Thermochemical study of natural pollucite

Experimental data on the heat capacities, enthalpies of formation from the oxides and elements of the natural pollucites: Cs0.77Na0.14Rb0.04Al0.91Si2.08O6·0.34H2O (Minas Gerais, Brazil) (I) and Cs0.84Na0.11Al0.88Si2.10O6·0.17H2O (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 810 K. A value of C°p,m(298.15 K)=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).