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

Abstract

The crystal structure of the mineral tveitite-(Y) (Y0.883Na0.106) (Ca0.841LREE0.159)(Ca0.716Na0.204HREE0.080)(Ca0.092Na0.074)F6.952 from amazonite pegmatites of Rov-Gora Mountain (Keivy, Kola Peninsula) is determined using X-ray diffraction (Stoe IPDS diffractometer, λMoKα, graphite monochromator, 2θmax = 63.5°, R = 0.051 for 1542 reflections). The main crystal data are as follows: a = 17.020(2) A, c = 9.679(2) A, V = 2428.2(4) A3, space group \(R\bar 3\), 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.