Crystal structure of natrite, γ-Na2CO3

A new occurrence of natrite, gamma-Na-2(CO3), was discovered as monomineral lenses (more than 20 cm in size) in pegmatites of Mt. Koashva, Khibiny alkaline massif, Kola peninsula, Russia. Electron microprobe, XRD, IR and optical data for natrite of this new occurrence are given. A single-crystal fragment isolated from polysynthetic twinned material was used for collecting X-ray single-crystal data: lambda(MoKalpha) = 0.71073 Angstrom, C2/m, a = 8.905 (4), b = 5.237(3), c = 6.045 (2) Angstrom, beta = 101.32(3)degrees, 1290 reflections. Satellite reflections on oscillation photographs indicate the presence of modulation in the structure of natrite. The average structure converged to R = 0.106; models with (i) all atomic positions split and (H) anharmonic thermal displacement parameters applied to unsplit atoms converged to R = 0.036 and 0.029 respectively. The structure of natrite contains [001] columns formed by face sharing Na octahedra and further 7-fold coordinated Na polyhedra; the columns are connected by CO3 groups.