Thermodynamic properties of lithium mica: Lepidolite

Abstract

Calorimetric measurements were made on natural sample of lepidolite having the composition (K 0.80Na 0.05Ca 0.07Rb 0.16Cs 0.03)(Li 1.34Al 1.40Fe 3+ 0.01)[Si 3.25Al 0.75O 10]F 1.80(OH) 0.20 from Na–Li-type rare-element-rich pegmatites of East Sayany, Russia. High-temperature enthalpy increments were measured with a Tian–Calvet calorimeter at 444–972 K using the drop method. The resultant C p , m o ( T) equation in the interval T = 298.15–972 K was calculated: C p , m o = 316.10 + 228.12 × 10 −3 T − 50.10 × 10 5 T −2 (J K −1 mol −1) [± 0.4%] and the value of C p , m o (298.15 K) = 327.8 J K −1 mol −1 was obtained. The standard molar enthalpy of formation from the elements was determined by high-temperature drop solution calorimetry in molten lead borate at T = 973 K. The value of Δ f H m o (298.15 K) for lepidolite was found to be −6201 ± 18 kJ mol −1. The thermodynamic properties of lepidolite of idealized composition KLi 1.5Al 1.5[Si 3AlO 10]F 2 were estimated based on the experimental data obtained.