Intrinsic elastic anisotropy of Westerly granite observed by ultrasound measurements, microstructural investigations and neutron diffraction

Westerly granite (WG) has been accepted as an isotropic homogeneous rock.
Here we return to WG and observe significant elastic anisotropy using
multidirectional ultrasonic sounding on spherical samples at pressures up
to 400 MPa. Thermal treatment of WG leads to formation of microcracks that
reduce elastic wave velocities and increase its elastic anisotropy. The 3D
distribution of P-wave velocities at low pressure is close to orthorhombic
symmetry. Application of hydrostatic pressure closes most of thermally
induced microcracks and decreases elastic anisotropy of WG, but at high
pressure the anisotropy is practically reversed compared to low pressure:
maximum P-wave velocity direction at low pressures is near minimum
velocity direction at high pressure and vice versa. To understand this
effect, microstructures of the rock were investigated by optical and
scanning electron microscopy. Preferred orientations of four major
rock-forming minerals – quartz, orthoclase, plagioclase and biotite – were
measured by time-of-flight neutron diffraction, which confirms significant
crystal alignment. All these data were used to numerically model
anisotropic elastic properties of WG. It is shown that WG possesses weak
intrinsic elastic anisotropy related mainly to the preferred orientation
of feldspars formed during igneous crystallization. Observed microcracks
are mostly related to the cleavage planes of feldspars and biotite, and
thus also demonstrate preferred orientation. Higher preheating
temperatures produce larger quantity of longer microcracks. These
microcracks act against the weak intrinsic elastic anisotropy of WG, and
define the elastic anisotropy at low pressures.