High-pressure behaviour and phase stability of Ca 2 B 6 O 6 (OH) 10 ·2(H 2 O) (meyerhofferite)

The high-pressure behaviour of meyerhofferite [ideally Ca2B6O6(OH)10·2(H2O), with a ~ 6.63 A, b ~ 8.34 A, c ~ 6.47 A, α = 90.8°, β = 102°, γ = 86.8°, Sp. Gr. P $$\overline{1}$$ ], 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., ΔV ~ 30 A3). The isothermal bulk modulus (KV0 = β−1P0,T0, where βP0,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.