Minerals of the feldspar group are the most common minerals in the Earth’s crust. Due to their high geological relevance, numerous X-ray diffraction studies at non-ambient conditions (high temperature (HT) and / or pressure (HP)) have been performed with detailed studies focused on orthoclase, sanidine, microcline, albite, and anorthite. Paracelsian, BaAl2Si2O8, is considered as a member of the feldspar group, despite the fact that its structure topology is different from that of the feldspar topology. Its crystal structure was first investigated by Smith (1953) in the Pnam space group, and later re-considered by Bakakin and Belov (1960) in the monoclinic P21/c structure type (Bakakin, Belov, 1960). The structure is based upon a three-dimensional framework formed by corner-sharing SiO4 and AlO4 tetrahedra forming four- and eight-member rings and channels running along the a axis. The channels outlined by the eight-membered rings are occupied by Ba atoms. The framework topology of paracelsian is identical to that observed in danburite, CaB2Si2O8, a borosilicate mineral, which is known as an ore mineral for boron. Recently, Pakhomova et al. (2017) investigated high-pressure evolution of danburite and found the HP-modification of danburite with Si in exclusively fivefold coordination. This prompted us to investigate the HP-behaviour of other minerals with the same structure topology and, in particular, paracelsian.The HP-study of paracelsian was undertaken using in situ single-crystal X-ray diffraction in a diamond anvil cell at the Extreme Conditions Beamline P02.2 of PETRA III (DESY, Hamburg) using the wavelength of 0.2896 Å. For the study, we have used crystals of natural paracelsian from the Benallt Mine, Gwynedd, Wales, UK (from a private collection of the fifth author) with the size of 0.010×0.010×0.005 mm3. Experiment was performed at ambient temperature in the pressure range from 0.0001 to 32 GPa with the pressure step no more than 4 GPa (10 pressure points in total).The study yielded surprising results with HP-induced phase transitions and three new modifications of paracelsian. The first phase transition (from paracelsian-I to paracelsian-II) occurs between 3 and 6 GPa. The transition is isosymmetric, i.e. the space group remains the same (P21/c). The structure of paracelsian-II is obtained from that of paracelsian-I by the formation of additional Al–O bond and the change of coordination number of Al from 4 to 5. As a result, the AlO4 tetrahedra transform into AlO5 trigonal bipyramids. The next phase transition occurs between 25 and 28 GPa and is accompanied by the symmetry change from monoclinic (P21/c) to orthorhombic (Pna21). The structure of paracelsian-III consists of SiO6 octahedra, AlO6 octahedra and distorted AlO4 tetrahedra. The third phase transition is observed between 28 and 32 GPa and results in the symmetry decreasing from Pna21 to Pn. The structure of paracelsian-IV is a distorted version of the structure of paracelsian-III.This study was supported by the Russian Foundation for Basic Research
