A NEW MECHANISM FOR TRANSITION METAL ISOTOPE FRACTIONATION BY SOLID-STATE ION CONDUCTION

The majority of all mined metal is housed in metal sulfide phases, but traditional isotope fractionation mechanisms have generally focused on the equilibrium/kinetics of reactions relevant to the silicate and oxide phases that constitute the bulk of the earth’s crust. As a consequence, the possibility of isotope fractionation mechanisms unique to metal sulfides has remained largely unexplored. Some sulfide and sulfosalt minerals are solid-state fast-ion conductors, meaning that they can conduct ions through their crystal lattice. In order to investigate whether isotope fractionation could be induced by superionic conduction, we chose to examine the mineral argentite (Ag₂S) which has perhaps the highest ionic conductivity of any known material. In nature, the ambient-stable phase, acanthite, is sometimes overgrown by a very unusual and relatively rare mineral habit of native silver that resembles locks of hair. Textural analysis of these “wire silvers” revealed that they are polycrystalline aggregates and likely the result of superionic conduction via the argentite phase. This constitutes the first recognition of mass migration by fast ion conduction in nature.

Based on our mineralogical observations, we developed experiments that enhanced the potential for superionic conduction in Ag₂S to occur, and grew wire silvers in the laboratory for Ag isotope analysis. We also collected a suite of natural wire silvers from around the world, as well as acanthite from the same deposits. Stable Ag isotopes were measured with MC-ICPMS, and compared to other Ag isotope data from the literature. Analysis revealed that natural wire silver is normally enriched in the heavy isotope ¹⁰⁹Ag, while common fractionation mechanisms would predict the opposite. In synthetic wires grown at high temperature (>450°C), this fractionation is amplified by an order of magnitude more than expected by any known isotope effect. This may indicate a previously unrecognized isotope fractionation mechanism associated with superionic conductors in nature and in general, which would have important implications for the geochemistry of sulfide ore deposits, as well as fast-ion technologies including atomic switches and solid-state ion batteries.