Studies on clathrasils. III.*

Abstract

Abstract The high temperature form of melanophlogite, 46 SiO2 · 6M14 · 2M12(M12 = CH4, N2; M14 = CO2, N2), is cubic with space group Pm3n and a = 13.436(3) Å at 200°C. The mineral is isostructural with the cubic gas hydrates of type I. Structure refinement with 667 independent reflections led to a weighted Rw = 0.040. Corner-sharing [SiO4] tetrahedra form a 3-dimensional framework which contains two types of cages: 2 pentagondodecahedra (V∼97 Å3) and 6 tetrakaidecahedra (V∼136 Å3) per unit cell. From difference Fourier syntheses it is concluded that the guest molecules M12 and M14 within the dodecahedral and tetrakaidecahedral cages are oriented in such a way that the van der Waals contacts between the guest molecules and the SiO2 framework are optimized. The mean value of the Si – O – Si angles (168.8°) is considerably higher and that of the Si – O distances (1.576 Å) is considerably lower than corresponding angles and distances in the known silica polymorphs.