Unveiling the high-pressure transitions in hydrated borates at ID15b, ESRF

Natural borates (e.g., ulexite, colemanite, kernite and borax) are the most common ore minerals of boron, strategic element in a series of technological applications. In hydrated borates, the main structural units are Bφx units (tetrahedra and planar trigonal group where φ is an anion, O2- or OH-), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly Na+, K+, Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OH- form a complex and pervasive hydrogen-bond network, often enhancing the connection between the polyions clusters and the cations-polyhedra, therefore playing a paramount role in the stability of the crystalline edifice. Hydrated borates can act as neutron-shielding materials, due to the isotope 10B (which accounts for about 20% of the natural boron) high cross-section for thermal neutrons (~3840 barns) [1]. Enhanced neutron radiation shielding capacity is achievable by using boron-containing minerals as aggregates in concretes. Notably, a comprehensive characterization of the crystal-chemistry, elastic parameters, phase-stability and structural behaviour (at the atomic scale) at different T and P conditions, is still missing for most hydrated borates. The aim of this contribution is to analyze and provide insides on the high-pressure behavior and structure evolution of a number of hydrate borate minerals, unveiling the phase transition driving deformation mechanisms that lead to the formation of their high-pressure polymorphs.