High-pressure phase transition and structural deformation in benitoite

Benitoite (ideally BaTiSi3O9) is a rare cyclosilicate characterized by a framework of three-membered silicate rings, representing an important mineralogical analogue for several high-pressure silicate and carbonate systems. A particularly noteworthy feature of benitoite is the presence of isolated (Si3O9)6– ring units, which are relatively uncommon among silicate minerals. Among naturally occurring silicates, only pseudowollastonite exhibits three-membered silicate rings with the same topology as benitoite [1]. Given the relevance of the CaSiO3 system for understanding mineral assemblages in the Earth’s interior, investigating the high-pressure behavior of benitoite provides an important comparative framework for understanding the stability and evolution of ring silicates under extreme conditions. The main aim of this contribution is to investigate the high-pressure behavior and structural evolution of benitoite through synchrotron single-crystal X-ray diffraction experiments conducted up to 22.3 GPa. The results reveal the occurrence of three pressure-induced phase transitions: the first between 3.93(5) and 4.40(5) GPa, corresponding to the transformation from benitoite to benitoite-II; the second between 12.44(5) and 13.05(5) GPa, leading to benitoite-III; and the third between 16.36(5) and 17.08(5) GPa, producing benitoite-IV. The crystal structures of the two new high-pressure polymorphs, benitoite-III (space group Pbn21) and benitoite-IV (space group R32), were successfully solved and refined. The structural evolution of benitoite under compression is mainly governed by a progressive distortion of the Si3O9 rings and TiO6 octahedra, accompanied by an increase in the coordination number of barium. In benitoite-IV, the topology of the silicate rings changes significantly, with the tetrahedral bases becoming nearly parallel, a modification associated with a marked increase in density. A remarkably similar behavior is observed in the CaSiO3 system, where the high-pressure polymorphs Breyite and Parabreyite, both characterized by distorted three-membered silicate rings, are considerably denser than low-pressure pseudowollastonite. These findings provide new insights into the mechanisms governing the stabilization of three-membered silicate rings at high pressure and offer a useful structural model for interpreting the behavior of high-pressure carbonate phases containing sp3-hybridized carbon.