High-pressure behavior of sillimanite and kyanite: compressibility, decomposition and indications of a new high-pressure phase

The high-pressure behavior of the two Al2SiO5 polymorphs sillimanite and kyanite was investigated up to 46 and 53 GPa, respectively, using a diamond-anvil cell and powder X-ray synchrotron diffraction. The bulk moduli of sillimanite and kyanite obtained from a fit to a second-order Birch-Murnaghan equation of state are B-0 = 176(11) GPa and B-0 = 190(3) GPa at fixed B' = 4, respectively. The samples were laser annealed several times at high pressures. After laser annealing at 46 GPa, sillimanite was decomposed into its oxide components corundum (Al2O3) and stishovite (SiO2). The high-pressure behavior of kyanite, however, is more complicated. Up to 20 GPa, kyanite remains stable. At 30 GPa a new high-pressure form is observed, which seems to be structurally related to kyanite. After laser heating, it partly decomposed into corundum and stishovite. However, when laser heating at much higher pressures of 47 to 53 GPa, the new phase remains stable.