Raslakite

Raslakite
General
Category Silicate mineral, Cyclosilicate
Formula
(repeating unit)		 Na15Ca3Fe3(Na,Zr)3Zr3(Si,Nb)(Si25O73)(OH,H2O)3(Cl,OH) (original form)
Strunz classification 9.CO.10
Crystal system Trigonal
Crystal class Pyramidal (3)
H-M symbol: (3)
Space group R3
Unit cell a = 14.23, c = 30.02 [Å]; Z = 3
Identification
Color Brownish-red
Crystal habit Grains
Cleavage Imperfect
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 5
Luster Vitreous
Streak White
Diaphaneity Transparent
Density 2.95 g/cm3 (measured)
Optical properties Uniaxial
Refractive index nω=1.61, nε=1.61 (approximated)
References [1][2]

Raslakite is a rare mineral of the eudialyte group with the formula Na15Ca3Fe3(Na,Zr)3Zr3(Si,Nb)SiO(Si9O27)2(Si3O9)2(OH,H2O)3(Cl,OH). This formula is based on the original one, and is extended to show the presence of cyclic silicate groups. The additional silicon and oxygen shown in separation from the cyclic groups (in parentheses) are in fact connected with two 9-fold rings.[1] The mineral has lowered symmetry (space group R3, instead of more specific for the group R3m one),[2] similarly to some other eudialyte-group members: aqualite, labyrinthite, oneillite and voronkovite.[1] The specific feature of raslakite is, among others, the presence of sodium and zirconium at the M2 site. Raslakite was named after Raslak Cirques located nearby the type locality.[2]

Occurrence and association

Raslakite was discovered, together with ikranite, in peralkaline pegmatites of the Mt. Karnasurt, Lovozero massif, Kola Peninsula, Russia. Raslakite is associated with aegirine, fluorcaphite, kazakovite, lamprophyllite, microcline, nepheline, and terskite.[2]

Notes on chemistry

Beside main elements given in the formula, raslakite contains some potassium, strontium, manganese, magnesium, cerium, titanium, and aluminium, with minor amounts of lanthanum and hafnium.[2]

Notes on crystal structure

The M(1) site in raslakite is split into two sub-sites, where Fe and Ca are located. The M(2) site is occupied by sodium, manganese (both with coordination number 5), and zirconium (tetrahedral coordination).[2]

References

  1. 1 2 3 Mindat, Raslakite, http://www.mindat.org/min-25687.html
  2. 1 2 3 4 5 6 Chukanov, N.V., Pekov, I.V., Zadov, A.E., Korovushkin, V.V., Ekimenkova, I.A., and Rastsvetaeva, R.K., 2003: Ikranite, (Na,H3O)15(Ca,Mn,REE)6Fe3+2Zr3([],Zr)([],Si)Si24O66(O,OH)6Cl·nH2O and raslakite Na15Ca3Fe3(Na,Zr)3Zr3(Si,Nb)(Si25O73)(OH,H2O)3(Cl,OH)-new eudialyte-group minerals from the Lovozero massif, Kola Peninsula. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva 132(5), 22–33 (in Russian, with English abstract)


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