Compressibility and crystal structure of andalusite at high pressure

The unit-cell dimensions and crystal structure of andalusite Al2SiO5 have been refined from X-ray data on single crystals mounted in a diamond anvil cell at pressures of 12,25, and 37 kbar. Structure refinements with anisotropic temperature factors yielded weighted R factors of 3.4, 4.9, and 5.2%o respectively. The bulk modulus of andalusite is 1.35t0.10 mbar and the axial compression ratios of orthorhombic unit-cell axes a:b:c are approxi-mately 2.1:1.5:1.0. The relatively greater compressibility of the A(IFOD bond results in a being the most compressible axis. Those bonds that compress)3obetween I bar and 37 kbar at room temperature, are the bonds that also expand significantly between 25 and 1000"C at room pressure. Polyhedral bulk moduli for the Al(l) octahedron, the Al(2) trigonal bipyramid and the Si tetrahedron are 1.3t0.2, 1.6!0.5, and 4.1+1.5 mbar' respectively. Thus, the aluminum polyhedra are significantly more compressible than the silicon tetrahedron. The omega step-scanning technique of X-ray intensity data collection results in a significant improvement in accuracy and is recommended for structure determination with the diamond-anvil high-pressure cell