Elastic behavior and pressure-induced structure evolution of topaz up to 45 GPa

The behavior of a natural topaz, Al2.00Si1.05O4.00(OH0.26F1.75), has been investigated by means of in situ single-crystal synchrotron X-ray diffraction up to 45 GPa. No phase transition or change in the compressional regime has been observed within the pressure-range investigated. The compressional behavior was described with a third-order Birch–Murnaghan equation of state (IIIBM- EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are as follows: KV = 158(4) GPa and KV′ = 3.3(3). The confidence ellipse at 68.3 % (Δχ2 = 2.30, 1σ) was calculated starting from the variance–covariance matrix of KV and K′ obtained from the III-BM-EoS least-square procedure. The ellipse is elongated with a negative slope, indicating a negative correlation of the parameters KV and KV′, with KV = 158 ± 6 GPa and KV′ = 3.3 ± 4. A linearized III-BM-EoS was used to obtain the axial-EoS parameters (at room-P), yielding: K(a) = 146(5) GPa [βa = 1/ (3K(a)) = 0.00228(6) GPa−1] and K′(a) = 4.6(3) for the a-axis; K(b) = 220(4) G Pa [βb = 0.00152(4) G Pa−1] and K′(b) = 2.6(3) for the b-axis; K(c) = 132(4) GPa [βc = 0.00252(7) GPa−1] and K′(c) = 3.3(3) for the c-axis. The elastic anisotropy of topaz at room-P can be expressed as: K(a):K(b):K(c) = 1.10:1.67:1.00 (βa:βb:βc = 1.50:1.00:1.66). A series of structure refinements have been performed based on the intensity data collected at high pressure, showing that the P-induced structure evolution at the atomic scale is mainly represented by polyhedral compression along with inter-polyhedral tilting. A comparative analysis of the elastic behavior and P/T-stability of topaz polymorphs and “phase egg” (i.e., AlSiO3OH) is carried out.