Raman Spectroscopy of the Amphibole Cummingtonite under Compression to 21 GPa—Ali Fuat Yuvali, Jason N. Ott, Elise Knittle and Quentin Williams

Abstract:

Amphiboles, found in igneous and metamorphic rocks, contain water in their chemical structure in the form of hydroxyl groups and contribute to water recycling processes in the interior of the Earth. In this study, we investigate the changes to the various vibrational modes of iron-rich natural cummingtonite ((Mg, Fe2+)2(Mg,Fe2+)5Si8O22(OH)2, a ferromagnesian amphibole mineral. Raman spectroscopy is used to characterize cummingtonite under pressures up to 20.8 GPa at 300 K. A single crystallite of cummingtonite along with ruby for pressure calibration were loaded in a symmetric type diamond anvil cell with methanol-ethanol-water solution as the pressure transmitting medium. Raman spectra were collected using a 633 nm excitation laser over the 50-1200 cm-1 and 3500-3800 cm-1 spectral regions. Mode splitting within the Raman spectra that occurs between 4.0 and 4.7 GPa provides evidence that there is a phase transition: based on past observations on more magnesian-rich compositions, this transition is likely associated with the C2/m to P21/m transition. This transition is most robustly manifested in the antisymmetric Si-Ob-Si stretching vibration in the range of 1045-1065 cm-1. Our results imply that enhanced iron-contents may elevate the pressure of this transition. The pressure shifts of 18 modes were monitored, and their least square fits were used to determine mode Grüneisen parameters. Three OH stretching peaks are resolved, associated with 3Fe, 2Fe/1Mg and Fe/2Mg cation configurations. The lowest frequency of these peaks (3Fe) shifts slightly negatively, while the two higher frequency vibrations shift weakly positively: this is in accord with the behavior of hydroxyl vibrations in other amphiboles observed using infrared spectroscopy. Decompression spectra are being collected to determine if the changes are reversible.