Calcium oxides of high reactivity

Abstract

A RECENT study of the kinetics of decomposition of calcite (CaCO₃) single crystals in vacuo showed that if the reaction was interrupted before completion a 30-µm layer of a poorly crystalline material was present between the undecomposed CaCO₃ and a layer of normal polycrystalline CaO¹. It was hypothesised that the material of this 30-µm layer is a metastable form of CaO that transforms irreversibly to the stable polycrystalline oxide when the accumulated strain exceeds a critical level. If so, the free energy of formation of the metastable oxide from the stable oxide must lie in the range between zero and +31,500−21 TJ mol⁻¹ and might well lie at the positive end of this range². Such a metastable oxide should be chemically more reactive than the stable oxide. If the hypothesis is true, then the principal product of decomposition in vacuo of calcite particles of diameter < 30 µm should be the metastable oxide. We present here evidence that this inferrence is correct, and we demonstrate that not only this oxide, but also a highly crystalline oxide which is produced when large calcite crystals are decomposed in vacuo, reacts more vigourously with water than does the product of calcite decomposition in air or nitrogen.