Effect of temperature and pressure on the crystal structure of topaz, Al ₂SiO ₄(OH, F) ₂

The effect of temperature and pressure was analyzed on the crystal structure of natural topaz from Gilgit division, Pakistan. The unit cell parameters at 298K, 423K, 573K, 723K, 873K, 1023K and 1173K, and X-ray diffraction intensity data at 298K, 573K, 873K and 1173K were collected using an imaging plate X-ray diffractometer equipped with rotating anode generator. Thermal expansion coefficients along the unit cell edges and of the volume are α _a=6.4(7)×10 ^-6K ^-1, α _b=5.5(6) ×10 ^-6K ^-1, α _c=8.1(6) ×10 ^-6K ^-1 and α _v =2.0(1) ×10 ^-5K ^-1. The results of structure refinement taking into account the anisotropic displacement parameters at these temperatures yielded R values of 2.48, 2.60, 2.56 and 2.70%. The increase of mean Al-0 distance with temperature was greater than those of Al-F and Si-O. The Al-F-Al bond angle decreased with the increase of displacement parameter of fluorine-atom. The unit cell parameters under pressures up to 6.8 GPa and X-ray diffraction intensity data under 0.0001 (ambient pressure), 3.7, 5.1 and 6.2 GPa were collected by a four-circle X-ray diffractometer. The bulk modulus determined by the Birch-Murnaghan equation-of-state is K _0T=154(2)GPa (assuming K'=4). The axial compressibilities of this sample are β _a=2.02(8)×l0 ^-3 GPa ^-1, β _b=1.42(5)×lO ^-3 GPa ^-1 and β _c=2.25(4)×l0 ^-3 GPa ^-1. The results of structure refinement at each pressure with anisotropic displacement parameters at ambient pressure and with isotropic displacement parameter at high pressures yielded R values of 2.66, 5.61, 5.43 and 5.91%, respectively. The mean Al-0 bond distance was significantly decreased by compression. The inverse relationships of variation against temperature and pressure are observed for both the unit cell parameters and bond distances.