Thermodynamics of natural tourmaline–elbaite

The first experimental data on the heat capacities, enthalpies of formation from the oxides and elements of a natural elbaite: (Na0.48Ca0.44K0.02)(Li1.45Mn0.05Fe0.033+Mg0.09)Al7.01Si6.00B3.05(OH)2.57F0.44O27.18 (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+366.2×10−3T−241.4×105T−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 ΔfH°m (298.15 K) of elbaite was found to be −15129±19 kJ mol−1. The value of ΔfG°m (298.15 K) was calculated as −14 213 kJ mol−1.