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

Abstract

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.