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Crystals of sodium heptasilicate (Na 8 Si 7 O 18 ) have been grown at 9 GPa, 1000  0 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) Å. α = 87.26(1)°, space group Rbu671, Z= I, 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  [6] Si: [4] Si =1:6, giving a structural formula of Na 8 Si[Si 6 0 18 ]. The mixed  [4] Si, [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 2(m-k) Si m-k [Si n-m+k O 2n+m ]. with k &lt; m &lt; ti and (n - m + k) ≥ (3/2)(m - k). The X-ray structure is consistent with microprobe analysis data and revises the earlier characterization of this phase as (ξ-Na 2 Si 2 O 5 . The sodium heptasilicate structure doesjiot fiilly_relax with decompression and undergoes incip­ient displacive transformation (R3̅m -&gt; R3̅) with minor positional disorder of Na cations.

Sodium heptasilicate: A high-pressure silicate with six-membered rings of tetrahedra interconnected by SiO 6 octahedra: (Na 8 Si[Si 6 O 18 ])