Raman spectroscopy of natural cordierite at high water pressure up to 5 GPa

The high‐pressure behaviour of cordierite, a widespread ring aluminosilicate with channels incorporating fluid compounds (H₂O, CO₂), is characterized by the absence of phase transitions up to 2.5 GPa. However, the distortion of the ring tetrahedra observed previously at 2.3 GPa is supposed to introduce a phase transition at higher pressure, which has not been checked so far. This work presents a high‐pressure Raman spectroscopic study of natural cordierite compressed in water medium up to 4.7 GPa in a diamond anvil cell. At P > 4 GPa, a disordering of both the framework and intrachannel H₂O subsystem is apparent from significant broadening of Raman peaks and the evolution of short‐range order parameters. This is followed by abrupt shifts of the framework and O–H stretching modes at about 4.5 GPa, indicating a first‐order phase transition. Its reversibility is seen from the recovery of the initial spectrum at P < 3 GPa. The shift amplitudes of different framework modes indicate the predominance of distortion over contraction of the framework polyhedra upon this transition. The disordering of the H₂O subsystem in the high‐pressure phase is likely a consequence of distortion of the channel‐forming framework elements, which is supposed to be a driving force of this transition. Copyright © 2011 John Wiley & Sons, Ltd.