High-pressure behaviour and phase stability of Ca₂B₆O₆(OH)₁₀·2(H₂O) (meyerhofferite)

Abstract

The high-pressure behaviour of meyerhofferite [ideally Ca₂B₆O₆(OH)₁₀·2(H₂O), with a ~ 6.63 Å, b ~ 8.34 Å, c ~ 6.47 Å, 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 ų). The isothermal bulk modulus (K_V0 = β⁻¹_P0,T0, where β_P0,T0 is the volume compressibility coefficient) of meyerhofferite, was found to be K_V0 = 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.