Crystal structure of tveitite-(Y): Fractionation of rare-earth elements between positions and the variety of defects
Authors: Yakubovich, O.1; Massa, W.; Pekov, I.; Gavrilenko, P.
Source: Crystallography Reports, Volume 52, Number 1, February 2007 , pp. 71-79(9)
Publisher: MAIK Nauka/Interperiodica
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Abstract:
The crystal structure of the mineral tveitite-(Y) (Y0.883Na0.106) (Ca0.841 LREE 0.159)(Ca0.716Na0.204 HREE 0.080)(Ca0.092Na0.074)F6.952 from amazonite pegmatites of Rov-Gora Mountain (Keivy, Kola Peninsula) is determined using X-ray diffraction (Stoe IPDS diffractometer, λMoK α, graphite monochromator, 2θmax = 63.5°, R = 0.051 for 1542 reflections). The main crystal data are as follows: a = 17.020(2) Å, c = 9.679(2) Å, V = 2428.2(4) Å3, space group <EquationSource Format="TEX"><![CDATA[ $$Rbar 3$$ ]]></EquationSource> , Z = 18, and ρcalcd = 4.00 g/cm3. The idealized structural formula of the mineral is represented as (Y,Na)6(Ca,LREE)6(Ca,Na,HREE)6(Ca,Na)F42 (Z = 3). The defect structure of the mineral manifests itself in a mixed occupation of all four independent cation positions and in a randomly disordered distribution of fluorine atoms over the majority of anion positions. It is shown that the crystal structure of tveitite-(Y) fulfills the function of an “Eratosthenes sieve” for yttrium cations and two groups of lanthanide cations, so that these cations are distributed over three different positions.
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