DECARBONATION AND RECARBONATION OF CALCITES HEATED IN CO2 .1. EFFECT OF THE THERMAL REGIME

Scanning electron micrographs and thermal analysis curves were obtained from a sample of synthetic calcite that was exposed to a series of heating and cooling cycles in CO2 at just above atmospheric pressure. The heating rate, the maximum temperature of heating and the residence time at maximum temperature were varied. Scanning electron micrographs revealed that the morphology of the sample changed drastically even after a single heating cycle, but subsequent changes were much smaller, whatever the heat treatment. In contrast, very different thermal analysis curves were obtained. In the first heating cycle calcite decomposed within a narrow temperature range, giving a single, sharp endotherm. An exothermic signal on cooling was due to recarbonation, which occurred abruptly, probably on the surfaces of the particles. An additional small exotherm on the cooling curve was attributed to recrystallisation of poorly crystallised calcium carbonate. In subsequent heating cycles recarbonation partly overlapped decomposition of calcite. Some CO2 was trapped in pores and more calcite formed as the temperature and pressure in the pores were raised. Depending on the heating regime, one, two or three endotherms resulted.