The atomic structure of chkalovite, Na2BeSi2O6, has been determined and refined by Simonov et al. (1976). It is orthorhombic, space group Fdd2, with cell parameters: a = 21.129 (5), b = 6.881 (2), and c = 21.188 (5)/Å. The structure is very similar to that of alpha-cristobalite (Dollase, 1965), comprising a framework of ordered BeO4 and SiO4 tetrahedra with the sodium atoms occupying sites within the cavities of the beryllosilicate framework. Taylor (1972) developed a tilting model for the cristobalite structure assuming regular tetrahedra. In addition to the single tilt system deduced for cristobalite (Taylor, 1972, 1984a) chkalovite has secondary tilt systems involving a slight adjustment of the structure by a small cooperative tilt or twist of some tetrahedra about one or more of their three-fold axes (see fig. 1 of Simonov et al., 1976).
