Raslakite

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]

Raslakite
Red glassy raslakite in nepheline-syenite from the type locality: Karnasurt Mountain, Lovozero Massif, Northern Region, Russian Federation.
General
CategorySilicate mineral, Cyclosilicate
Formula
(repeating unit)		Na15Ca3Fe3(Na,Zr)3Zr3(Si,Nb)(Si25O73)(OH,H2O)3(Cl,OH) (original form)
Strunz classification9.CO.10
Crystal systemTrigonal
Crystal classPyramidal (3)
H-M symbol: (3)
Space groupR3
Unit cella = 14.23, c = 30.02 [Å]; Z = 3
Identification
ColorBrownish-red
Crystal habitGrains
CleavageImperfect
FractureConchoidal
TenacityBrittle
Mohs scale hardness5
LusterVitreous
StreakWhite
DiaphaneityTransparent
Specific gravity2.95 (measured)
Optical propertiesUniaxial
Refractive indexnω=1.61, nε=1.61 (approximated)
References[1][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. Mindat, Raslakite, http://www.mindat.org/min-25687.html
  2. 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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