Phase transitions during compression of thaumasite, Ca3Si(OH)6(CO3)(SO4)?12H2O: A high-pressure synchrotron X-ray powder diffraction study

The high-pressure behaviour of the thaumasite structure was investigated using synchrotron X-ray powder diffraction, up to 19.5 GPa. Based on Rietveld refinements, thaumasite retained the room pressure P63 space group throughout the whole investigated pressure range while the pressure dependence of the refined unit cell parameters can be cast into three different compression regimes, each corresponding to a different thaumasite phase (th-I, th-II, and th-III) related by isosymmetric phase transitions. In particular, the phase transition in the 7.40-15.02 GPa P-range (i.e. from th-II to th-III) is associated to an inversion of the axial bulk moduli which, by analogy with ettringite, can be rationalized as due to a change in the relative strengths of the iono-covalent bonds along the [Ca3Si(OH)6(H2O)12]4+ columns parallel to the c-axis vs. the oxygen-hydrogen bonds linking the columns within the ab-plane. The linear inverse relationship between the low- and high-temperature data from literature with those collected under high-pressure conditions reveals that the same bonding regime governs the anisotropic expansion and contraction of thaumasite up to about 1.4 GPa and 400K (HP-HT stability limits of th-I phase).