Thermodynamics of vivianite-group arsenates M₃(AsO₄)₂?8H₂O (M is Ni, Co, Mg, Zn, Cu) and chemical variability in the natural arsenates of this group

In this work, we investigated theM(3)(AsO4)(2)& sdot;8H(2)O end members annabergite (M is Ni), erythrite (M is Co), and hornesite (M is Mg) and their solid solutions. Acid-solution calorimetry and relaxation calorimetry were used to determine the solubility products ( log K sp ) for annabergite ( - 33.7 ), erythrite ( - 32.1 ), and hornesite ( - 22.3 ). Solubility products for other end members of this group were extracted from the literature and critically evaluated. The enthalpies of mixing are complex, related to subsystems M(1)(3)(AsO4 )(2)& sdot;8H(2)O-M(1)M(2)(2)(AsO4)2 & sdot;8H(2)O and M(1)M(2)(2)(AsO4)(2)& sdot;8H(2)O-M(2)(3)(AsO4)2 & sdot;8H(2)O. They are small and positive for the annabergite-erythrite solid solution and small and negative for the annabergite-hornesite solid solution. Autocorrelation analysis of Fourier-transform infrared (FTIR) spectra shows correlation of strain decrease in the structure with the negative enthalpies of mixing in the annabergite-hornesite solid solution. A set of more than 600 electron microprobe analyses of the M-3(AsO4)(2)& sdot;8H(2)O minerals documents the variability and complexity in this group. Most common compositions are those dominated by Ni, Co, or Ni-Co. The analytical results were used to calculate the maximal configurational entropies which could be a factor that compensates for the small enthalpies of mixing in the annabergite-erythrite solid solution. The data presented here can be used to model sites polluted with metals and arsenic and to enhance our understanding of complex solid solutions.