The role of evaporation on the formation of secondary Cu-hydroxy minerals in the arid Namaqualand soil system of South Africa

Abstract

The soils surrounding the Spektakel mine in the arid Okiep copper district of the Northern Cape Province, South Africa have been exposed to Cu containing acid mine drainage (AMD) for well over a century. This has led to acute soil contamination with Cu concentrations present as high as 10wt%. Despite the sulphate-rich environment, associated with AMD, the Cu-hydroxy chloride mineral, atacamite [Cu2(OH)3Cl] is the exclusive secondary Cu mineral identified in the soils. Brochantite [Cu4(OH)6SO4], the sulphate equivalent was not detected. Evaporation is a major hydrologic process in the arid soil system, thus the chemical evolution of the saline, metal containing brine was determined both experimentally and using PHREEQC modelling in the presence and absence of calcite. In samples that evaporated in the absence of calcite, the pH of the evaporating solution progressively decreased to below 3.5, whereas the solution in contact with calcite retained a circumneutral pH throughout the evaporation process. Thus in the absence of calcite, evaporation alone will not favour the formation of Cu-hydroxy minerals. Chemical activities of the evaporating solutions were modelled using the SIT.dat database of PHREEQC. This allowed the inclusion of aqueous complexes into the activity calculations of the evaporating solutions. Contrary to the conservative molality evolution of sulphate, the evolution of the sulphate activity was substantially suppressed due to the formation of MgSO40 aqueous complexes. We propose that these MgSO40 aqueous complexes are responsible for the absence of brochantite whereas the conservative chloride ion is more available for secondary Cu mineral formation. This suggests that ion pairs and aqueous complexes play an important role in determining the speciation of evaporite minerals and should be included in the modelling of such systems.