Abstract

The crystal structure of the low temperature form of natural melanophlogite, 46SiO2 · 6M14 · 2M12 (M14 = N2, CO2; M12 = CH4, N2), was determined using single-crystal X-ray diffraction data at room temperature. The structure is tetragonal with space group P42/nbc and unit cell a = 26.818(2) and c = 13.365(1) Å, 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 SiO4 tetrahedra forming large internal voids represented by distorted tetrakaidecahedra and pentagondodecahedra, which accommodate CO2 or N2 and CH4 or N2 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) Å 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) Å2. 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.

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