Elasticity and Anisotropy of Common Crustal Minerals

Full interpretation of crustal seismology in terms of composition and fabric is difficult in the absence of high quality elasticity data for the constituent minerals. Over the last forty years, a limited data set based on 1-bar ultrasonic determinations has served as the principal foundation for such discussions. We have now measured single crystal elastic properties of several dominant crustal minerals in both feldspar and amphibole mineral series. We use impulsively stimulated light scattering to determine body wave or surface wave velocities as a function of propagation direction on crystals having dimensions of tens to hundreds of microns. Full tensor elastic constants for these low symmetry (monoclinic and triclinic) crystals were obtained by inverting the velocity data. We observe greater velocity anisotropy and find constants that are 10 to 25% larger than previously reported constants. Some of the constants that appear biased low in the older data set are associated with strains in common cleavage directions; this underscores the need for a full re-examination of crustal scale elasticity modeled from properties of constituent minerals. In particular, the new data suggest that minerals less abundant than feldspars and amphiboles have a significant role in obtaining an adequate fit to crustal velocity profiles and that large-scale elastic anisotropy of the crust must be addressed in the next generation of crustal velocity models.