The rate of dissociation and the degree of stability of MnCO3 in air and in inert gases (nitrogen and argon) were studied over the temperature range 100 to 720° C. In the temperature range under consideration, the solid end-product is manganosite (Mn1-xO), hausmannite (Mn3O4), partridgeite or bixbyite (α-Mn2O3), tetragonal γ-Mn2O3 or occasionally a combination of two or even three of these products depending on the temperature, the gaseous medium, and the grain size of the initial MnCO3.
The rate of dissociation of MnCO3 is practically independent of the initial amount of material and is mainly a function of the grain size, temperature, and composition of the gaseous medium in which the reaction takes place. Variations of rhodochrosite concentration are of minor importance. The order of the reaction is very close to one. The activation energies calculated for 71 medium-to high-temperature experiments and extrapolated down to 120° C vary between 18.1 and 27.2 kcal mole−1. The instability or metastability of MnCO3 in air (depending on the grain size) at 110°C is established. MnCOs appears to be stable at room temperature (25°C) in air. The metastability of MnCO3 at 110°C is explained by a thin layer of α-Mn2O3 in the same way as the metastability of Al surrounded by a coating of Al2O3.