Crystals of sodium heptasilicate (Na 8 Si 7 O 18 ) have been grown at 9 GPa, 1000 degrees 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 a = 7.180(1) Aa, alpha = 87.26(1) degrees , space group Rbu671, Z = 1, and D x = 3.009 g/cm 3 . The structure contains isolated six-membered UDUDUD rings of SiO 4 tetrahedra interconnected by SiO 6 octahedra with (super [6]) Si: (super [4]) Si = 1:6, giving a structural formula of Na 8 Si[Si 6 O 18 ]. The mixed (super [4]) Si, (super [6]) Si framework is undersaturated because only one of the non-bridging O atoms in the SiO 4 tetrahedron is shared with the SiO 6 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 Na 2k Na (sub 2(m-k)) Si (sub m-k) [Si (sub n-m+k) O (sub 2n+m) ], with k<m<n and (n-m+k) > or = (3/2)(m-k). The X-ray structure is consistent with microprobe analysis data and revises the earlier characterization of this phase as zeta -Na 2 Si 2 O 5 . The sodium heptasilicate structure does not fully relax with decompression and undergoes incipient displacive transformation (R3m-->R3) with minor positional disorder of Na cations.

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