A Raman spectroscopic study of the phase transition in omphacite

Omphacites are very important in petrology because of their occurrence in rocks metamorphosed under high-pressure conditions. They have a wide thermobaric range of stability. Raman spectra of a natural, iron-bearing, ordered omphacite have been studied and compared with the experimentally disordered spectra. The order–disorder phase transition causes extra modes in the spectra. The ordering state affects the full width at half maximum (FWHM) of the main omphacite peak at 680 cm−1. Ordered omphacite shows a split peak around 1010 cm−1 caused by the slightly different orientation of tetrahedra in the monoclinic chain.