Thermodynamics of natural tourmalines—Dravite and schorl

Abstract

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=270K to 700K. High-temperature enthalpy increments were determined on the Tian–Calvet microcalorimeter at interval of 712–973K using the “drop” method. The values of Cp,m° (298.15 K)=780.6 J K−1 mol−1 for dravite and 794.2JK−1mol−1 for schorl were obtained and the resultant Cp,m°(T) equation in the interval T=298.15–973K were calculated: Cp,m°=1166.3+80.5×10−3T−364.2×105T−2, J K−1 mol−1, (±2.4%) JK−1mol−1 and Cp,m°=1016.5+346.8×10−3T−289.5×105T−2, J K−1 mol−1, (±3.7%) at 298.15–973KJK−1mol−1, respectively. The standard molar enthalpies of formation from the elements were determined by high-temperature solution calorimetry in molten lead borate at T=973K on the Tian–Calvet microcalorimeter. The values of ΔfHm° (298.15 K) were found to be −15,410±24kJmol−1 for dravite and −14,500±44kJmol−1 for schorl. The estimation of thermodynamic properties of six tourmaline end-members was made.