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

The high-pressure behaviour of meyerhofferite [ideally Ca2B6O6(OH)(10)center dot 2(H2O), with a similar to 6.63 angstrom, b similar to 8.34 angstrom, c similar to 6.47 angstrom, alpha = 90.8 degrees, beta = 102 degrees, gamma = 86.8 degrees, Sp. Gr. P (1) over bar], a B-bearing raw material (with B2O3 approximate to 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., Delta V similar to 30 angstrom(3)). The isothermal bulk modulus (K-V0 = beta(-1)(P0,T0), where beta(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) similar to 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.