With the use of ab initio DFT and electron diffraction calculations, the crystal structures of a carbon ‘‘C8 phase’’ are found. Total energies and electron diffraction patterns are calculated for all possible known to date candidates, namely supercubane, tetrahedral, BC-8, and sodalite (SOD) structures as nearest neighbours in unit cell size. The results of calculation agree well with only one phase. The structure can be attributed to sp3-hybridized carbon SOD zeolite with 12 atoms in a unit cell. Carbon sodalite belongs to space group No. 229, with a unit cell of a=4.34 A, and a calculated crystal density of 2.927 g/cm3. The existence of a tetrahedral phase (a=3.86 A) is discussed. A simple formula of relative structural stability is proposed for all sp3-hybridized phases with unit cells of 3.5–5.0 A.