Using the example of quartz (SiO 2 ), we demonstrate the possibility of gaining insight into the mechanism of structural phase transitions by studying the determinant of the dynamical matrix. Analytical expressions for the determinant are given in the framework of simple lattice-dynamical models. The analysis of these expressions enables us to predict the inherent instability of a system of close-to-regular tetrahedra arranged as in the β-quartz structure. The origin of this instability can be traced to the equilibrium condition of the regular tetrahedron. Already small violations of this condition can lead to a change-of-sign of the determinant with all its consequences for crystal stability. Due to the presence of two strongly differing force scales in the quartz system (strong silicon-oxygen bonds on the one hand and comparatively weak oxygen-oxygen as well as inter-tetrahedral interactions on the other), all other influences on the determinant are comparatively weak.