A Raman Spectroscopic Study of Kernite to 25 GPa

Abstract:

A Raman spectroscopic study of kernite to 25 GPa

Marcus Silva, Earl O'Bannon III, and Quentin Williams

Department of Earth & Planetary Sciences, University of California Santa Cruz

The Raman spectrum of kernite (Na₂B₄O₆(OH)₂·3(H₂O)) has been characterized up to ~25 GPa in order to explore pressure-induced changes in a structurally novel mineral that contains mixed coordination borate groups (three- and four-fold), and both hydroxyl units and water. During compression, all of the ~30 modes monitored shift positively and monotonically until ~2.2 GPa where a few low frequency modes disappear and tetrahedral borate modes merge. The low frequency modes that disappear at ~2.2 GPa are likely associated with Na vibrations, and their disappearance suggests that dramatic changes occur in the Na sites at ~2.2 GPa. The merging of the boron bending and stretching modes at ~2.2 GPa suggests that the local symmetry of the BO₄ tetrahedra changes at this pressure, and likely becomes more symmetric. The remaining modes shift positively up to ~7.4 GPa where a second notable change occurs. All but 5 modes (with initial frequencies of 150, 166, 289, 307, and 525 cm^-1) disappear at ~7.4 GPa. This indicates that a second phase transition has occurred which affects both the BO₃H and BO_4­ groups: based on the loss of modes, this transition may be associated with disordering of the crystal. These 5 modes persist and shift monotonically up to ~25 GPa. On decompression, the 5 modes shift smoothly down to ~2.0 GPa where a few new modes appear in the spectrum. When fully decompressed to room pressure, the Raman spectrum of the recovered sample is significantly different from the ambient spectrum of the initial sample. Thus, our results are suggest a phase transition occurring at 2.2 GPa with changes in the Na and tetrahedral boron sites, followed by an additional transition at 7.4 GPa that may involve disordering of the crystal. In the latter transition, at least the BO₃H groups appear to be destabilized, implying that the three-fold coordination of boron groups is, in contrast to the case of carbon, unstable in crystalline phases at relatively modest pressure conditions.