Scanning calorimetric measurement of heat capacity during incongruent melting of diopside

Scanning calorimetric measurements (in step-scan mode) using a Setaram HT1500 calorimeter were performed on initially crystalline diopside (CaMgSi₂O₆) between 1403 and 1762 K and provide a direct measurement of heat capacity. Incongruent melting begins approximately 59 K below the melting point reported previously of 1665 K. Backscattered electron imaging confirms that the starting crystalline sample is a single phase at a resolution ≥1 μm. The diopside is stoichiometric to ±2% (resolution of electron microprobe analysis). The observed incongruency, which has been noted previously, is therefore intrinsic to pure CaMgSi₂O₆ and not related to the presence of other phases or to deviations in the Ca/Mg ratio from unity. -from Authors