Direct measurement of the enthalpy of fusion of diopside

Crystalline diopside, CaMgSi₂O₆, was dropped directly into a Setaram "HT1500" calorimeter operating at high temperature. At 1575 < T < 1624 K, the heat content of the crystals was in excellent agreement with previously published results. Above 1634 K, a rise in the enthalpy was seen, supporting the incongruent melting reported by Kushiro. The total enthalpy of fusion at 1665 K, the nominal melting point, is 138.5 kJ/mol, in excellent agreement with measurements which used a cycle that involves dropping the liquid to form a glass and measuring heats of solution of glass and crystals. The heat content of a glassy diopside starting material was also measured. Because the calorimetric experiment lasts only about 8 minutes, these measurements could be extended into the supercooled liquid range (to 1170 K) before the onset of rapid crystallization. A change in slope in the measured heat contents gives a glass transition temperature of 933 K, about 70 K lower than that reported by other methods. A linear fit of all the data above T₈ (supercooled liquid at 970 to 1170 K, stable liquid at 1668 to 1766 K) gives an average heat capacity for the liquid of 332.8 J/mol · K, comparable to the value of 334.6 J/mol · K reported by Richet and Bottinga.