Mineral matter transformation and the behavior of mineral matter in the coal during gasification, provide more information on the suitability of a specific coal source for combustion or gasification purposes. Therefore, the chemistry and mineral interactions have to be understood in order to determine the suitability for fixed bed gasification purposes with regards to mineral matter transformations and slagging properties. Although a suite of minerals important for the gasification process were identified [Van Dyk JC, Melzer S, Sobiecki A. Mineral matter transformations during Sasol-Lurgi fixed bed dry bottom gasification - utilization of HT-XRD and FactSage modelling. Minerals Engineering 2006; 19: 1126-35], some of the minerals, i.e. anorthite and calcite, with a specific behavior at different concentrations in the mineral structure and the transformation thereof was not studied and highlighted in detail. A number of other researchers [Reifenstein AP, Kahraman H, Coin CDA, Calos NJ, Miller G, Uwins P. Behavior of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite. Fuel 1999; 78: 1449-61], [Kondratiev A, Jaks E. Predicting coal ash slag flow characteristics (viscosity model for the Al2O3-CaO-'FeO'-SiO2 system). Fuel 2001; 80: 1989-2000] and [Kondratiev A, Jak E. Applications of the coal ash slag viscosity model for the slagging gasification technologies (viscosity model in the Al2O3-CaO-'FeO'-SiO2 system), 18th Pittsburgh Coal Conference, Newcastle, Australia, December 2001]) also did not investigate these gasification changes and mineralogical deformation during specific gasification conditions in detail. The principle aim of this paper is to identify the role of Ca-containing mineral species towards the in situ capture Of CO2 during gasification, as well as understanding the chemistry and interpret the mechanism Of CO2 capture by means of high temperature X-ray diffraction (HT-XRD), in combination with FactSage modeling. The CaO content of a South African and another coal source investigated in the present study, were 6 mass% and 30 mass% respectively. The basic components present in the coal, or specifically CaO, only act as a fluxing component up to a specific percentage, where after the ash fusion temperature starts to increase again. At this turning point the (Si+Al):Ca molar mass ratio is 2.75, which implies that after the turning point, the formation of anorthite is maximized and can thereafter only remain at the same level. The anorthite formation, when the Ca content increases, follows the inverse trend of the ash flow temperature prediction curve with the coal containing 6% CaO. The decrease in anorthite formation, with increasing Ca content, after the turning point in the graph, can be explained by the fact that more of the crystalline phase becomes a liquid (slag), and thus also the increase in the amount CaO in the slag will be observed. At the turning point, it is also interesting to note the stabilisation of the amount of other Ca-containing species. These are the minerals that are responsible and available for the mechanism where CO2 can be captured on Ca to form CaCO3. The formation of CaCO3 can also be observed from the turning point where the (Si+Al):Ca molar mass ratio is <2.75, which corresponds with the formation of other Ca-containing species. Thermodynamic modeling with FactSage results indicated that anorthite can only form to the point where the (Si+Al):Ca molar mass ratio is >2.75. Anorthite (CaSi2Al2O8) forms within the gasification zone and all non-reacted Ca react with CO2 to form CaCO3 further down in the combustion zone. (C) 2008 Elsevier Ltd. All rights reserved.