The crystal structure of low melanophlogite

The crystal structure of the low temperature form of natural melanophlogite, 46SiO 2 · 6M 14 · 2M 12 (M 14 = N 2 , CO 2 ; M 12 = CH 4 , N 2 ), was determined using single-crystal X-ray diffraction data at room temperature. The structure is tetragonal with space group P 4 2 / nbc and unit cell a = 26.818(2) and c = 13.365(1) A, which is the (2 ×2 × 1) superstructure of high-temperature cubic melanophlogite and includes four formula units. The structure with 335 variable parameters including anisotropic temperature factors (or atomic displacement factors) was refined to R = 0.0288 for 2706 observed reflections. The main silica framework consists of nearly regular SiO 4 tetrahedra forming large internal voids represented by distorted tetrakaidecahedra and pentagondodecahedra, which accommodate CO 2 or N 2 and CH 4 or N 2 guest molecules, respectively. This low temperature form is a displacive variant of the cubic high-temperature form. The mean bond length is 1.588(4) A for Si-O, and the bond angles for Si-O-Si are distributed over a large range from about 145 to 171° with a mean value of 159.4(3)°. The thermal vibrations of Si are nearly isotropic with amplitudes approximately equal to the average mean square displacement of 0.0119(4) A 2 . The thermal vibrations of the O atoms are highly anisotropic with a wide range of mean square displacements. There is a positive correlation between the Si-O-Si bond angles and the mean-square displacements of the O atoms.