Phase stability of hydrated borates at high pressure

Hydrated borates are a class of minerals made by clusters or chains of Bφx groups (φ represents an oxygen, an H2O molecule or an OH-) organized either in tetrahedra or in planar trigonal groups. Hydrated borates are believed to be a cheaper alternative to B4C for radiation-shielding concretes (Okuno et al., 2005), due to the large cross section (~3840 barns) for thermal neutrons of the isotope 10B, which represents about 20% of the boron in nature. A comprehensive characterization of the crystal-chemistry, elastic properties, stability and structural behavior of natural borates at varying T and P conditions is advisable for modelling and understanding their role when utilized as aggregates in radiation-shielding concretes (Torrenti et al., 2010), in which the components are subject to pressure (by static compression) and temperature (by irradiation). Interestingly, all hydrated borates studied so far at high-pressure display one (or more) phase transition, and the pressure at which the phase transitions occur seems to be correlated to the H2O content of the minerals (e.g., Comboni et al., 2020, 2021). During the phase transitions, the most dramatic structural change is the increase of the coordination number of part of the IIIB to IVB, by the interaction between the IIIB and one H2O molecule or OH- group, underlying the importance of the hydrogen bond network in the stability of the crystalline structure. Comboni D., Pagliaro F., Gatta G.D., Lotti P., Milani S., Merlini M., Battiston T., Glazyrin K. & Liermann H.P. (2020) - High-pressure behavior and phase stability of Na2B4O6(OH)2·3H2O (kernite). J. Am. Ceram. Soc., 103, 5291-5301. Comboni D., Poreba T., Pagliaro F., Battiston T., Lotti P., Gatta G.D., Garbarino G. & Hanfland M. (2021) - Crystal structure of the high-P polymorph of Ca2B6O6(OH)10·2(H2O) (meyerhofferite). Acta Crystallogr., B77, 940-945. Okuno K. (2005) - Neutron shielding material based on colemanite and epoxy resin. Radiat. Prot. Dosim., 115, 258-261. Torrenti J. & Nahas G. (2010) - Durability and Safety of Concrete Structures in the Nuclear Context. Int. Conf. Concr. under Sev. Cond., Merida, Mexico, 3-18.