UC Berkeley

With the advent of x-rays in the late 1800s and crystal x-ray diffraction in the early 1900s, the field of crystallography took off and by now, the crystal structures of most minerals have been well-characterized. When subjected to stress, the response of the crystal is dependent upon the material properties. When an aggregate of randomly oriented anisotropic crystals is subjected to a differential stress, crystallographic preferred orientation can develop. Seismic anisotropy detected in areas of high deformation, such as proximal to subducting slabs and along the core-mantle boundary, has been attributed to the development of crystallographic preferred orientation (CPO), a process in which anisotropic crystals rotate into a similar alignment in response to differential stress, creating an anisotropic bulk aggregate. Stress can also distort the crystal lattice by creating defects in the lattice, such as point defects and dislocations, which create areas of localized elastic strain surrounding the defect. The orientation of the macrostress may affect the residual strain in the crystal lattice. And the evolution of this plasticity around a feature, such as a fracture, can provide information regarding fracture mechanics of the material, how the material responds to a free surface, and elucidate healing mechanics. In this dissertation, three experiments studying microstructure in geologic materials using synchrotron x-rays and their implications for Earth’s systems are explored.

In Chapter 1, the texture development of high-pressure hydrous trigonal phase D [MgSi2O4(OH)2], monoclinic phase Egg [AlSiO3(OH)], and orthorhombic δ-AlO(OH) are investigated using radial diamond anvil cell (DAC) x-ray diffraction. Phase D, synthesized from serpentine, developed an (001) maximum of 7.4 m.r.d, indicating basal slip, and aligning the basal planes perpendicular to the principal axis of compression. Phase Egg, synthesized from kaolinite, developed a strong (001) maximum of 10.3 m.r.d., also indicative of basal slip as the primary deformation mechanism. δ-AlO(OH) developed an (010) maximum of 3.0 m.r.d. parallel to the compression direction. For all phases, the sheet-like planes aligned themselves perpendicular to the compression axis. Visoplastic self-consistent polycrystal plasticity modeling confirmed that (001) slip in phase D at 50% strain produces a texture very similar to the texture observed in the deformed aggregate. The hydrous phases investigated here have P-wave anisotropy and shear-wave splitting values comparable or greater than other mantle phases. The stiffness coefficients of deformed bulk aggregate were also calculated.

In Chapter 2, scanning Laue microdiffraction is used to measure the residual strain in thin sections of natural and experimentally deformed quartzites from which the paleostress orientation is inferred. Samples with low levels of plastic deformation, which possess small, uniform diffraction peak shape return principal strain orientations reasonable with the known or expected deformation conditions, such as a vein quartzite from a hydrothermal structure in Hong Kong (HSM), showing the principal axis of compression perpendicular to the long axes of the grains, and rolled Ti, which shows the principal axis of compression parallel to the normal direction and principal axis of extension parallel to the rolling direction. In samples with high levels of plastic deformation, such as quartzite from a hydrothermal explosion structure in Hong Kong (Ko1) or quartzite deformed in a piston apparatus (H2), the principal strain axes of compression and extension show two maxima oriented along the vertical axis inclined at 45◦ to the Z-direction. This strain pattern, seen across four different rock samples, is believed to be an artifact that results from the inability to detect subpixel shifts in the peak position due to the non-Gaussian intensity distribution of the peak shape. However, the source of this exact strain pattern remains a point of investigation.

In Chapter 3, scanning Laue microdiffraction is used to measure the elastic strain around a Mode I crack along (10-14) in a slice of calcite in a time series of three consecutive scans over the course of 44 hours following load removal. An increasingly compressive strain perpendicular to the fracture surface that migrates and accumulates around the crack plane is observed. Correspondingly, an increasingly tensile strain coplanar to the fracture surface is also observed. Peak broadening is recorded in the areas showing high strain. This suggests the movement of plasticity such as defects toward the fracture surface over time, in addition to the increased compressive strain perpendicular to the crack plane with relaxation of the bulk of the crystal, indicative of healing. The orientation of common low-temperature glide planes r = {10-14} <20-2-1> and f = {-1012} <0-22-1> systems, and twinning on e = {-1018} <40-41> in calcite are oriented to intersect with the crack along (10-14) and could facilitate such movement.