Atomic-scale deformation mechanisms at high-pressure in hydroboracite, CaMg[B₃O₄(OH)₃]₂·3H₂O

The high-pressure behaviour of hydroboracite [ideally CaMg[B₃O₄(OH)₃]₂·3H₂O, Sp. Gr. P2/c with a ~ 11.769, b ~ 6.684, c ~ 8.235 Å, and β ~ 102.6° at room conditions] has been studied by two in-situ single-crystal synchrotron X-ray diffraction experiments up to about 15 GPa, using He as pressure-transmitting medium. Between 14.51(5) and 14.72(5) GPa, hydroboracite undergoes a first-order phase transition to its high-pressure polymorph, hydroboracite-II (likely monoclinic with a ~ 11.29, b ~ 6.297, c ~ 7.48 Å, and β ~ 106°, space group unknown). The isothermal bulk modulus (KV0 = β−1P0,T0, where βP0,T0 is the volume compressibility coefficient) of hydroboracite was found to be KV0 = 41.4(6) GPa. The destructive nature of the phase transition prevented any structure resolution of hydroboracite-II or even the continuation of the experiments at pressures higher than 15.45(5) GPa. In the pressure range 0–14.45(5) GPa, the compressional anisotropy of hydroboracite, indicated by the ratio between the principal components of the unit-strain ellipsoid, is ε1:ε2:ε3 = 2.2:1.3:1. The P-induced deformation mechanisms at the atomic scale in hydroboracite are here described.