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 approximately 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. Introduction High pressure structure determination contain valuable data on the equations of state, interatomic forces and chemical bonding in minerals. The Al2SiO5 polymorphs, andalusite, sillimanite and kyanite, provide an interesting system where aluminum occurs in three types of coordinations; in addition to the octahedral coordination found in all three minerals, aluminum also occurs in tetrahedal coordination in sillimanite and in five-fold trigonal bipyramidal coordination in andalusite. Knowledge of the response to pressure and temperature of these different types of Al-O bonds in the presence of relatively rigid SiOa tetrahedra is necessary for an understanding of the stability relations and phase transformation within the Al2SiO5 system. The temperature efects on the crystal structures of andalusite, sillimanite and kyanite have been determined by Winter and Ghose (1979). The elastic constants of andalusite and sillimanite have been determined by Vaughan and Weidner (1978), who used the Brillouin scattering technique. From infrared and Raman 0003-004)v84l0506-05 l 3$02 . 00 5 I 3 spectroscopic data, the phonon spectra ofandalusite and their temperature dependence have been determined and interpreted on the basis of a rigid-ion model by Iishi et al. (1979). In this paper are presented data on the high pressure structural response of andalusite to 37 kbar, as well as correlations between high-pressure changes and hightemperature response, elasticity, and phonon spectra. It has been possible to observe directly the structural elements of andalusite that are responsible for the compressibility and, hence, the elastic constants. The present data on andalusite at high-pressure, combined with high-temperature data of Winter and Ghose (1979), may be used to test the inverse relationship betwedn structural responses due to temperature versus pressure (Hazen and Finger, 1982). Andalusite is a light-atom structure with a small unit-cell and orthorhombic symmetry and thus also constitutes a test case for the improvement in accuracy of Xray intensity data collected at high pressure by the omega step-scanning technique. The resulting improvement in 514 RALPH ET AL.: ANDALUSITE AT HIGH PRESSURE the data resulted in a successful refinement of the anisotropic temperature factors in a complicated silicate structure at high pressure.