EQUATIONS OF STATE AND CRYSTAL STRUCTURES OF ANDALUSITE AND SILLIMANITE TO 10 GPA

Volume 22 of the Reviews in Mineralogy (RiM) Series was devoted to the Al₂SiO₅ polymorphs [1], largely because of their importance as thermobarometric indicators in metamorphic rocks. From a crystal chemical point of view, the aluminosilicate polymorphs are also of interest because of the different coordinations of Al in the structures, as described by P.H. Ribbe in RiM Volume 5 [2]. In andalusite and sillimanite, for example, one-half of the aluminum is 6-coordinated and the other half is 5-coordinated and 4-coordinated, respectively. We report here the equations of state and structures of andalusite and sillimanite at various pressures to ~10 GPa and 298K determined from single-crystal X-ray diffraction. A third-order Birch-Murnaghan equation of state fit to the P-V data results in a isothermal bulk modulus (K_T) of 144(1) GPa and K'=6.8(3) for andalusite and K_T=163.8 (1.0) and K'=5.0(3) for sillimanite. The values of K' are significantly greater than 4 and values of K_T are approximately 4-5% lower than previous determinations from single-crystal X-ray diffraction [3,4]. Both minerals exhibit anisotropic compressibility with [001] in andalusite and [010] in sillimanite exhibiting the least compressibility and [100] exhibiting the greatest compressibility in both structures. The [SiO4] tetrahedra are the most rigid units in the structures whereas the [AlO6] octahedra are the most compressible. The compression of the [A11O6] octahedron in both structures is anisotropic with the longest Al1 – OD bond shortening by 2.9% between 1 bar and 9.8 GPa in andalusite, and by 3.3% between 1 bar and 8.5 GPa in sillimanite.

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