Thermal stability of thaumasite

The thermal stability of thaumasite, Ca₃Si(OH)₆(SO₄)(CO₃)·12H₂O, has been determined and new thermodynamic data defining its stability and solubility are reported. The absolute upper limit of stability of thaumasite in a saturated aqueous solution at 1 bar pressure is 68 ± 5 °C. This numerical value apparently conflicts with the widespread view that thaumasite is only stable at low temperatures, <20 °C. The belief that thaumasite preferably forms at low temperatures is also encouraged by thermodynamic calculations: its free energy of formation enlarges rapidly with decreasing temperatures. Thaumasite solubility increases rapidly with rising temperature so it is destabilised in many mineralogical phase assemblages, often at temperature much below 68 °C. These so called conditional limits, i.e., conditional on thaumasite coexisting in particular mineralogical assemblages, are important in practice to limit its occurrence at high temperatures but should nevertheless not be confused with absolute stability, as determined from the pure compound. Hence two types of stability exist, absolute and conditional. The possibility of stabilising thaumasite to higher temperatures, >68 °C, by forming solid solution with ettringite components is discussed: it remains a theoretical but as yet unproven possibility.