Publication Date:
2014-02-15
Description:
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 (H 2 O 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 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,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 β a –1 β b –1 〉 β c –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 (β b –1 )/ P . The diversification between the a - and b -axis is also expressed by the pressure-depending increase of the distortion parameter . The pronounced elastic softening in both the b -axis and c -axis directions (β b –1 )/ P = –4.3±0.9, (β c –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.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
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