High-pressure behavior of kyanite: Compressibility and structural deformations

The lattice parameters of kyanite were measured at various pressures up to about 60 kbar by single-crystal X-ray diffraction in a diamond anvil cell. Unit-cell dimensions decreased linearly with an almost uniform rate: beta(a) = 2.00(8) x 10(-4), beta(b) = 1.90(4) x 10(-4), beta(c) = 2.00(4) x 10(-4) kbar(-1). The principal compressibility coefficients were beta(1) = 2.23 x 10(-4), beta(2) = 2.04 x 10(-4), beta(3) = 1.65 x 10(-4) kbar(-1), with beta(1) forming an angle of 35 degrees with the c axis. K-0, calculated by fitting pressure-volume data to a third-order Birch-Murnaghan equation of state, was 1560(100) kbar, with K' 5.6(5.5); when K' was set at 4, K-0 became 1600(30) kbar. Structural refinements were carried out on data collected at 0.001 kbar with the crystal in air and at 0.1, 25.4, 37, and 47 kbar with the crystal in the diamond anvil cell. Whereas the Si tetrahedra and A14 octahedron were incompressible in this P range, the polyhedral bulk modulus for A11 and A12 was 1280(150) kbar and 2380(200) kbar for A13. These octahedra became more regular with increasing pressure. The almost isotropic compression pattern was due to the many shared edges between the polyhedra, uniformly distributed in the cell. The evolution of Al-Al separation showed that the largest reduction regarded the A12-A13 and A12-A14 distances, whereas the average A11-A12 distance was almost unchanged, resulting from linkage with Si tetrahedra having rigid edges. The result was that the largest reduction did not occur along the c axis but along the A14-All-A12-A13 directions. The geometrical structural invariance, expressed by the beta(v)/alpha(v) ratio and obtained from the average compressibility and average thermal expansion of the cell volume (Winter and Chose 1979), was 23 degrees C/kbar. The following equation of state, which applies in crustal P-T conditions, may be defined as: V/V-0 = 1 + 3.00(7) x 10(-5) T - 5.8(1) x 10(-4) P, where Tis in degrees C and P in kbar. The present volume-pressure data support multi-anvil experiments by Schmidt et al. (1997) defining the P-T conditions necessary for decomposition of kyanite into stishovite + corundum.