The tetragonal and monoclinic polytypes of tschernichite (the natural counterpart of zeolite beta) from Mt. Adamson (Antarctica) were isolated and studied by single-crystal X-ray diffraction using a diffractometer equipped with a CCD detector. Diffraction patterns were interpreted with the help of the OD theory, and synthetic precession images allowed us to overcome ambiguities regarding the symmetry assignment. The two crystalline phases isolated are characterized as two MDO polytypes, tetragonal and monoclinic with different crystal sizes (small and large crystals, respectively), morphologies (single tetragonal dipyramids and radiating hemispherical groups, respectively), and chemistries (high and low Si/Al ratio, respectively). Both refined structures showed regular T-O distances (in the range 1.56-1.68 Angstrom), and extraframework ions spreading over a large number of sites with low occupancy. Many independent sites were identified in both polytypes: in the monoclinic type three sites are occupied by Ca, whereas in the tetragonal three are occupied by Ca and one by Mg. The location and coordination of the site with the highest Ca occupancy in the monoclinic polytype were satisfactorily reproduced by potential energy calculations.