Excess heat capacity and entropy of mixing in high structural state plagioclase

Low- and high-temperature heat capacities for a series of synthetic high structural state plagioclase crystals (Ab-An) were measured using both a relaxation calorimeter and a differential scanning calorimeter. The measurements were performed at temperatures between 5 and 800 K on milligrain-sized polycrystalline samples that had been characterized in a previous study. The data show positive excess beat capacities of mixing at temperatures below 300 K with a maximum Value of similar to 2 J/(mol.K). Below similar to 70 K, the excess heat capacities exceed two standard deviations and are thus significant. Above 300 K, the measurements indicate negative excess heat capacities with a maximum of ca. -1.5 R(mol.K) at about 400 K, and do not exceed two standard deviations. The excess vibrational entropies of mixing are positive with an asymmetric variation. At T = 298.15 K, the largest deviation from ideal behavior occurs at Ab(20)An(80) amounting to Delta S(vib)(ex) = 2.8 +/- 2.4 J/(mol.K). An asymmetric Margules mixing model was found to adequately describe the vibrational entropy-composition behavior, yielding W(AbAn)(Svib) = 16.4 J/(mol.K) and = W(AbAn)(Svib) 4.7 J/(mol.K).