Fluid-Enhanced Coarsening of Mineral Microstructures in Hydrothermally Synthesized Bornite–Digenite Solid Solution

Symplectic microstructures are abundant in copper–iron–sulfide minerals and are conventionally considered to form by solid-state diffusion processes. Here we experimentally demonstrate that coarsening of exsolution lamellae occurs ∼1000 times faster in the presence of a fluid compared to the equivalent dry system. Bornite–digenite solid solutions (Cu5FeS4–Cu8.52Fe0.11S4.88) were synthesized hydrothermally via the replacement of chalcopyrite, and we compared the microtextures in the product subjected to different cooling histories: (i) dry annealing after synthesis; (ii) cooling to an annealing temperature immediately following hydrothermal synthesis; and (iii) annealing in a hydrothermal fluid following quenching to room temperature and then reheating. We interpret the rapid coarsening of the exsolution lamellae in the presence of a fluid phase to result from recrystallization associated with healing of the open porous microstructure in the parent phase. The porosity is a consequence of the synthesis of t...