Abstract
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: V0=345.57(7) A3, KT0=164(2) GPa and K′=2.9(4). The axial-EoS parameters are: a0=4.6634(3) A, KT0(a)=152(2) GPa, K′(a)=2.8(4) for the a-axis; b0=8.8349(5) A, KT0(b)=224(3) GPa, K′(b)=2.6(6) for the b-axis; c0=8.3875(7) A, KT0(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−P0=10.55 GPa, the ratios e1:e2:e3 are 1.00:1.42:1.56 (with e1||b, e2||a, e3||c and |e3| > |e2| > |e1|). 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.
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