Investigation of the High-pressure Behaviour of Clinochlore by X-ray Diffraction and Infrared Spectroscopy

Chlorite plays an essential role in transporting water into the deep Earth.This study investigates the high-pressure behaviour of clinochlore up to ~20 GPa at room temperature, using synchrotron X-ray diffraction and infrared spectroscopy.We identified a polytypic phase transition (from IIb-2 to IIa-1) at ~9 GPa and a structural distortion at ~12 GPa.The polytypic phase transition at ~9 GPa is characterised by a significant reorganization of interlayer hydrogen bonding, transitioning to states of weakly bonded or non-hydrogen-bonded hydroxyl groups.The phase transition showed reversibility during the experiment.The structural distortion near 12 GPa indicates adjustments in the compression mechanism related to the hydroxyl groups across both the brucite-like and 2:1 sheets.The unit-cell volume of the low-pressure IIb-2 phase was fitted to the second-order Birch-Murnaghan equation of state, and the resulting parameters are V 0 = 710.9(3) 3 and K 0