A vibrational spectroscopic study of kernite to 25 GPa: Implications for the high-pressure stability of borate polyhedra

摘要

The Raman and infrared spectra of kernite [Na2B4O6(OH)(2)center dot 3H(2)O] have been characterized up to similar to 25 and 23 GPa, respectively, to explore pressure-induced changes in a structurally novel mineral that contains mixed coordination borate groups (threefold and fourfold), as well as both hydroxyl groups and water molecules. BO3 and BO4 vibrational modes are characterized in both the Raman and infrared spectra of kernite, and reassignments of some modes are made based on observed pressure shifts. Under compression to similar to 25 GPa, kernite undergoes three phase transitions: one initiates near similar to 2.5 GPa, the second occurs at similar to 7.0 GPa, and the third near 11.0 GPa. The first transition is characterized by a loss of both a subset of the Na-associated modes and a sharp OH peak, and it is fully reversible. The second transition is characterized by the loss of most of the BO3 modes, some of the BO4 modes at similar to 7 GPa, and further broadening of the H2O and OH peaks. This transition is partially reversible on decompression, but the Raman spectra indicate that the high-pressure structure and its reversion products are likely disordered. The third transition is characterized by the loss of most of the Raman and infrared modes, and it is not reversible on decompression. The decompression products from similar to 25 GPa have markedly different infrared and Raman spectra from the starting material, and appear to involve different bonding environments of boron than are present in the starting materials: thus, pressure treatment of boron-rich materials may generate novel quenchable structures. Notably, the BO3 groups in kernite are destabilized under compression, with the BO4 groups appearing to be more stable under compression. This shift in coordination may be endemic among borate glasses and crystalline borates under compression. The coordination conversion (and its products on decompression) is likely to highly depend on the detailed t