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The heterogeneous chemical environment which develops in the heap leaching of some pyrrhotite-containing copper ores can promote covellite and chalcopyrite formation particularly in acid-depleted regions of a heap. In such circumstances, copper recovery will be delayed until the acid and oxidation fronts move through the bed of ore and these secondary copper sulfides are re-leached. The transition from pyrrhotite to chalcopyrite most probably follows the sequence, pyrrhotite to copper-pyrrhotite to unnamed mineral CuFe3S4 to isocubanite to chalcopyrite, with a major structural expansion occurring prior to CuFe3S4. The mechanism is one in which copper is incorporated into pyrrhotite, which maintains its NiAs-type structure up to a stability limit, above which the structure rearranges to a chalcopyrite-like structure followed by isomorphic substitution of copper for iron. The structural rearrangement proceeds with significant expansion in one of the hexagonal axis directions and contractions in the other directions. Depending on the orientation, this expansion induces different levels of strain in the product chalcopyrite. The level of strain subsequently impacts on the rate of chalcopyrite metathesis to covellite. The depth of chalcopyrite formation into the pyrrhotite varies with pyrrhotite orientation.

The distribution of vacancies throughout the underlying NiAs structure of pyrrhotite 5Cwas analysed through the application of vacancy avoidance and the closeness condition in conjunction with order–disorder layering. Two crystallographically equivalent structure solutions (chiral enantiomers) were produced consisting of layers containing one vacancy in every eight iron sites broken by a fully occupied layer every fifth iron layer, and best described by monoclinic statistical models. The statistical 5Cstructures were verified using synchrotron powder diffraction data as well as published electron-diffraction patterns. An order–disorder structure description is proposed for the intermediate pyrrhotites of which pyrrhotite 5Cis an end-member.