Elastic behaviour and structural evolution of topaz at high pressure

The elastic behaviour and the high-pressure structural evolution of a natural topaz, Al2.00Si1.05O4.00(OH0.26F1.75), have been investigated by means of in situ single-crystal X-ray diffraction up to 10.55(5) GPa. No phase transition has been observed within the pressure range investigated. Unit-cell volume data were fitted with a third-order Birch-Murnaghan Equation of State (III-BM-EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are: V (0)=345.57(7) angstrom(3), K (T0)=164(2) GPa and K'=2.9(4). The axial-EoS parameters are: a (0)=4.6634(3) angstrom, K (T0)(a)=152(2) GPa, K'(a)=2.8(4) for the a-axis; b (0)=8.8349(5) angstrom, K (T0)(b)=224(3) GPa, K'(b)=2.6(6) for the b-axis; c (0)=8.3875(7) angstrom, K (T0)(c)=137(2) GPa, K'(c)=2.9(4) for the c-axis. The magnitude and the orientation of the principal Lagrangian unit-strain ellipsoid were determined. At P-P (0)=10.55 GPa, the ratios epsilon(1):epsilon(2):epsilon(3) are 1.00:1.42:1.56 (with epsilon(1)||b, epsilon(2)||a, epsilon(3)||c and |epsilon(3)| > |epsilon(2)| > |epsilon(1)|). Four structural refinements, performed at 0.0001, 3.14(5), 5.79(5) and 8.39(5) GPa describe the structural evolution in terms of polyhedral distortions.