Thermodynamics and crystal structures of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O

Abstract Synthetic samples of krautite, Mn[AsO3(OH)]⋅H2O, koritnigite, Zn[AsO3(OH)]⋅H2O and cobaltkoritnigite, Co[AsO3(OH)]⋅H2O, were used for calorimetric experiments. For krautite and koritnigite, single-crystal X-ray diffraction was used to determine positions of all atoms, including the H atoms. These data allowed the hydrogen-bond network and the function of H2O molecules in these structures to be determined. The structural formulae are Mn4(H2[3]O)4[AsO3(OH)]4 and Zn4(H2[3]O)2[AsO3(OH)]4(H2[4]O)2, where [3]H2O and [4]H2O are the ‘transformer’ and ‘non-transformer’ H2O groups, respectively. Even though the principal features of these structures are identical, the details, especially those regarding the H2O groups, differ from one structure to another structure in this group. The solubility products (log Ksp) were determined from calorimetric data, that is, from the experimentally measured enthalpies of formation and entropies. They relate to the reaction M[AsO3(OH)]⋅H2O → M2+ + HAsO42– + H2O and are –6.10 for krautite, –6.88 for koritnigite and –6.83 for cobaltkoritnigite. We also estimated the log Ksp for magnesiokoritnigite as –2.0. Calculation of phase diagrams shows that all these phases originate under acidic conditions from solutions with high metal and arsenate concentration. They are restricted to local environments, to pockets that maintain such high concentrations over the time necessary for crystallisation of the krautite-group phases.