Crystal structure of tveitite-(Y): Fractionation of rare-earth elements between positions and the variety of defects

The crystal structure of the mineral tveitite-(Y) (Y_0.883Na_0.106) (Ca_0.841 LREE _0.159)(Ca_0.716Na_0.204 HREE _0.080)(Ca_0.092Na_0.074)F_6.952 from amazonite pegmatites of Rov-Gora Mountain (Keivy, Kola Peninsula) is determined using X-ray diffraction (Stoe IPDS diffractometer, λMoK _α, graphite monochromator, 2θ_max = 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) ų, space group $Rbar 3$ , Z = 18, and ρ_calcd = 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.