Compressibility and reversible amorphization of thaumasite Ca3Si(OH)6(CO3)(SO4)·12H2O pressurized in methanol–ethanol–H2O up to 5 GPa

Abstract

The elastic and structure behavior of natural thaumasite compressed in methanol–ethanol–H2O up to 5 GPa was studied by synchrotron powder diffraction with a diamond anvil cell. In the pressure range between 0.0001 and 4.5 GPa, the compression is regular and slightly anisotropic, with a more rigid ab-plane coinciding with the orientation of hydrogen bonds and S–O, C–O bonds in anion groups. The corresponding bulk moduli derived from the third-order Birch–Murnaghan EoS fit are K a = 43(2), K c = 35(2), K T = 39(2) GPa. Rietveld refinements reveal some general features of the structure evolution of thaumasite, which are consistent with the observed elastic anisotropy. The compression within the ab-plane proceeds mainly at the expense of shortening of hydrogen bonds and much lesser decrease of C–O and S–O bonds. In the range of 0.0001–3 GPa the Ca–O polyhedra contract more rapidly along the c-axis as compared to the ab-plane. At about 5 GPa, thaumasite undergoes a reversible transformation to an amorphous phase. The observed behavior differs drastically with that studied previously using helium as the pressure medium, which suggests the effect of He penetration increasing the structure stiffness. Without helium support, the thaumasite structure is preserved only up to 4.5 GPa.