Recent research on the polymorphism of calcium orthosilicate has broadened and to some extent modified the picture of the phase relations of this substance as presented in the pioneer studies of Day and Shepherd (1906) and Rankin and Wright (1915). These investigators recognized three distinct forms, α, β, and γ, related by transformation as follows: α⇄β at 1420° C. and β⇄γ, at 675° C. The high-temperature α form was described as monoclinic or triclinic and intimately twinned, the β form orthorhombic, and the low-temperature γ form as probably monoclinic. The optical properties of these α and β forms as reported are so closely similar that were it not for the twinning, regarded as diagnostic, the two forms would be distinguishable only with difficulty. This optical similarity was later found to be paralleled by a close agreement of the X-ray powder diffraction patterns of the two phases early noted by Hansen (1928) and confirmed among others by Insley (1936).