ID15b and Hydrated borates at Highpressure

Hydrated borates, such as colemanite, kernite, ulexite, and borax, represent the primary ore minerals of boron—a critical geochemical marker for petrogenetic processes and a strategic resource for advanced technological applications. Classified as critical raw materials by the EU in 2017[1], these minerals are increasingly investigated as aggregates in neutron-shielding concretes due to their high thermal neutron absorption capacity, stemming from their elevated H and 10B content. Structurally, hydrated borates consist of Bφx units (tetrahedra and trigonal planes), where φ represents an oxygen atom or a OH- group or a H2O molecule, organized into polyion clusters, which are linked to alkali or alkaline-earth polyhedra (e.g., Na+, Ca2+, Mg2+). Within these frameworks, H2O molecules and OH- groups form an extensive hydrogen-bond network that is fundamental to the stability of the crystalline edifice [2, 3]. This contribution explores the high-pressure deformation mechanisms and structural evolution of various hydrated borates, investigated via synchrotron X-ray diffraction at the ID15b beamline (ESRF). Our results show that these minerals undergo several high-pressure phase transitions. These involve the conversion of trigonal Bφ3 units into Bφ4 tetrahedra through the formation of additional B-φ bonds, or the rearrangement of the coordination spheres surrounding the divalent/trivalent cations (e.g., Fig. 1). Interestingly, a recurrent structural pattern appears to emerge, potentially providing a basis for a predictive framework regarding the high-pressure stability of this mineral class (Fig. 2).