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

The behavior of a natural topaz, AlSiO(OHF), 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 = 158(4) GPa and K ′ = 3.3(3). The confidence ellipse at 68.3 % (Δχ = 2.30, 1σ) was calculated starting from the variance-covariance matrix of K 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 and K ′, with K = 158 ± 6 GPa and K ′ = 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 [ β = 1/(3 K( a)) = 0.00228(6) GPa] and K′( a) = 4.6(3) for the a-axis; K( b) = 220(4) GPa [ β = 0.00152(4) GPa] and K′( b) = 2.6(3) for the b-axis; K( c) = 132(4) GPa [ β = 0.00252(7) GPa] 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 ( β: β: β = 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., AlSiOOH) is carried out. [ABSTRACT FROM AUTHOR]