Using polarized Raman spectroscopy to study the stress gradient in mineral systems with anomalous birefringence

Polarized Raman spectroscopy was applied to garnet hosts which exhibit anomalous birefringence around inclusions of zircon and quartz to elucidate the spatial distribution of the anisotropic strain fields in the vicinity of the host-inclusion boundary. We show that there is a direct relationship between the stress-induced birefringence and the Raman scattering generated by the fully symmetric phonon modes (the A1g modes in cubic crystals). Our experimental results coupled with selected finite element models show that the ratio between the measured Raman peak intensity collected in cross and parallel polarized scattering geometries of totally symmetric modes represents a useful tool to constrain the radial stress profile in the host around the inclusions. Further, we demonstrate how group-theoretical considerations and tensor analysis of the morphic effect (external-field-induced change of the symmetry) on the phonons and the optical properties of the host can help to derive useful information on the symmetry of the stress field. Finally, we show experimentally that, under the same amount of applied stress, this approach is more sensitive than the commonly used approach of measuring differences in phonon frequencies and provides better opportunities to map the spatial variations of strain. This approach is an alternative technique to study structural phenomena associated with anomalous birefringence in host crystals surrounding stressed inclusions and could be applied to other systems in which similar optical effects are observed.