Powdered mixtures of synthetic uvarovite [a = 12.010(l)Å n; = 1.855(5)] and synthetic spessartite [a = 11.615(1)Å; n = 1.800(5)] held at 27.5 kbar and l450°C for 48 hours yielded continuous solid solutions for which the cell edge and refractive index varied linearly with composition. At one atmosphere pressure in air, partial solubilities were found up to 20 mole percent from each end member. In air uvarovite is stable up to 1375(5)°C, spessartie to 116°C, and a 50–50 percent mixture breaks down at 1050°C, the minimum melting temperature in the solid solution series. The partial solubility observed at one atmosphere in air might be caused by the instability of Mn2+ and the low temperature of reaction.

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