A mixture with the mole ratio of Mg₂₊/Si₄₊=3 : 2 was prepared by the simultaneous hydrolysis of magnesium and silicon alkoxides. The phases and thermal properties of the mixture were studied by means of differential thermal analysis, thermogravimetric analysis and X-ray diffraction. The results obtained are summarized as follows:
1) 2 MgO-SiO₂ and MgO·SiO₂ were formed at 770°C and 865°C, respectively.
2) For the three modifications of MgO·SiO₂, it is presumed that orthoenstatite and clinoenstatite are the low temperature phases, on the other hand, protoenstatite is the high temperature phase.
3) It was considered that the formation of 2 MgO·SiO₂ and MgO-SiO₂ was followed by the mechanism described by Ball and Taylor for surpentine (3 MgO·2 SiO₂·2 H₂O).
4) The rate of formation of 2 MgO-SiO₂ was determined. Reaction isotherms were best described by the first-order plots. The nucleation process maybe the rate-determing step. The values of the activation energies are about 82 kcal/mol and 27 kcal/mol for initial and final stages, respectively.