Omphacite’s Hidden Clues: A Step Forward in Raman Geobarometry

Omphacite, a mineral commonly trapped inside garnet, could be a powerful tool for Raman elastic geobarometry—a technique that helps scientists determine the pressures ancient rocks experienced deep within the Earth. But to make this method reliable, we need to understand how cation order affects phonon compressibility, or how the vibrational properties of omphacite respond to pressure.

In this study, Lisa Baratelli et al analyzed high-pressure Raman spectra of six omphacite crystals with the same composition (~Jd52Di48) but different cation arrangements. Their results were compared with previous data on ordered and disordered omphacite (~Jd43Di57).

The Raman peak near 688 cm⁻¹, linked to Si-O-Si bond bending, is highly sensitive to pressure but remains unaffected by cation order or composition—making it a reliable pressure marker in omphacite-garnet systems.

However, to get accurate results, the omphacite grain must be exposed to measure the peak position at atmospheric pressure, as composition can still influence the wavenumber.

The peak width near 688 cm⁻¹ depends on cation order, not pressure, meaning it can help estimate the temperature at which the mineral was trapped—providing insights into the rock’s thermal history.

These findings refine the use of omphacite-in-garnet systems in geobarometry, but further studies are needed to fully unlock its potential. This research brings us one step closer to deciphering the extreme conditions recorded in metamorphic rocks!