[S-MP27] Physics and Chemistry of Minerals

[SMP27-05] The stability of sodium borate hydrates

*Atsushi Kyono¹, Wataru Nishiyasu¹ (1.Division of Earth Evolution Sciences, Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:Borax Na2B4O5(OH)4.8H2O, Fundamental Building Block, Thermal analysis, X-ray diffraction, Quantum chemical calculation

Boron concentrations in the continental crust have been increased by continental growth and the boron cycle. The diversity of the fundamental building block (FBB) of borates leads to increase the variety of boron minerals, which causes the formation of new boron minerals by continental collision events. It is however unclear whether FBB is actually changed under high-temperature and high-pressure conditions. To examine the relationship between the change of the FBB’s connection mode and the phase change, borax Na₂B₄O₅(OH)_4.8H₂O, one of the most common borate minerals, was investigated under hydrothermal and high-temperature conditions by thermal analysis, powder X-ray diffraction, single-crystal X-ray diffraction, and quantum chemical calculations. The results showed that borax was extremely stable under the hydrothermal conditions. The crystal structure of borax expanded along the b-axis with increasing temperature, but the FBB almost remained unchanged. The FBB in borax was changed at the temperature of 600 °C and further changed to another FBB at 650 °C. These FBBs possessed energetically unstable configurations. Since the FBBs in borates are more likely to change under the high-temperature conditions, new boron minerals would be formed under the high-temperature conditions.