High pressure phase transitions of paracelsian BaAl2Si2O8

Abstract

Three new polymorphs of aluminosilicate paracelsian, BaAl 2 Si 2 O 8 , have been discovered using synchrotron-based in situ high-pressure single crystal X-ray diffraction. The first isosymmetric phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The phase transition is associated with the formation of pentacoordinated Al 3+ and Si 4+ ions, which occurs in a stepwise fashion by sequential formation of Al-O and Si-O bonds additional to those in AlO 4 and SiO 4 tetrahedra, respectively. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic ( P 2 1 / c ) to orthorhombic ( Pna 2 1 ). The structure of paracelsian-III consists of SiO 6 octahedra, AlO 6 octahedra and distorted AlO 4 tetrahedra, i.e. the transition is reconstructive and associated with the changes of Si 4+ and Al 3+ coordination, which show rather complex behaviour with the general tendency towards increasing coordination numbers. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna 2 1 to Pn . The transition has a displacive character. In the course of the phase transformation pathway up to 32 GPa, the structure of polymorphs becomes denser: paracelsian-II is based upon elements of cubic and hexagonal close-packing arrangements of large O 2− and Ba 2+ ions, whereas, in the crystal structure of paracelsian-III and IV, this arrangement corresponds to 9-layer closest-packing with the layer sequence ABACACBCB .

A High-Pressure Investigation of the Synthetic Analogue of Chalcomenite, CuSeO3∙2H2O

“…This behavior resembles the HP behavior of related hydrated minerals, like FeSO4•H2O and MgSO4•H2O [11,12], which also undergo phase transitions at similar pressures. A similar behavior has been recently observed in the minerals danburite and paracelsian [31,32]. For the pressure-dependence of the c-axis, we observe a nearly linear pressure evolution, with experiments and calculations in qualitative agreement.…”

“…In particular, at 4.8 GPa, the length of the sixth Cu-O bond is 2.80 Å: i.e., only 27% larger than the fifth Cu-O bond. This behavior of the Cu-O bonds is very similar to the behavior observed in danburite and parcelsian [31,32], which also undergo isostructural phase transitions under compression. In order to understand the changes that occur in the crystal structure at the phase transition, we have analyzed what happened with the different polyhedral units.…”

“…In particular, at 4.8 GPa, the length of the sixth Cu-O bond is 2.80 Å: i.e., only 27% larger than the fifth Cu-O bond. This behavior of the Cu-O bonds is very similar to the behavior observed in danburite and parcelsian [31,32], which also undergo isostructural phase transitions under compression. becomes stiff with increasing pressure would make that the crystal prefers to compress along the a-axis instead of the b-axis (as it does below the transition pressure) changing the compressibility of both axes and favoring the structural reorganization associated with the phase transition observed in CuSeO 3 •2H 2 O.…”

Polymorphism of feldspars above 10 GPa

“…For most of the studied frameworks [27][28][29] , the formation of an additional T-O bond is accompanied by a pronounced jump in the unit cell parameters, as also found in the current study and, accordingly, an increase in the densities of the feldspar-derived phases (Fig. 3).…”

“…However, increases in the distortion parameters of the AlO 4 tetrahedra (BAV up to~128˚and QE up to~1.04) indicate that anorthite-II → anorthite-III phase transition is also induced by continuous distortion of AlO 4 tetrahedra. A similar mechanism of pressure-induced densification has recently been observed for tetrahedral framework compounds with different connectivities of TO 4 units (T=Si 4+ , Al 3+ , P 5+ , Be 2+ ) [27][28][29] . Similar to feldspars, the displacive TO 4 →TO 5 transitions are induced by continuous approach of an additional O atom to the coordination spheres of T atoms and are accompanied by severe geometrical distortion of initial TO 4 tetrahedra.…”