Sodium heptasilicate: a high-pressure silicate with six-membered rings of tetrahedra interconnected by SiO6 octahedra: (Na8Si[Si6O18])

Abstract

Crystals of sodium heptasilicate (Na8Si7O18) have been grown at 9 GPa, 1000°C using the MA6/8 superpress at Edmonton. The X-ray structure was determined at room pressure (R = 5.8%). Sodium heptasilicate is trigonal with [see original article for formula]. The structure contains isolated six-membered UDUDUD rings of SiO4 tetrahedra interconnected by SiO6 octahedra with (6)Si:(4)Si = 1:6, giving a structural formula of Na8Si[Si6O18]. The mixed (4)Si,(6)Si framework is undersaturated because only one of the non-bridging O atoms in the SiO4 tetrahedron is shared with the SiO6 octahedron. Six Na cations per formula unit (pfu) are in eightfold coordination with O, and two are in a distorted and compressed octahedral coordination. Sodium heptasilicate belongs to the homologous series [see original article for formula]. The X-ray structure is consistent with microprobe analysis data and revises the earlier characterization of this phase as ζ-Na2Si2O5. The sodium heptasilicate structure does not fully relax with decompression and undergoes incipient displacive transformation (R3̄m → R3̄) with minor positional disorder of Na cations.