Environmental factors controlling the precipitation of Cu-bearing hydrotalcite-like compounds from mine waters. The case of the “Eve verda” spring (Aosta Valley, Italy)

In the Cu-Fe-sulphide mining district of Servette-Chuc (Saint-Marcel, Aosta Valley, Italy), a streambed called Eve Verda and covered with a colloidal blue-green precipitate has been known since the 18th century. X-ray and chemical analyses reveal that the precipitate is a mixture of a nanocrystalline Cu-Al-sulphate hydrotalcite-like compound close to hydrowoodwardite (Cu1-xAlx [SO4]x/2[OH]2·mH2O) (49 ± 10 vol.%), and amorphous Al and Si oxyhydroxides. The Al /(Al + Cu) ratio of hydrowoodwardite (i.e., x) ranges between 0.10 and 0.30 with a mean value of 0.17 ± 0.04. The average formula may be expressed as follows: Ci0.81Zn0.02 Al0.17(SO4)0.08 (OH)2·1.10 H2O. In order to construct phase diagrams, we estimated the thermodynamic properties of the hydrowoodwardite solid solution starting from calorimetric measurements available in the literature, i.e., those for Mg, Co, Ni and Zn hydrotalcites. At least for x < 0.35, hydrowoodwardite may be treated either as an ideal or as a non-ideal solid solution between the two end-members Cu(OH)2 (spertiniite) and Al(SO4) 0.5(OH)2 (aluminite). For x = 1/3 (i.e., woodwardite sensu stricto following IMA nomenclature) and m = 0, the ideal solid solution model provided a Gibbs free energy of formation (ΔGf0) of -684.4 kJ/mol and an enthalpy of formation (ΔHf0) of -795.0 kJ/mol, while the non-ideal solution model yielded ΔGf0 = -658.6 kJ/mol and ΔHf0 = -767.5 kJ/mol. The estimated data agree well with the solubility measurements reported for hydrotalcites. Since hydrowoodwardite precipitates where mineralised, acidic waters from an ephemeral stream are mixing with alkaline, diluted waters of a perennial spring, we calculated pH-Xw pseudosections (i.e., isochemical phase diagrams), where Xw is the molar proportion of the two mixing waters. The thermodynamic modelling indicates that the formation of hydrowoodwardite is related to a geochemical barrier represented by the perennial spring waters. It also demonstrates that the composition of hydrowoodwardite (i.e., x) is mainly a function of pH, while the modal composition of the precipitate depends instead on Xw. This explains the variations observed in the precipitate composition at Eve Verda.