Crystal structure of melanophlogite, a natural clathrate compound of silica**

The high temperature form of melanophlogite, 46 Si02. 6M14 .2M12(M12=CH4, N2; M14=C02, N2), is cubic with space group Pm3n and a = 13.436(3) A 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 [Si04] tetrahedra form a 3dimensional framework which contains two types of cages: 2 pentagondodecahedra (V ~97 A3) and 6 tetrakaidecahedra (V ~ 136 A3) 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 Si02 framework are optimized. The mean value of the Si -0 -Si angles (168.8°) is considerably higher and that of the Si -0 distances (1.576 A) is considerably lower than corresponding angles and distances in the known silica polymorphs.