Collaborative Research: A New Mechanism for Metal Isotope Fractionation Induced by Natural Solid-State Ion Conduction

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Naturally occurring metallic silver is sometimes found with a wire-, string- or rope-like morphology, commonly called “wire silver”. Until recently, very little was known about the structural characteristics of wire silver and even less about how it forms. Surprisingly, this morphology can be created in the laboratory under simple conditions very different from those found in nature, yet their structural characteristics are very similar to natural samples. From the study of natural and synthetic specimens, it has been found that wire silver formation occurs by a solid-state process called “ion conduction”, where metal ions can move directly through a crystal structure, and in this case, silver(I) ions move through the mineral argentite. Even more surprising, this ion transport during wire growth causes an enrichment of the heavy stable isotope 109Ag in the wire. This is opposite of that predicted by mechanisms of isotope enrichment known to occur in nature. Thus, further study of wire silver growth may elucidate a previously unrecognized mechanism for isotope separation in nature. Understanding this mechanism and the degree of possible isotope enrichment is significant for our basic understanding of physical and chemical phenomena. These in turn have direct application to the study of geochemical and geological processes such as metal transport and the isotopic signatures found in ore deposits. Broader impacts include possible technological applications where the isotope chemistry of materials can affect their physical properties. This study involves scientists from multiple academic departments and different disciplines, and will include student training and participation at multiple levels, from undergraduates through doctoral candidates. This project integrates experimental mineralogy, analytical geochemistry and computational methods in the study of wire silver formation under highly controlled conditions as a means to elucidate the fundamental mechanism of the observed isotope effect. Isotope fractionation is hypothesized to result from the process of predissociation during superionic conduction, occurring preferentially for 109Ag+ ions as they move through the silver sulfide structure. The study will also test whether this isotope effect extends to other transition metals such as copper, zinc and nickel. Understanding the mechanisms, magnitude and direction of isotope fractionation is essential for the use and accurate interpretation of isotope data in geological studies, and allows for the engineered development of materials with “tuned” isotope chemistries. Finally, this unexploited physical phenomenon might be leveraged to improve the characteristics of technological devices that utilize fast ion conduction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.