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The high-pressure behaviour of meyerhofferite [ideally Ca2B6O6(OH)10ꞏ2(H2O), with a ~6.63 Å, b ~8.34 Å, c ~6.47 Å, α = 90.8°, β = 102°, γ=86.8°, Sp. Gr. P1 ], a B-bearing raw material (with B2O3  46 wt%) and a potential B-rich aggregate, has been studied by single-crystal synchrotron X-ray diffraction up to 9 GPa, under hydrostatic conditions. Meyerhofferite undergoes a first-order phase transition to meyerhofferite-II, likely iso-symmetric, bracketed between 3.05 and 3.48 GPa, with a spectacular unit-cell volume discontinuity (i.e., V~ 30 Å 3 ). The isothermal bulk modulus (KV0=  -1 P0,T0, where P0,T0 is the volume compressibility coefficient) of meyerhofferite was found to be KV0 =31.6(5) GPa, and a marked anisotropic compressional pattern, with K(a)0 : K(b)0 : K(c)0 ~1.5:1:3, was observed. The bulk modulus of meyerhofferite-II increases to 55(2) GPa and, differently to the majority of the borates studied at high pressure so far, the anisotropic compressional pattern of meyerhofferite decreases markedly in the high-pressure form. The P-induced deformation mechanisms controlling, at the atomic scale, the bulk compression of meyerhofferite are here described. Considerations about the use of meyerhofferite as a potential B-based aggregate in concretes, mortars or resins, are provided.

The crystal structure of the high-pressure polymorph of meyerhofferite, ideally Ca2B6O6(OH)10·2(H2O), has been determined by means of single-crystal synchrotron X-ray diffraction data. Meyerhofferite undergoes a first-order isosymmetric phase transition to meyerhofferite-II, bracketed between 3.15 and 3.75 GPa, with a large volume discontinuity. The phase transition is marked by an increase in the coordination number of the boron B1 site, from III to IV, leading to a more interconnected and less compressible structure. The main structural differences between the two polymorphs and the P-induced deformation mechanisms at the atomic scale are discussed.

The high-pressure behavior of kernite [ideally Na2B4O6(OH)2•3H2O, a ~7.02 Å, b ~9.16 Å, c ~15.68 Å, β = 108.9°, Sp. Gr. P21/c, at ambient conditions], an important B-bearing raw material (with B2O3  51 wt%) and a potential B-rich aggregate in radiation shielding materials, has been studied by single-crystal synchrotron X-ray diffraction up to 14.6 GPa. Kernite undergoes an iso-symmetric phase transition at 1.6-2.0 GPa (to kernite-II). Between 6.6-7.5 GPa, kernite undergoes a second phase transition, possibly iso-symmetric in character (to kernite-III). The crystal structure of kernite-II was solved and refined. The isothermal bulk modulus (KV0= -1 P0,T0, where P0,T0 is the volume compressibility coefficient) of the ambient-pressure polymorph of kernite was found to be KV0 = 29(1) GPa and a marked anisotropic compressional pattern, with K(a)0 : K(b)0 : K(c)0 ~ 1:3:1.5., was observed. In kernite-II, the KV0 increase to 43.3(9) GPa and the anisotropic compressional pattern increases pronouncedly. The mechanisms, at the atomic scale, which govern the structure deformation, have been described.