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)(2.85)F0.27O27.59 and schorl (Na0.82Ca0.13K0.03)(Fe2.232+Fe0.223+Al5.90Mg0.28Mn0.09Li0.02Ti0.11)Si6.04B3.11(OH)(3.00)F0.02O27.80 have been obtained. The heat capacities were measured by DSC method from T= 270 K to 700K. Hightemperature enthalpy increments were determined on the Tian-Calvet microcalorimeter at interval of 712-973K using the drop method. The values of C-p,C-m degrees (298.15 K) = 780,6J K-1 mol(-1) for dravite and 794.2J K-1 mol(-1) for schorl were obtained and the resultant C-p,C-m degrees (T) equation in the interval T = 298.15-973 K were calculated: C-p,C-m degrees = 1166.3 + 80.5 x 10(-3)T - 364.2 x 10(5)T(-2). J K-1 mol(-1), (+/- 2.4%) J K-1 mol(-1) and C-p,C-m degrees = 1016.5 + 346.8 x 10(-3)T -289.5 x 10(5)T(-2). J K-1 mol(-1). ( 3.7%) at 298.15-973 KJ 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 Delta H-f(m)degrees-1 (298.15K) 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. (C) 2012 Elsevier B.V. All rights reserved.