Effects of crystal orientation on unpolarized Raman spectra of chrysotile and their implication in polymorph identification of serpentines

Polymorphs of serpentines play key roles in various geodynamic processes including deep Earth water cycling, subduction zone deformation and forearc fluid activity so that their accurate identification is essential. In recent years, Raman spectroscopy has become increasingly important in identifying them. Here, we have taken advantage of our newly discovered megacrystals of chrysotile (Ctl), probed the effects of different crystallographic orientations on its Raman peaks, and examined the influences of the resulting Raman spectroscopic variations on the identification of the serpentine polymorphs. We first established a novel geometric method to estimate the Ctl fiber inclination angle θ , which is equal to arcsin( L 2 / L 1 ), with L 1 and L 2 respectively as the major and minor axes of the elliptical Ctl thin section. We secondly collected unpolarized Raman spectra from 14 Ctl single crystals on a petrographic thin section prepared from an extensively serpentinized jianite specimen, with the θ values ranging from ∼1.5° to 80.2°, as determined by our geometric method. We thirdly correlated the Raman features with θ . The results show that most Ctl Raman features systematically vary with the Ctl fiber inclination angle θ . This is the first time that the effects of the Ctl crystallographic orientations on the Ctl Raman features have been studied on Ctl crystals with fiber diameters on a scale of microns rather than nanometers. Furthermore, the implications of our results to the identification of Ctl have been addressed.