The analysis of the mechanical behavior was carried out using the elastic properties by performing finite distortions of the lattice. The phase is then described by the bulk (B) and the shear (G) moduli obtained by averaging of the elastic constants. Here we used Voigt's method (cf. original and modern works) based on a uniform strain. The calculated sets of elastic constants C ij (i and j corresponding to directions) are given in Table . All C ij values are positive. The elastic constants of qtz C 6 have the largest values, close to diamond , and smaller magnitudes were obtained for qtz BN and qtz SiC. The bulk (B V ) and shear (G V ) moduli (see the last two columns of Table ) were calculated using the equations for the hexagonal system :