Thermodynamics of vivianite-group arsenates M 3 (AsO 4 ) 2 ⋅  8H 2 O ( M is Ni, Co, Mg, Zn, Cu) and chemical variability in the natural arsenates of this group

In this work, we investigated the M 3 (AsO 4 ) 2 ⋅  8H 2 O end members annabergite ( M is Ni), erythrite ( M is Co), and hörnesite ( 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 hörnesite ( - 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 (AsO 4 ) 2 ⋅  8H 2 O– M (1) M (2) 2 (AsO 4 ) 2 ⋅  8H 2 O and M (1) M (2) 2 (AsO 4 ) 2 ⋅  8H 2 O– M (2) 3 (AsO 4 ) 2 ⋅  8H 2 O. They are small and positive for the annabergite–erythrite solid solution and small and negative for the annabergite–hörnesite 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–hörnesite solid solution. A set of more than 600 electron microprobe analyses of the M 3 (AsO 4 ) 2 ⋅  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.