High pressure behavior and phase transitions in benitoite

Benitoite (ideally BaTiSi3O9) is a rare cyclosilicate characterized by a framework of three-membered silicate rings, which represents a significant mineralogical analogue for several high-pressure silicate and carbonate systems. Interestingly, benitoite has (Si3O9)6– silicate-ring units, which is a relatively uncommon feature in silicates. Only CaSiO3 pseudowollastonite present a structure with 3-fold silicate ring units with the same topology as benitoite [1]. Considering the interest of the CaSiO3 system for the Earth’s inner mineralogy, high pressure study on benitoite is a rather relevant comparative system to be investigated. The main aim of this contribution is to show the high-pressure (HP) behaviour and structural evolution of benitoite through synchrotron single-crystal X-ray diffraction experiments up to 22.3 GPa. Our results show the occurrence of three phase transitions: between 3.93(5) and 4.40(5) GPa from benitoite to benitoite -II, between 12.44(5) and 13.05(5) GPa, to benitoite-III and between 16.36(5) and 17.08(5) GPa to benitoite-IV. The crystal structures of the two new HP-polymorphs, benitoite-III (space group Pbn21) and benitoite-IV (space group R32), were successfully solved and refined. The structural evolution is driven by a progressive deformation of the Si3O9 rings and the Ti-octahedra, coupled with an increase in the barium coordination number. In benitoite-IV, the topology of the silicate rings becomes distinct, with the tetrahedral bases becoming almost parallel, a modification that correlates with a significant increase in overall density. A remarkably similar behaviour is observed in the CaSiO3 system, where the high-pressure polymorphs breyite and parabreyite, which feature distorted three-membered rings, are significantly denser compared to the low-pressure pseudo-wollastonite. The findings on benitoite provide valuable insights into the stabilization mechanisms of three-fold rings under extreme conditions, offering a structural model that may also explain the behaviour of high-pressure carbonates with sp3-hybridized carbon.