The thermodynamic properties of natural margarite

Abstract

Thermal, IR spectroscopic, and thermochemical studies of natural brittle mica, margarite Ca1.00Na0.10Mg0.02Al3.89Fe0.013+Si2.03Ti0.01O10(OH)1.74F0.26, were performed. The enthalpy of formation of natural margarite from the elements (−6269 ± 12 kJ/mol) was determined by melt solution calorimetry on a high-temperature heat-conducting Calvet microcalorimeter (Setaram, France). Enthalpy growth over the temperature range 298.15–973 K was determined by the drop method. Equations for the temperature dependences of the enthalpy and heat capacity were obtained, H°(T)−H°(298.15 K), J/mol = 435.21T + 36.46 × 10−3T2 + 109.91 × 105/T − 169863 and C°p, J/(mol K) = 435.21 + 72.92 × 10−3T − 109.91 × 105/T2. The experimental data were used to estimate the thermodynamic properties of margarite of the theoretical composition, CaAl2[Al2Si2O10](OH)2.