Cation order in omphacitic clinopyroxenes: implications for Raman elastic geothermobarometry

Omphacite is a common mineral found in many geological settings, thus its widespread occurrence makes it a valuable candidate for Raman elastic geothermobarometry. This approach uses the deformation recorded by mineral inclusions to determine the pressure and temperature conditions under which they were entrapped [1]. Raman scattering, highly sensitive to structural deformations, provides crucial insights into crystal structure variations that occur due to heating or compression. While many host-inclusion systems have been studied, clinopyroxene inclusions remain relatively unexplored. Therefore, accurate calibration of Raman-peak positions against hydrostatic pressure is essential for applying Raman elastic geobarometry to omphacites in different mineral hosts. Natural omphacite crystals exhibit cationic ordering associated with crystallization temperature, which affects their elastic properties. To investigate these aspects, in situ high-pressure Raman spectroscopy measurements were performed on natural ordered and experimentally disordered omphacites from Münchberg Massif (Germany). By examining omphacite crystals with varying degrees of order achieved through isothermal annealing experiments, it was observed that increasing cationic disorder leads to peak broadening, while pressure variations influence Raman peak positions. However, the peak position is also affected by the chemical composition. Therefore, Fe3+-rich crystals from Lugros and Camarate (SE Spain), and Voltri massif eclogites (Italy), already characterized by single crystal X-ray diffraction [2], along with synthetic Fe-free omphacites [3], were analyzed, as an initial step towards the chemical calibration of omphacites using Raman spectroscopy. By considering the influence of chemical composition and cationic ordering, we have expanded our understanding of the elastic behavior of omphacite obtained through Raman spectroscopy and its potential use for geobarometric calculations, important for determining the pressure and temperature conditions involved in geological processes where omphacite growths. References: [1] Angel R J et al. (2019) Z Kristallogr Cryst Mater 234(2):129-140 [2] Cámara F (1995) PhD thesis, University of Granada, 504 pp. [3] Pandolfo F et al. (2015) Phys Chem Minerals 42:1-14