Abstract
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 of ≥ 1 μm. The diopside is stoichiometic 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. The first 20% of melting occurs over an interval of 44 K and the remaining 80% within 15 K. Measurements of heat capacity of crystalline and liquid diopside (outside the melting interval) scatter within 5 and 12%, respectively, of the accepted literature values. The integrated heat contents, in both the crystalline and liquid regions, are within 0.3% of reported values. The enthalpy of fusion is 137.7 kJ/mol, in excellent agreement with previous determinations (Stebbins et a1., 1983; Richet and Bottinga, 1984;Ziegler and Navrotsky, 1986). Continuous scanning measurements during cooling of diopside liquid at different rates produce a large heat effect at temperatures well below the melting point reported previously. This heat effect can be correlated with the simultaneous crystallization of a clinopyroxene solid solution and wollastonite. Scanning measurements at a constant cooling rate of l0 K/h were repeated three times under identical conditions; each time the observed heat effect, indicating crystallization, took place at a different temperature (1576,1534, and 1566 K). These observations are in general agreement with the kinetic crystallization study of Kirkpatrick et al. (1981), although metastable crystallization of forsterite was not detected in our cooling experiments.
