The thermal decomposition of natural mixtures of huntite and hydromagnesite has been investigated. Hydromagnesite decomposes endothermically giving off water and carbon dioxide, the mechanism is dependent on heating rate. Mass losses measured by TGA are consistent with the loss of four water molecules, from the loss of water of crystallisation, and one water molecule from the decomposition of the hydroxide ion, followed by the loss of four carbon dioxide molecules from the decomposition of the carbonate ions. The magnesium carbonate, remaining after the dehydration of hydromagnesite, recrystallises exothermically in response to higher heating rates. This causes the decomposition of the carbonate ions to split into two stages with the second stage moving to a higher temperature. The magnitude of each stage is dependent on the heating rate. Huntite decomposes endothermically, at a higher temperature, giving off carbon dioxide in two stages. Mass losses measured by TGA are consistent with the loss of three carbon dioxide molecules, from the decomposition the carbonate ions associated with the three magnesium ions, followed by the loss of a single carbon dioxide molecule associated with the decomposition of the carbonate ion associated with the calcium ion. (C) 2011 Elsevier B.V. All rights reserved,