Ferraris, G., Gula, A., Ivaldi, G., and Nechelyustov, G.N., 2003: Zirsilite-(Ce), (Na,[])12(Ce,Na)3Ca6Mn3Zr3Nb(Si25O73)(OH)3(CO3)•H2O, and carbokentbrooksite (Na,[])12(Na,Ce)3Ca6Mn3Zr3Nb(Si25O73)(OH)3(CO3)•H2O - two new eudialyte-group minerals from the Dara-i-Pioz alkaline massif, Tajikistan. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva 132(5), 40–51 (in Russian, with English abstract); in: Jambor, J.I, and Roberts, A.C., 2004: New mineral names. American Mineralogist 89(11-12), 1826-1834
  4. Johnsen, O., Ferraris, G., Gault, R.A., Grice, D.G., Kampf, A.R., and Pekov, I.V., 2003. The nomenclature of eudialyte-group minerals. The Canadian Mineralogist 41, 785-794
  5. "Darai-Pioz Glacier (Dara-i-Pioz; Dara-Pioz), Alai Range (Alayskiy), Tien Shan Mtn, Region of Republican Subordination, Tajikistan - Mindat.org". Mindat.org. Retrieved 2016-03-11.


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