The incommensurately modulated structures of natural natrite at 120 and 293 K from synchrotron X-ray data

Abstract

The incommensurately modulated structure of the mineral natrite has been refined for the first time. Two single-crystal grains, Lv and Kh, from two different occurrences [Mt. Karnasurt, Lovozero massif (Lv), Kola peninsula, Russia, and the pegmatite of Mt. Koashva, Khibiny massif (Kh)], have been investigated at 293 and 120 K using synchrotron X-ray data. The average structures of both minerals are identical and the basic features of the structural modulations are similar to the synthetic γ-Na 2 CO 3 phase previously published. The γ (incommensurate) → δ (lock-in) phase transition reported at low temperature for the synthetic compound was not observed down to 120 K in natural natrite. Crystalchemical aspects, especially about the second coordination sphere for the carbon atoms, are examined to explain the different structural behaviors observed at low temperature. The possible role played by the minor isomorphous substitutions in natural natrite specimens is also discussed.

Thermal behaviour of natrite Na2CO3 in the 303–1013 K thermal range

“…The structural features of natrite have been thoroughly investigated in the recent past [4][5][6]. The more relevant feature of this compound is that at room temperature (RT) it is an incommensurately modulated phase characterized by very strong satellite reflections.…”

“…Such reflections result from a displacive modulation, in other words, from a stationary lattice wave in which each atom of the unit cell has its proper wave form, amplitude and phase, but all having the same modulation vector q ¼ q 1 a*þq 3 c* [7]. At RT, q 1 and q 2 take the values of ca 0.18 and 0.32, respectively [4,[6][7][8]. A continuous dependence of q on T has been reported in the 296-603 K thermal range [8][9][10].…”

Aperiodic mineral structures

“…This group seemed to be a completely rigid unit in the average structure of natrite refined by Zubkova et al (2002). Arakcheeva et al (2010), however, showed that the CÀO distances and the CÀO2ÀO1ÀO1 torsion angle (describing the deviation from planarity of the CO 3 entity) are slightly modulated along t, the additional coordinate in superspace. The reasons for such a modulation were described in detail by Arakcheeva and Chapuis (2005) for synthetic Na 2 CO 3 .…”

“…Karnasurt) and the Khibiny alkaline massif (Mt. Koashva), was studied in detail by Arakcheeva et al (2010) using the superspace approach. The average structure of natrite can be mainly described as graphite-like layers formed by Na3 and CO 3 ions, stacked along the c axis (Fig.…”

“…Modulations of the interatomic distances along the t axis in the structure of natrite from Khibiny (at both RT and 120 K) and Lovozero, compared with the behaviour observed for synthetic Na 2 CO 3 (Arakcheeva and Chapuis, 2005 À last column). In the C-Na vs. t plots, the grey areas mark the regions with 7 Na atoms in the 3.1 Å vicinity of each C (reproduced from Arakcheeva et al (2010) with the permission of the Mineralogical Society of America).…”