Compressibility and crystal structure of andalusite at high pressure Available to Purchase

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% respectively. The bulk modulus of andalusite is 1.35±0.10 mbar and the axial compression ratios of orthorhombic unit-cell axes a:b:c are approximately 2.1:1.5:1.0. The relatively greater compressibility of the Al(1)–O_D bond results in a being the most compressible axis. Those bonds that compress >3σ between 1 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(1) octahedron, the Al(2) trigonal bipyramid and the Si tetrahedron are 1.3±0.2, 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.