Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: I. Volume and heat capacity of mixing

Abstract

The unite cell parameters and heat capacities of a series of synthetic clinopyroxenes on the join Ca-Tschermak (CaTs)−diopside (Di) were measured using X-ray powder diffraction and calorimetric methods, respectively. The volume of mixing at 298 K shows a negative asymmetric deviation from ideality. A two-parameter Margules fit to the data yields W ^V_CaTs−Di  = −0.29 ± 0.11 cm³ mol⁻¹ and W ^V_Di−CaTs  = −1.14 ± 0.14 cm³ mol⁻¹. Heat capacities were determined between 5 and 923 K by heat-pulse at 5−302 K and differential-scanning calorimetry at 143−923 K. The precision of the low and high temperature C _p data is better than ±1%. Polynomials of the form C _p  = a + bT ^−1/2 + cT ⁻² + dT ⁻³ were fitted to the C _p data in the temperature range between 250 and 925 K. Thermal entropy values [S ₂₉₈ − S ₀] and [S ₉₀₀  −S ₀] as well as enthalpies [H ₂₉₈ − H ₀] and [H ₉₀₀ − H ₀] were calculated for all members of the solid solution series. No significant deviation from ideal mixing behavior was observed.