Low temperature measurements of the heat capacity and thermodynamic functions of pseudo-malachite Cu5(PO4)2(OH)4

Abstract

The investigation of the heat capacity of a natural specimen of copper phosphate—pseudo-malachite Cu5(PO4)2(OH)4 in the temperature range between 4.2K and 320K has been carried out by the method of low-temperature adiabatic calorimetry. Tabulated values of the heat capacity and thermodynamic functions of the mineral including the changes of entropy and enthalpy and the Gibbs function of free energy have been calculated. The standard values of thermodynamic functions of pseudo-malachite at T=298.15K are Cp,m°=(385.43±0.41) J mole−1 K−1, Δ0TSm°=(412.16±0.61) J mole−1 K−1, Δ0THm°=(63681.5±57.0) J mole−1, Fm°=(198.57±0.47) J mole−1 K−1. In the low-temperature area 0<(T/K)<22.69 an anomaly has been registered in the variation of the heat capacity of the mineral, for which the lattice constituent has been determined. The contribution of anomalous component into entropy and enthalpy of the mineral is ΔStr=(5.772±0.081)Jmole−1K−1, ΔHtr=(29.94±0.42)Jmole−1.