Recently, a thermodynamically stable phase between a (low) andΒ (high) phases of quartz has teen found. This paper explains the three phases by a statistical theory of Bragg-William’s type by assuming three states of SiO4 tetrahedron along a-axis. Adjusting the interaction parameters one can predict the successive transitions of the first-order (Tc) and the nearly second-order (TQ. Also with an assignment of Tc = 573‡C, one obtains TQ = Tc+ 1.2 degree which fits to the experiment. The difference in energy between Dauphiné twin bonding and the normal bonding is estimated to be 0.525 eV.