Thermodynamics of natural tourmaline–elbaite

Abstract

The first experimental data on the heat capacities, enthalpies of formation from the oxides and elements of a natural elbaite: (Na 0.48Ca 0.44K 0.02)(Li 1.45Mn 0.05Fe 0.03 3+Mg 0.09)Al 7.01Si 6.00B 3.05(OH) 2.57F 0.44O 27.18 (Malkhan pegmatite field, east Siberia, Russia) have been obtained. The heat capacities of elbaite were measured by DSC method from T=270 to 540 K. High-temperature enthalpy increments were determined on the Tian–Calvet microcalorimeter at interval of 569–1023 K using the drop method. A value of C° p,m (298.15 K)=757.3 J K −1 mol −1was obtained and the resultant C° p,m( T) equation in the interval T=298.15–1023 K was calculated: C° p,m=919.7+366.2×10 −3 T−241.4×10 5 T −2 ( K −1 mol −1) (±1.4%). The standard molar enthalpy of formation from the elements was determined by high-temperature solution calorimetry in molten lead borate at T=973 K on the Tian–Calvet microcalorimeter. The value of Δ f H° m (298.15 K) of elbaite was found to be −15129±19 kJ mol −1. The value of Δ f G° m (298.15 K) was calculated as −14 213 kJ mol −1.