In situ Raman spectroscopic investigation on the phase transition of grunerite at high pressures

Abstract Amphiboles are key rock-forming minerals and play a significant role in the subduction process. Grunerite (Mg,Fe)7Si8O22(OH)2, as an important ferromagnesian amphibole, is known to undergo a phase transition from monoclinic C2/m to P21/m structure at elevated pressures. This study investigates the phase transition of grunerite under high pressure using a diamond-anvil cell combined with in situ Raman spectroscopy, aiming to identify pressure-induced phase transitions and associated structural changes. We have identified mode splits of the OH stretching modes indicative of the C2/m to P21/m phase transition at ∼1.46 GPa and observed another possible phase transition suggested by a discontinuity in the OH modes at ∼17.81 GPa. The splitting of the OH stretching modes resulted from the distinct environments of two OH positions in the P21/m structure, in comparison to the crystallographically identical OH positions in the C2/m structure. We have also identified additional M-O modes in the lower-wavenumber spectrum range. The results provide new insights into the structural stability of grunerite under extreme geological conditions, contributing to a deeper understanding of metamorphic petrology and geodynamics.