Re-Examination of the Polymorphism of Dicalcium Silicate

Samples of Ca₂SiO₄ were prepared by three different methods. These samples were examined by differential thermal analysis, high-temperature X-ray diffraction, and air quenching of pellets. It was found that the β modification, during cooling, converted completely and rapidly to the γ modification and "dusted" only if it had been heated above 1420?± 10? C. If the sample was not heated above this critical temperature, a mixture of the β and γ forms always resulted at room temperature. The addition of 0.5 weight % of CaO, SiO₂, Al₂0₃, Fe₂0₃, or MgO had no effect on the sequence of phases for samples cooled from above the critical temperature. When, however, these same samples were cooled from below 1410?C, the final room-temperature product was not the same for all samples. On the other hand, the additional 0.5% of Na₂0, K₂O, Cr₂O₃, or B₂0₃ prevented the formation of the β modification regardless of the temperature from which the sample was cooled. The critical heating temperature correlated with the transformation temperature for the α-α inversion. A theory relating this inversion to the completeness of the β→γ transformation is presented.