Mechanical properties of quartz at the a-b phase transition: Implications for tectonic and seismic anomalies

[1] The anelastic response of single-crystal and polycrystalline quartz and quartz-rich sandstones to applied stress at low frequencies has been investigated by dynamic mechanical analysis and force torsion pendulum measurements. The dynamic modulus was measured on heating and cooling through the a-b phase transition under shear and bending stress applied at low frequency (~1Hz). Single crystal quartz shows highly anisotropic properties in both the real and imaginary part of the modulus. Polycrystalline novaculite quartz displays the a-b phase transition temperatures (Tc) around 8 C higher than seen in single crystal quartz, due to the pinning stress effect of grain boundaries. The properties of the sandstone vary with heating and cooling cycles as the cementation and microfracturing of the grain structure changes at high temperature. The phase transition in quartz is ferrobielastic, and as such energy differences between domains in the transformation microstructure arise under conditions of applied shear stress. Pre-transition softening and an increase in anelastic loss above Tc are attributed to ferrobielastic switching of incipient domain microstructures that arise in the silica microstructure. The influence of temperature on this microstructure is to induce increased switching of domains as a response to thermal stress inherent in the anisotropic thermal expansion of the polycrystalline structure. These results indicate that ferrobielastic switching in quartz is an important mechanism in controlling changes in mechanical properties. As such, we anticipate that it will induce measurable anomalies in seismic signatures of quartz-rich portions of the Earth’s crust in regions of high thermal flux. The transition may also be responsible for observed seismic velocity reductions and tectonic weakening in western North America and in certain parts of Tibetan plateau.