COPPER ISOTOPE FRACTIONATION DURING THE OXIDATIVE PHASE TRANSITION OF SULFIDE MINERALS, CHALCOCITE TO COVELLITE, USING TIME-RESOLVED SYNCHROTRON X-RAY DIFFRACTION

The focus of this research is to understand Cu isotope fractionation associated with the weathering of secondary copper minerals. As shown by previous work, chalcocite (Cu₂S) transforms to covellite (CuS) by the oxidation of Cu¹⁺ to Cu²⁺ and the loss of Cu from the mineral structure. Mathur et al. (2005) have demonstrated that the Cu isotope composition of the solids becomes lighter during this reaction. In the current study, we have explored time-resolved X-ray diffraction as a means of monitoring the many intermediate phases that are part of the reaction sequence.

Pure, naturally occurring chalcocite powders were placed in flow-through capillary reaction cells and exposed to aqueous solutions of 0.01 M and 0.1 M ferric sulfate for ~2 hrs. Real-time diffraction data were collected at intervals of 2 min at beam line X7B, National Synchrotron Light Source, using a MAR345 imaging plate. The δ⁶⁵Cu values of the starting and final powders were measured using a Finnigan Neptune multi-collector inductively coupled plasma mass spectrometer.

The powder diffraction data clearly revealed that chalcocite passes through a complex series of partially oxidized intermediate phases during its transformation to covellite. Reaction kinetics for the 0.1 M ferric sulfate experiments were sufficiently rapid that covellite appeared within 10 min at room temperature, whereas the same reaction progress was only achieved after 90 min with the 0.01 M solution. The greater time resolution afforded by the slower reaction rates allowed observations of the following intermediate phases: djurleite (Cu_1.94S), digenite (Cu_1.80S), anilite/roxybite (Cu_1.75S), geerite (Cu_1.6S), spionkopite (Cu_1.4S), and yarrowite (Cu_1.13S). These intermediate reaction steps were characterized by the presence of 2 or more phases. Preliminary isotope results indicated that the original chalcocite powders had δ⁶⁵Cu values of 0.45+/-0.04‰ and the residual covellite powders had δ⁶⁵Cu values of -14.15+/-0.04‰. Coupling isotopic analyses obtained during the entire reaction sequence with our time-resolved XRD results will make it possible to associate Cu fractionation behavior with specific mineral phases.