A thermochemical study of vaterite

Vaterite is shown to be unstable with respect to calcite at 25°C by measurements of the enthalpies of solution in 0·1 N HCl under 0·97 atm CO₂ and the solubilities in water under 0·97 atm CO₂ of the two polymorphs. For a pure, synthetic vaterite ΔH (tr) = ?1036 ±16 cal mol^?1 and ΔG(tr) = ?790 ± 25 cal mol^?1 for the transition to calcite. For other vaterites aged longer during preparation ΔH(tr) is smaller and shows a linear relationship with the X-ray line broadening which extrapolates to ΔH(tr) = ?545 ± 30 cal mo^?1 for zero broadening. The use of X-ray line broadening as a measure of crystal imperfection and stability is discussed for various synthetic and natural vaterites.