The excess enthalpy of magnesite accumulated by vibration grinding at low specific grinding energy consumption is due predominantly to an increase in specific surface area; at higher energy supply, it is caused by changes in the X-ray amorphous phase content, and when the supplied energy exceeds ca 2000 kJ kg−1 it is a result of the generation of other kinds of defects. The generated defects are relatively stable below 800 K and are the reason for a broad range of distribution of local molar Gibbs energies. Thus, at low temperatures only the ‘active’ portion of samples is able to decompose. Defects relax above ca 800 K, with rates comparable with the rate of decomposition itself. Accordingly, the efficiency of mechanical activation is impressive only below this temperature.