ELASTICITY OF PLAGIOCLASE FELDSPARS

Over the last decade the elasticity and compressional behavior of most major crustal and mantle minerals has been measured, thus allowing the link to be made between the properties of mantle rocks and the properties of their consitutent minerals. The glaring exception is the feldspars. Although they comprise some 60% the Earth's crust the only existing and widely-used elasticity data for feldspars comes from the pioneering work of Rhyzova (1964). That data was collected from twinned samples and the data was therefore reduced as if the feldspars had monoclinic symmetry. This paucity of data is because most feldspars are triclinic. A full evaluation of their elasticity therefore requires measurement of 21 independent components of the elastic constant tensor. This is a time-consuming task for most measurement methods, such as Brillouin scattering and ultrasonic interferometry. Thus no complete set of elastic constants for a triclinic mineral has previously been published, as far as we are aware. The development of the Impulse Stimulated Light Scattering technique (Abramson et al., 1999) allows the relatively rapid measurement of surface wave velocities of crystals, independent of symmetry, from which differences between elastic tensor components can be constrained. When these data are combined with compressional moduli from high-pressure X-ray diffraction studies (Angel, 2004) the full elastic tensor of a triclinic crystal can be determined.

We will present the first determinations of the full elastic moduli of plagioclase feldspars. Several individual moduli differ significantly from the pseudo-monoclinic constants from Rhyzova (1964). The data show that plagioclases are as elastically and compressionally anisotropic as micas, which has implications for the development of texture in rocks and for seismic wave anisotropy in the crust. Changes in the moduli with composition are associated with sub-solidus phase transitions on the plagioclase join.

Abramson et al. (1999) Ann. Rev. Phys. Chem. 50:279-313. Angel (2004) Contribs. Min. Pet. 146:506-512. Rhyzova (1964) Akad SSSR Izv Ser Geofiz 7:1049-1051.