The thermal energy storage properties of dolomite, CaMg(CO₃)₂, from three sources (commercial, mined, and synthetic) are investigated as a potential solid–gas thermochemical energy store for concentrating solar thermal power (CSP) plants. The reversible carbon dioxide release/absorption cycle can be used to store and release large quantities of thermal energy that can be harnessed near 550 °C. To enable carbon dioxide absorption, a novel molten salt eutectic NaCl : MgCl₂ mixture was added to the dolomite, which proved effective. Curiously, the mined (unrefined) sample proved to have the best calcination/carbonation properties with a stable cyclic stability of ∼50 mol% CO₂ over 10 cycles. A morphological investigation by scanning electron microscopy reveals the mined sample contains impurities (e.g. quartz) that prevent detrimental agglomeration of dolomite and its reaction products (CaCO₃ and MgO). High levels of porosity in the mined dolomite also provide short gas–solid reaction pathways during carbonation. A commercial dolomite source contained too many impurities, resulting in the formation of high levels of Ca₂SiO₄ that acted as a Ca-sink, lowering CO₂ capacity. Finally, a high purity synthetic dolomite sample displayed high levels of agglomeration and phase segregation, lowering reversible CO₂ capacity, which was attributed to a lack of impurities that restrict agglomeration.