Thermodynamic properties of lithium mica: Lepidolite

Calorimetric measurements were made on natural sample of lepidolite having the composition (K0.80Na0.05Ca0.07Rb0.16Cs0.03)(Li1.34Al1.40Fe3+0.01)[Si3.25Al0.75O10]F1.80(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 image(T) equation in the interval T = 298.15–972 K was calculated: image = 316.10 + 228.12 × 10−3 T − 50.10 × 105 T−2 (J K−1 mol−1) [± 0.4%] and the value of image (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 Δfimage(298.15 K) for lepidolite was found to be −6201 ± 18 kJ mol−1. The thermodynamic properties of lepidolite of idealized composition KLi1.5Al1.5[Si3AlO10]F2 were estimated based on the experimental data obtained.