The solubility of the synthesised mimetite was measured in a series of dissolution experiments at 5-55 degrees C and at pH values between 2.00 and 2.75. The solubility product logK(SP) for the reaction Pb-5(AsO4)(3)Cl <-> 5Pb(2+)+3AsO(4)(3-)+ Cl- at 25 degrees C is -76.35 +/- 1.01. The free energy of formation Delta G(f,298)(0) calculated from this measured solubility product equals -2634.3 +/- 5.9 kJ mol(-1). The temperature dependence of the logK(SP) is non-linear, indicating that the enthalpy of the reaction depends on the temperature. The enthalpy of the formation of mimetite Delta H-f,(0) is -2965.9 +/- 4.7 kJ mol(-1), the entropy, Delta S-0, is 39.5 J mol(-1) K-1, and the heat capacity, Delta C-p,f(0) is -6172 +/- 105 J mol(-1) K-1. Hydrochemical modelling indicates that regardless of the composition of the background solution, Pb-5(AsO4)(3)Cl is most stable at neutral to weakly alkaline pH.
