Atomic-scale deformation mechanisms at high-pressure in inderborite, CaMg[B3O3(OH)5]2(H2O)4⋅2H2O

The high-pressure behavior of inderborite [ideally CaMg[B3O3(OH)5]2(H2O)4·2H2O, Sp. gr. C2/c with a~12.14, b ~7.43, c ~ 19.23 Å, β ~90.3° at room conditions] has been studied by two in-situ single-crystal synchrotron X-ray diffraction experiments up to about 10 GPa, using He as pressure-transmitting fluid. Between 8.11 (5) and 8.80(5) GPa, inderborite undergoes a first-order phase transition to its high-pressure polymorph, inderborite-II (with a~11.37, b~6.96, c~17.67, β~96.8° ΔV⁓ 7.0%, space group unknown). The isothermal bulk modulus (KV0 = β−1P0,T0, where βP0,T0 is the volume compressibility coefficient) of inderborite was found to be KV0 = 41(1) GPa. The destructive nature of the phase transition prevented any structure resolution of inderborite-II or even the continuation of the experiments at pressures higher than 10.10(5) GPa. In the pressure range 0-8.11(5) GPa, the compressional anisotropy of inderborite, indicated by the ratio between the principal components of the Eulerian finite unit-strain ellipsoid, is ε1:ε2:ε3 = 1.4: 1.05: 1. The deformation mechanisms at the atomic scale in inderborite are here described. Our findings support the hypothesis of a quasi-linear correlation between the total H2O content and P-stability range in hydrated borates, as the pressure at which inderborite undergoes the phase transition falls in line with most of the hydrate borates studied at high-pressure so far.