The thermal decomposition mechanisms and the intermediate morphology of MgCl 2 •6H 2 O and MgCl 2 •H 2 O were studied using integrated thermal analysis, X-ray diffraction, scanning electron microscope and chemical analysis. The results showed that there were six steps in the thermal decomposition of
by simultaneous dehydration and hydrolysis at 203 • C, the dehydration of Mg(OH)Cl•0.3H 2 O to MgOHCl at 235 • C, and finally the direct conversion of MgOHCl to the cylindrical particles of MgO at 415 • C. To restrain the sample hydrolysis and to obtain MgCl 2 •H 2 O, MgCl 2 •6H 2 O was first calcined in HCl atmosphere until 203 • C when MgCl 2 •H 2 O was obtained; HCl gas was then turned off and the calcination process continued, producing Mg 3 Cl 2 (OH) 4 •2H 2 O calcined at 203 • C, Mg 3 (OH) 4 Cl 2 at 220 • C and MgO at 360 • C. The temperature of producing MgO from calcination of MgCl 2 •H 2 O was lower (360 • C) than that from MgCl 2 •6H 2 O (415 • C) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg 3 Cl 2 (OH) 4 •2H 2 O particles and the uneven surface porous Mg 3 (OH) 4 Cl 2 particles. The MgO particles obtained at 360 • C had a flake structure.