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The equations of state and structures of andalusite and sillimanite have been determined using high-pressure single-crystal X-ray diffraction. A third-order Birch-Murnaghan equation-of-state fit to 14 P-V data points measured between 1 bar and 9.8 GPa for andalusite yields values of K T0  = 144.2(7) GPa and K’ = 6.8(2). A similar analysis for sillimanite involving a fit to 13 P-V data points between 1 bar and 8.5 GPa results in K T0  = 164(1) GPa and K’ = 5.0(3). The axial compression of both structures is nonlinear and highly anisotropic (~60%) with the c-axis being the least compressible axis in both structures. The axial moduli determined with a parameterized form of the third-order Birch-Murnaghan equation of state are: K a ₀ = 163(1) GPa, K b ₀ = 113.1(7) GPa, and K c ₀ = 297(1) GPa with K’ a ₀ = 2.1(3), K’ b ₀ = 5.08(19), and K’ c ₀ = 11.1(4) for sillimanite, and K a ₀ = 99.6(7) GPa, K b ₀ = 152.2(9) GPa, and K c₀  = 236(3) GPa with K’ a₀  = 5.83(19), K’ b₀  = 7.6(3), and K’ c₀  = 5.5(9) for andalusite. The major compression mechanism in both structures involves shortening of bond lengths within the AlO 6  octahedra with volume reductions of 7.4% and 5.1% in sillimanite and andalusite, respectively, over the pressure ranges studied. In andalusite there is also significant compression of the AlO 5  polyhedra and, to a lesser degree, the SiO 4  tetrahedra that display reductions of 5.0% and 3.1% in volume, respectively. In sillimanite there is no significant compression of either the AlO 4  or SiO 4  tetrahedra which behave as rigid, incompressible units.

Equations of state and structures of andalusite to 9.8 GPa and sillimanite to 8.5 GPa