Incommensurate-modulated structure of nosean, a sodalite-group mineral

High-resolution transmission electron microscopy has provided evidence for ordering of [Na₄.SO₄]²⁺ and [Na₄.H₂O]⁴⁺ clusters in nosean, ideally Na₈[Al₆Si₆O₂₄]SO₄.H₂O. Reflections of type hhl, l = 2n + 1, indicate that space group P4̄3n is an average for nosean. Space group P23, a subgroup of P4̄3n, applies to one type of domain that occurs in nosean and allows for a good model structure for nosean. These domains are based on ordering of the above clusters, and the domains are separated by antiphase domain boundaries. Domains that are based on positional modulations of the framework oxygen atoms also occur, some parts of the crystal show varying superstructure fringes that are absent in other parts. The cluster ordering, which is enhanced by the net charge differences between the clusters, accounts for the two well-defined framework oxygen-atom positions in nosean. -from Authors