Refinement of the crystal structures of low-rare-earth and “typical” burbankites by the Rietveld method

Abstract

The crystal structures of two samples of burbankite from the Khibiny massif, namely, low-rare-earth burbankite (Na1.82Ca1.02Y0.02)2.86(Sr2.32Ba0.43Ca0.17La0.06Ce0.02)3.00(CO3)5 from pectolite metasomatites and burbankite of the characteristic composition (Na2.22Ca0.65Y0.03)2.97 × × (Sr2.10Ba0.33Ce0.23Ca0.15La0.12Nd0.05Pr0.02)3.00(CO3)5 from alkaline hydrothermolites, were refined by the Rietveld method. The experimental data were collected on an ADP-2 diffractometer (λCuK α radiation, Ni-filter; 15.00° < 2θ < 155.00°; 2θ scan with steps 0.02°; the exposure time per step, 15–20 s; the number of (α1 + α2) reflections 556–570). All the calculations were performed using the WYRIET program (version 3.3) in the sp. gr. P63 mc. For low-rare-earth burbankite: a = 10.5263(1) A, c = 6.5392(1) A, R P = 3.52, R wp = 4.49, R B = 4.10, and R F = 4.11. For burbankite from alkaline hydrothermolites: a = 10.5313(1) A, c = 6.4829(1) A, R P = 2.54, R wp = 3.23, R B = 3.06, and R F = 3.44. The structures were refined using the anisotropic thermal parameters of cations.