Thermal analysis studies of the dolomite-ferroan dolomite-ankerite series. II. decomposition mechanism in flowing CO 2 atmosphere

Abstract

Ankerites and ferroan dolomites [Ca(Mg,Fe)(CO 3) 2] dissociate in three stages to give characteristic three-endothermic-peaked DTA curves which correspond to three weight loss steps on TG curves. Pure dolomite decomposes in only two steps. The definition of these phenomena is greatly enhanced by determination in flowing CO 2. During this investigation a series of well-characterized minerals covering the dolomite-ferroan dolomite-ankerite series was studied using TG-DTG, DTA-EGA, TM and continuous heating X-ray powder photography (“continuous XRD”), backed up by X-ray diffractometry of products cooled from intermediate temperatures (“static” XRD). Limited Mössbauer spectroscopic determinations (reported in detail elsewhere) were also made on intermediate products. The first stage of the thermal decomposition of dolomite results in the formation of Mg-calcite ((CaMg)CO 3) and periclase (MgO), with the liberation of CO 2. In the cases of ankerite and ferroan dolomite, the initial reaction is similar except that a ferroan periclase or magnesian wustite ((Mg,Fe)O) is produced respectively. However, this phase is rapidly oxidized by CO 2 to magnesioferrite (MgFe 2O 4). A ferrous spinel (Fe 3O 4 - MgFe 2O 4 solid solution) may also be formed as an unstable transient intermediate compound during this oxidation, depending on experimental conditions. As a result, CO is liberated by reduction of CO 2. The second decomposition step arises out of a solid-state reaction between calcite and magnesioferrite to produce dicalcium ferrite (2CaO · Fe 2O 3) and periclase with the evolution of CO 2. The remaining unreacted calcium carbonate dissociates to CaO and CO 2 in the third decomposition step.