Phase transitions in benitoite at high-pressure

The rare cyclosilicate benitoite (ideally BaTiSi3O9) represents a fundamental structural analogue for highpressure (HP) silicate and carbonate systems, primarily due to its uncommon framework of three-membered silicate rings. Among silicates, this [Si3O9]6- topology is shared only by CaSiO3 pseudowollastonite, making benitoite an ideal comparative system for investigating the mineralogy of the Earth’s interior (Yang & Prewitt, 1999). This study explores the structural evolution and compressional behaviour of benitoite via synchrotron single-crystal X-ray diffraction up to 22.3 GPa. Our experimental data reveal a complex polymorphic sequence involving three distinct phase transitions: to benitoite-II at ∼4.2 GPa, to benitoite-III at ∼12.7 GPa, and finally to benitoite-IV at ∼16.7 GPa. The crystal structures of the previously unknown polymorphs benitoite-III (space group Pbn21) and benitoite-IV (space group R32) were solved and refined. The structural transformation is governed by a progressive distortion of the Si3O9 rings and TiO6 octahedra, alongside an increase in the Ba coordination number. Notably, benitoite-IV exhibits a unique ring configuration where the tetrahedral bases become nearly parallel, leading to a marked increase in overall density. This densification mechanism closely mirrors the behaviour observed in the CaSiO3 system, where the HP-polymorphs breyite and parabreyite - both featuring distorted three-fold rings - are significantly denser than low-pressure pseudowollastonite. Ultimately, these findings provide critical insights into the stabilization of three-membered rings under extreme conditions and offer a robust structural model to interpret the behaviour of high-pressure carbonates containing sp3- hybridized carbon.