Measurement of polarizability tensor for muscovite and potential application to microstructural characterization

An accurate quantification of the crystallographic-preferred orientation of weak minerals is the key step to decipher the rock deformation history. Measuring the crystallographic orientations in a thin section provides detailed 2-D information about the microtexture. An established method for this task is the electron backscattered diffraction technique. However, the high-level polishing required by EBSD analysis can produce a bias especially in the modal percentage of fragile minerals such as mica. An alternative method is Raman spectroscopy, which requires minimal amount of sample preparation and surface polishing. This method relies on the fact that the intensity of Raman bands varies depending on the mutual orientation between the polarization direction of the incident laser and the crystallographic axes. Given the polarizability tensor, it is possible to inversely calculate the crystallographic orientation by collecting the band intensity as a function of laser polarization direction. In order to calculate the polarizability tensor of the A and B mode of muscovite, we measured the Raman band intensity as a function of orientation around 360° for a muscovite sample cut parallel to the c-axis. We found that the 196 cm^-1 and the 263 cm^-1 bands are most suitable for orientation measurements. The polarizability tensor can be fitted, which makes it possible to perform mapping of the crystallographic orientation on a thin-section, such as mica schist deformed under deviatoric stress.