EFFECT OF HIGH-LOW QUARTZ TRANSITION ON COMPRESSIONAL AND SHEAR WAVE VELOCITIES IN ROCKS UNDER HIGH-PRESSURE

The compressional and shear wave velocities in quarzite, granite, and granulite are determined at a fixed confining pressure of 2 kb as a function of temperature up to 720° C. The high-low quartz transition of the constituent quartz minerals is associated with a pronounced decrease in velocity of the compressional waves when approaching the transition and with a significant velocity increase after the transition. In contrast, the effect of the α-β transition on shear wave velocities is small. The drop of VP is explained by the elastic softening of structure of the constituent quartz minerals near the α-β transition and the opening of grain-boundary cracks, caused by the very high volumetric thermal expansion of the quartz relative to the other component minerals. The velocity increase in the β-field may be attributed to an elastic hardening of the quartz structure. Poisson ratios computed from the velocity data are anomalous for a solid: they become negative within the transition regime. The transition temperature, as indicated by the minimum velocities, is higher in the polycristalline rocks than is expected on grounds of single crystal behavior, and the discrepancy is more marked in granite than in quartzite. The shift of the transition temperature to higher values is explained by internal stresses that arise from the anisotropy of the thermal expansion and compressibility of individual grains and the differences in thermal expansion and compressibility between different component minerals. The role of the α-β quartz transition as a possible cause of low-velocity layers is discussed. © 1979 Springer-Verlag.