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 (III-BM-EoS). The III-BM-EoS parameters, simultaneously refined using the data weighted by the uncertainties in P and V, are as follows: K (V) = 158(4) GPa and K (V) ' = 3.3(3). The confidence ellipse at 68.3 % (Delta chi(2) = 2.30, 1 sigma) was calculated starting from the variance-covariance matrix of K (V) 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 K (V) and K (V) ', with K (V) = 158 +/- A 6 GPa and K (V) ' = 3.3 +/- A 4. A linearized III-BM-EoS was used to obtain the axial-EoS parameters (at room-P), yielding: K(a) = 146(5) GPa [beta (a) = 1/(3K(a)) = 0.00228(6) GPa(-1)] and K'(a) = 4.6(3) for the a-axis; K(b) = 220(4) GPa [beta (b) = 0.00152(4) GPa(-1)] and K'(b) = 2.6(3) for the b-axis; K(c) = 132(4) GPa [beta (c) = 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 (beta (a):beta (b):beta (c) = 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.