Cordierite under hydrostatic compression: Anomalous elastic behavior as a precursor for a pressure-induced phase transition

The high-pressure behavior of cordierite was investigated by means of in situ experiments using piston-cylinder press and diamond-anvil cell. Static compression in diamond-anvil cells was conducted with various penetrating and non-penetrating pressure media (H2O up to 2 GPa, argon and 4:1-methanol-ethanol up to 7 GPa). The measurement of lattice parameters revealed neither a significant influence on the elasticity nor any indication for effects in analogy to over-hydration within the experimental pressure ranges. Volumetric compression experiments at constant rates up to 1.2 GPa in a piston-cylinder apparatus insinuate subtle irregularities in the low-pressure range at around similar to 0.35 and similar to 0.85 GPa. The Delta V/V contribution related to the anomalous compression behavior in that pressure range is of the order of 5 x 10(-4). The results obtained from single-crystal X-ray diffraction between 10(-4) and 7 GPa revealed an unexpected and anomalous linear volume decrease, corresponding to K-T,K-298 = 131 +/- 1 GPa for the bulk modulus and K' = -0.4 +/- 0.3 for its pressure derivative for a third-order Birch-Murnaghan equation of state. The compressional behavior of the main axis directions is anisotropic with beta(-1)(a) approximate to beta(-1)(b) > beta(-1)(c) for an initial pressure regime up to similar to 3 GPa. At pressures above similar to 4 GPa, the compression of the a- and b-axis starts to differ significantly, with the b-axis showing elastic softening as indicated by negative values for partial derivative(beta(-1)(b))/partial derivative P. The diversification between the a- and b-axis is also expressed by the pressure-depending increase of the distortion parameter Delta. The pronounced elastic softening in both the b-axis and c-axis directions partial derivative(beta(-1)(b))/partial derivative P = -4.3 +/- 0.9, partial derivative(beta(-1)(c))/partial derivative P = -1.2 +/- 0.8) are responsible for the apparent linear bulk compression, which indicates the structural instability and precedes a so far not reported ferroelastic phase transition to a triclinic polymorph, following a primitive lattice above the critical transition at similar to 6.9 GPa.