Thermodynamics of natural tourmalines—Dravite and schorl

The experimental data on the heat capacities and enthalpies of formation from the elements of two natural tourmalines: dravite (Na0.53Ca0.31K0.03)(Mg2.72Al6.22Li0.02Ti0.08Fe0.122+)Si5.92B2.92 (OH)2.85F0.27O27.59 and schorl (Na0.82Ca0.13K0.03)(Fe2.232+Fe0.223+Al5.90Mg0.28Mn0.09Li0.02Ti0.11)Si6.04B3.11(OH)3.00F0.02O27.80 have been obtained. The heat capacities were measured by DSC method from T = 270 K to 700 K. High-temperature enthalpy increments were determined on the Tian–Calvet microcalorimeter at interval of 712–973 K using the “drop” method. The values of image for dravite and 794.2 J K−1 mol−1 for schorl were obtained and the resultant image equation in the interval T = 298.15–973 K were calculated: image, (±2.4%) J K−1 mol−1 and image, (±3.7%) at 298.15–973 K J K−1 mol−1, respectively. The standard molar enthalpies of formation from the elements were determined by high-temperature solution calorimetry in molten lead borate at T = 973 K on the Tian–Calvet microcalorimeter. The values of image were found to be −15,410 ± 24 kJ mol−1 for dravite and −14,500 ± 44 kJ mol−1 for schorl. The estimation of thermodynamic properties of six tourmaline end-members was made.