Thermochemical study of natural pollucite

Abstract

Experimental data on the heat capacities, enthalpies of formation from the oxides and elements of the natural pollucites: Cs0.77Na0.14Rb0.04Al0.91 Si2.08O6·0.34H2O (Minas Gerais, Brazil) (I) and Cs0.84Na0.11Al0.88Si2.10 O6·0.17H2O (East Siberia, Russia) (II) have been determined. The heat capacities of the pollucite (II) were measured by differential scanning calorimetry (DSC) method from T = 250 to 810K. A value of C°p,m(298.15K) = 172.2 J K-1 mol-1 was obtained and the resultant C°p,m(T) equation in the interval T = 298.15-610 K was calculated: C°p,m = 131.37 + 181.97 × 10-3T - 11.84 × 105T-2 J K-1 mol-1 (±0.36%). The standard enthalpies of formation were determined by a set of thermochemical cycles using decomposition and subsequent dissolution of the dehydration products in molten lead borate at T = 973 K. Values of ΔfH°m (298.15 K) are found to be -3104 ± 13 kJ mol-1 (pollucite I) and -3090 ± 14 kJ mol-1 (pollucite II). The values of ΔfG°m (298.15 K) were calculated as -2921 kJ mol-1 (pollucite I) and -2911 kJ mol-1 (pollucite II). © 2003 Elsevier Science B.V. All rights reserved.