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.
ASJC Scopus subject areas
- Geochemistry and Petrology