At the pressure and temperature conditions of the lower crust, quartz undergoes a displacive phase transition from a trigonal ( $$\alpha $$ ) to a hexagonal phase ( $$\beta $$ ). At room pressure, the $$\alpha $$ – $$\beta $$ quartz transition occurs at 574.1 °C and it is associated with large changes in the thermodynamic and elastic properties. For that reason, it is interpreted as the cause of significant seismic velocity co...