An NMR study of structure and ordering in synthetic K2MgSi5O12, a leucite analogue

Abstract

Phases with the composition K2MgSi5O12, belonging to the leucite structure group were synthesised under dry and hydrothermal conditions and studied using 29Si NMR. The 29Si spectrum for the dry-crystallised material (which is cubic) consists of a single broad line, suggesting a high degree of disorder. The hydrothermally crystallised material (which is probably monoclinic, with a distorted leucite lattice) has a 29Si spectrum which consists of ten lines of equal intensity, two of which have small chemical shift anisotropies and are therefore assigned to Q4(4Si) sites. These data have been interpreted in terms of a structure with 12 distinct tetrahedral sites over which 2 Mg atoms and 10 Si atoms are fully ordered. A 2-dimensional COSY spectrum shows correlations between some Q4(3Si) silicon atoms and two other Q4(3Si) silicon atoms. This fully constrains the topology of the unit cell. Two schemes of Si/Mg ordering over the unit cell can give good fits to the COSY spectrum. Using the tetrahedral (T) site notation defined for natural tetragonal leucite, the first of these arrangements involves Mg and Q4(4Si) silicon atoms each occupying one T1-type site and one T3-type site, and Q4(3Si) silicons occupying the remaining sites, i.e. four T2-type sites, two T1-type sites and two T3-type sites. In the second arrangement, the T2-type sites are occupied by Mg atoms and Q4(4Si) silicon atoms and all the T1-and T3-type sites are occupied by Q4(3Si) atoms.