On the thermal decomposition of nesquehonite

Among the phases in the MgO–CO2–H2O system, nesquehonite, MgCO3·3H2O, attracts particular attention because of its potential application to carbon capture. However, its stability and the reported sequence of phases formed during the course of its thermal decomposition differ between authors and the corresponding decomposition mechanisms find various explanations. To improve the knowledge on the thermal decomposition of nesquehonite, new thermal data are presented and X-ray diffraction was used to follow the evolution of the solid products of decomposition. During thermal decomposition, nesquehonite loses its water below 300–350 °C whereas CO2 is lost above 300–350 °C, but the mechanism of thermal decomposition process is influenced by the choice of experimental conditions. The first loss of water, between ~55 and ~135 °C, results in a partial collapse of the nesquehonite structure and in the formation of a crystalline phase which is referred to as “phase X” (approximately MgCO3·2H2O) and this gradually converts into an amorphous phase upon further heating and water loss. The regeneration of nesquehonite upon rehydration of either “phase X” or the amorphous phase suggests that sufficient structural elements persist throughout the initial stages of decomposition to reconstitute nesquehonite.