  General features of the $\alpha-\beta$ transition of quartz are investigated.
Molecular dynamics methods are mainly used, an analytic treatment being
deferred to a work in preparation. A basic preliminary observation is that the
transition involves only a subsystem of four normal modes on which the
remaining ones just act as a reservoir. The dynamics of the relevant subsystem
turns out to be Hamiltonian, being governed by an effective potential that
depends on the specific energy of the total system. The effective potential is
actually calculated through time averages. It describes the transition as a
pitchfork bifurcation, and also explains the phenomenon of the soft mode, since
it exhibits a frequency that vanishes at the transition. The critical exponent
too is estimated.

Statistical Mechanics