Pressure-Induced Phase Transitions in Danburite-Type Borosilicates

The high-pressure behaviors of two isotypic borosilicates (maleevite, BaB2Si2O8, and pekovite, SrB2Si2O8) have been studied using in situ single-crystal X-ray diffraction and Raman spectroscopy. Maleevite undergoes one reconstructive phase transition between 36 and 38 GPa with the formation of a triclinic phase, maleevite-II, featuring octahedrally coordinated silicon. In contrast, pekovite undergoes two phase transitions: first, an isosymmetric order-disorder phase transition to pekovite-II (between 18 and 23 GPa) and then a reconstructive phase transition with the formation of triclinic pekovite-III (between 29 and 33 GPa). The structure of pekovite-II is characterized by the splitting of the Si site into two sites. The results have been confirmed by Raman spectroscopy and density functional theory (DFT) calculations. Raman spectra indicate that the reconstructive phase transitions of both borosilicates are irreversible. Upon decompression, the triclinic phases persist metastably at least down to 12 and 17 GPa, for pekovite and maleevite, respectively. The comparison of the high-pressure behavior of danburite-group minerals with the general formula MB2Si2O8 (M = Ca, Sr, Ba) reveals that increasing size of an extraframework cation for M = Sr and Ba governs the stability of the danburite-type structure and prevents the formation of pentacoordinate silicon species observed in danburite (M = Ca).