The effects of non-hydrostatic stress on the structure and properties of alpha-quartz

The study of the effects of non-hydrostatic stresses on rock-forming minerals is fundamental for understanding how minerals respond to tectonic stresses in the Earth’s interior. Larger deviatoric stresses of the order of GPa can arise from physical interaction between minerals (e.g. host–inclusion systems, compositional gradients) at the micrometric scale, even if at the large-scale large stresses are annealed by plastic flows. Thus, experimental data under hydrostatic pressure do not provide us access to all the possible modifications and structural changes experienced by minerals subjected to deviatoric stresses. Therefore, we carried out ab initio hybrid Hartree–Fock/Density Functional Theory simulations to determine the properties of alpha-quartz, since it is one of the most abundant minerals in the Earth’s crust with a very simple chemistry and structure. We calculated its structure, elastic parameters and Raman-active vibrational modes as a function of different applied strains, which allow us to show how phonon frequencies and structural parameters (bond lengths and angles) are affected by deviatoric stress conditions rather than hydrostatic pressure.