Pressure-driven phase transitions in hydrated borates

Hydrated borates (e.g., colemanite, ulexite, kernite and borax) are the most common ore minerals of boron, an important geochemical marker, in pegmatitic and granitic systems, for petrogenetic processes and a strategic element in a series of technological applications. Hydrated borates, which have been listed as critical raw materials by the EU [1], could be used as aggregate in neutron-shielding Sorel or Portland concretes, enhancing the adsorption towards thermal neutrons. 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-polyhedrons, therefore playing a paramount role in the stability of the crystalline edifice [2, 3]. The aim of this contribution is to analyze and provide insides on the high-pressure behavior and structure evolution of several hydrate borate minerals, unveiling the phase transition driving deformation mechanisms that lead to the formation of their high-pressure polymorphs. A common pattern, that could be used to predict the high-pressure phase stability of this class of minerals, has been detected.