Polymorphism of Calcium Orthosilicate

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Abstract
Therecentobservation of orthorhombic α'‐Ca 2 Si0 4 (bredigite) at all temperatures between about 850° and 1450°C. leads to a rational interpretation of the polymorphism of this substance, which is very satisfactory from the crystal‐chemical point of view. The so‐called β phase, of as yet unknown, complex structure, exists only meta‐stably, and not above but below about 675°C, where γ is the stable phase. The monotropic β phase forms on cooling from α’near 675°C. as the result of the inhibition of the α’→γ inversion, at 850°C. The inhibition is caused by the need for a considerable atomic rearrangement and a 12% volume increase which accompanies the change of the coordinations CaO 2 and CaOlo, in α', to CaO 6 , in γ. Among the solid phase equilibria with, Mg 2 SiO 4 and Ca 3 (PO 4 ) 2 , unlimited solid solubility between γ‐Ca 2 SiO 4 and Mg 2 SiO 4 is predicted, whereas the solubility of Mg 2 SiO 4 is a limited one in α’and still more so in a‐Ca 2 SiO 4 , as a result of the substitution, for calcium, of the smaller magnesium.
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