Feklichevite

Feklichevite is a rare mineral of the eudialyte group with the formula Na11Ca9(Fe3+,Fe2+)2Zr3NbSi(Si3O9)2(Si9O27)2.[3][2] The original formula was extended to show the presence of cyclic silicate groups and presence of silicon at the M4 site, according to the nomenclature of eudialyte group.[4] When compared to other minerals of the group, feklichevite characterizes in the presence of ferric iron (thus similar to ikranite, mogovidite and fengchengite[2]) and dominance of calcium at the N4 site. Calcium is ordered in the structure and is also present at the M1 site.[3][4] Other iron-bearing minerals of the group are eudialyte, ferrokentbrooksite, georgbarsanovite, khomyakovite, labyrinthite, oneillite and rastsvetaevite, but they rather contain ferrous iron[2] Feklichevite name honors Russian mineralogist and crystallographer, V. G. Feklichev.[3]

Feklichevite
General
CategoryCyclosilicate
Formula
(repeating unit)		Na11Ca9(Fe3+,Fe2+)2Zr3Nb[Si25O73](OH,H2O,Cl,O)5 (original form)
IMA symbolFek[1]
Strunz classification9.CO.10
Dana classification64.1.7.
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
H-M symbol: (3m)
Space groupR3m
Unit cella = 14.26, c = 30.17 [Å] (approximated); Z = 3
Identification
ColorDark brown
Crystal habitisometric, thick-tabular; grains
Cleavage(001), perfect
TenacityBrittle
Mohs scale hardness5.5
LusterVitreous
StreakBrownish
DiaphaneityTransparent or Translucent
Density2.87 (approximated)
Optical propertiesUniaxial (+)
Refractive indexnω=1.61, nε=1.61 (approximated)
PleochroismNone
Ultraviolet fluorescenceNo
Common impuritiesMn, F
References[2][3]

Occurrence and association

Feklichevite was found in cancrinite syenite vein of pegmatoid type, that occurs in the Kovdor alkaline massif, Kola Peninsula, Russia. Feklichevite coexists with aegirine-diopside, cancrinite, potassic feldspar, pectolite, titanite, hematite, and руrrhоtitе.[3]

Notes on chemistry

Feklichevite has quite high admixtures of manganese and fluorine, with minor hafnium, strontium, titanium, cerium, and lanthanum.[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, Feklichevite, http://www.mindat.org/min-11029.html
  3. Pekov, I.V., Ekimenkova, I.A., Chukanov, N.V., Rastsvetaeva, R.K., Kononkova, N.N., Pekova, N.A., and Zadov, A.E., 2001. Feklichevite Na11Ca9(Fe3+,Fe2+)2Zr3Nb[Si25O73](OH,H2O,Cl,O)5, a new mineral of the eudialyte group from Kovdor Massif, Kola peninsula. Zapiski Vserossijskogo Mineralogicheskogo Obshchestva 130(3), 55-65
  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


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