Tschortnerite, a copper-bearing zeolite from the Bellberg volcano, Eifel, Germany

Abstract

Crystals of sodium heptasilicate (Na8Si7O18) have been grown at 9 GPa, 1000 0C 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 Dx = 3.009 g/cm3. 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[Si6018]. 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 Na2kNa2(m-k)Sim-k[Sin-m+kO2n+m]. with k < m < 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-Na2Si2O5. The sodium heptasilicate structure doesjiot fiilly_relax with decompression and undergoes incip­ient displacive transformation (R3̅m -> R3̅) with minor positional disorder of Na cations.