Thermodynamic properties of synthetic calcium-free carbonate cancrinite

Abstract

Calcium-free carbonate cancrinite with formula unit Na8.28[Al5.93Si6.07O24](CO3)0.93(OH)0.49·3.64H2O (CAN) has been synthesized under hydrothermal conditions. The product has been characterized by the methods of scanning electronic microscopy and energy dispersive X-ray analysis, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis with FTIR of evolved gases (TGA–FTIR), and X-ray powder diffraction. The heat capacity of CAN has been measured from 6 to 259 K via low-temperature adiabatic calorimetry. A linear combination of Einstein functions has been used to approximate the obtained data on the heat capacity. The thermal contributions to the entropy and enthalpy of CAN in the temperature range 0–300 K have been calculated from these data. The heat capacity and third-law absolute entropy of CAN at 298.15 K are 1,047 ± 30 and 1,057 ± 35 J mol−1 K−1, respectively. High-temperature oxide-melt solution calorimetry has been used to determine the enthalpy of formation from elements of CAN at 298.15 K; the value equals −14,684 ± 50 kJ mol−1. The Gibbs energy of formation from elements at 298.15 K has been calculated and totaled −13,690 ± 51 kJ mol−1.