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 ~0.35 and ~0.85 GPa. The ΔV/V contribution related to the anomalous compression behavior in that pressure range is of the order of 5 × 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 KT,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 βa−1 ≈ βb−1 > βc−1 for an initial pressure regime up to ~3 GPa. At pressures above ~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 ∂(βb−1)/∂P. The diversification between the a- and b-axis is also expressed by the pressure-depending increase of the distortion parameter Δ. The pronounced elastic softening in both the b-axis and c-axis directions ∂(βb−1)/∂P = −4.3±0.9, ∂(βc−1)/∂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 ~6.9 GPa.

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