The crystal structure of gottardiite, a new natural zeolite

Gottardiite is a new high-silica zeolite found in the Jurassic dolentes of Mt. Adamson (Victoria Land, Antarctica). Its unit-cell content is: Na2.5K0.2Mg3.1Ca4.9Al 18.8Si117.2O272·93 H2O. The mineral is orthorhombic with a = 13.698(2), b = 25.213(3), c = 22.660(2) Å, topological symmetry Fmmm and real symmetry Cmca. In the Fmmm symmetry there are oxygens on centres of symmetry, which cause energetically unfavourable T-O-T angles of 180°; in the Cmca symmetry the sets of T, on which these oxygens lie, disappear. The topology of gottardiite, which has not been found in other natural zeolites, is the same as that of synthetic zeolite NU-87; the latter, however, is characterized by a monoclinic symmetry P21/c (a = 14.32, b = 22.38, c = 25.09 Å, β = 151.5°). The framework of gottardiite can be described by the polyhedral units 5462 and 54. These units developing along [010] give rise to chains which, sharing edges with other analogous chains, form impermeable sheets parallel to (001). Each sheet is bonded to other analogous sheets through 4-rings of tetrahedra between, and parallel to the sheets. A 2-D channel system parallel to (001) is present in gottardiite. Straight 10-ring channels run parallel to [100], whereas 12-ring channels "snake" in the [010] direction. 10-ring and 12-ring channels are connected through 10-ring windows. The ion sites localized in the channels all have weak electron density and large distances (>2.7Å) from framework oxygens. An enrichment in Al in the tetrahedra of 4-rings can be envisaged.