ABSTRACT METAL ISOTOPE FRACTIONATION INDUCED by FAST ION CONDUCTION in NATURAL and SYNTHETIC WIRE SILVER by Calvin J. Anderson A

ABSTRACT METAL ISOTOPE FRACTIONATION INDUCED BY FAST ION CONDUCTION IN NATURAL AND SYNTHETIC WIRE SILVER by Calvin J. Anderson An unusual metal isotope fractionation has been observed in association with the growth of wire silver, whose unique texture and morphology can be explained by superionic + conduction of Ag in Ag2S. This constitutes the first recognition of mass migration by fast ion conduction in nature. Stable Ag isotope analysis revealed 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 ore deposits, as well as fast-ion technologies including atomic switches and solid-state ion batteries. METAL ISOTOPE FRACTIONATION INDUCED BY FAST ION CONDUCTION IN NATURAL AND SYNTHETIC WIRE SILVER A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science by Calvin J. Anderson Miami University Oxford, Ohio 2018 Advisor: John Rakovan Reader: Mark Krekeler Reader: Elisabeth Widom ©2018 Calvin J. Anderson This Thesis titled METAL ISOTOPE FRACTIONATION INDUCED BY FAST ION CONDUCTION IN NATURAL AND SYNTHETIC WIRE SILVER by Calvin J. Anderson has been approved for publication by The College of Arts and Science and Department of Geology and Environmental Earth Science ____________________________________________________ John Rakovan ______________________________________________________ Mark Krekeler _______________________________________________________ Elisabeth Widom Table of Contents Introduction 1 Methods Wire Silver Synthesis 1 Texture Analysis 2 Isotope Analysis 2 Results Texture Analysis 3 Isotope Analysis 6 Discussion Wire Silver Growth Mechanism 7 Isotope Fractionation Mechanism 9 Significance 12 References 13 Appendix: Supplementary Information 17 iii List of Tables Table S1 Stable Ag isotope data for natural and synthetic wire silver 26 Table S2 Sample statistics of δ109Ag analysis 29 iv List of Figures Figure 1 Neutron transmission spectrum, whole specimen 3 Figure 2 Hypothesized wire silver growth process 4 Figure 3 Stable Ag isotope analysis of natural and synthetic wire silver 7 Figure 4 Hypothesized isotope fractionation by predissociation 10 Figure 5 Operation of a CBRAM device 11 Figure S1 SEM image of flame-grown synthetic wire silver 17 Figure S2 Flame-growth apparatus 18 Figure S3 SEM image of natural wire silver 19 Figure S4 SEM image of a flame-grown synthetic wire silver apex 20 Figure S5 SEM image of a flame-grown synthetic wire silver base 21 Figure S6 SEM image of a hydrothermal synthetic wire silver 22 Figure S7 SEM image of real-time in-situ wire growth 23 Figure S8 Relative neutron transmission spectrum of single grain areas 24 Figure S9 Neutron transmission image of natural wire silver 25 v Dedication For Lily, more precious than silver. vi Acknowledgments I would like to thank my colleagues Ryan Mathur (Juniata College) and Anton Tremsin (University of California at Berkeley) for their essential contributions to this research. I would also like to acknowledge Peter Megaw (IMDEX) for helping to make the connection between the mineralogical and geochemical aspects of this work, my collaborators Volker Lüders (German Research Center for Geosciences) and Thomas Böellinghaus (Federal Institute for Materials Research and Testing, Berlin) for providing important insight, Takenao Shinohara and Kenichi Oikawa (Japan Spallation Neutron Source) for their support with neutron imaging experiments, Matthew Gonzales (Laboratory for Isotopes and Metals in the Environment, Penn State University) for assistance with ICP-MS, and Richard Edelmann and Matthew Duley (Center for Advanced Microscopy and Imaging, Miami University) for assistance with SEM and real-time EBSD. I am grateful to Antonio Arribas (Akita University, Japan) and Linda Godfrey (Rutgers University) for reviewing this manuscript. I would also like to thank the following people for providing, and helping to acquire, specimens for analysis: Bryan Lees, Daniel Trinchillo, Andreas Massanek, Debra Wilson, Tomasz Praszkier, Wolfgang Wendel, Terry Huizing, Jack and Pete Heckscher, John Jaszczak, Dan Weinrich, Mark Mauthner. This study was funded by the Mineralogical Research Initiative. vii Introduction Wire silver is an unusual and relatively rare mineral habit of native silver that resembles locks of hair, and which is often intimately associated with acanthite (Ag2S). Wire silver occurs naturally in some of the world’s most historic and productive silver ore districts, including Kongsberg, Norway, and Freiberg, Germany, where it has been known to reach impressive proportions, on the order of tens of cm in length and multiple cm thick1. Anthropogenic wire silver has been known since at least 1574, as it can be grown simply by heating acanthite over a charcoal fire2. More recently, wire silver was found growing spontaneously en masse on certain silver-based industrial power transfer switches where they caused catastrophic electrical failure3. Despite intermittent study over nearly five centuries by researchers from a wide range of disciplines2,4–8, wire silver has never been thoroughly characterized, and consequently has remained poorly understood: only two diffraction patterns9, one crystal section10, and no geochemical data have ever been published. This study presents textural and isotopic characteristics of wire silver that, for the first time, indicate the action of solid-state superionic conduction by natural processes as well as an inverse fractionation of Ag isotopes that cannot be explained by known mechanisms. These findings hold both geological and technological significance, and may lead to the discovery of a new isotope fractionation mechanism. Methods Wire Silver Synthesis. More than a dozen mm-scale synthetic wire silvers were grown at high temperature with methods inspired by traditional and contemporary experiments2,5,11. These provided ideal, clean surfaces for textural comparisons (see Fig. S1). Natural acanthite (Ag2S) was heated to above a threshold temperature of 450°C inside the inner cone of a Bunsen burner flame. To accommodate larger pieces of acanthite, three Bunsen burners were arranged such that their individual flames coalesced into a single one (Fig. S2). This provided the necessary thermal gradient and temperature regime for rapid wire growth, such that samples 3-8 mm long could be grown in as little as a half hour (Fig. S1). Due to the nature of an open flame, it was not possible to exactly monitor the local temperature of the sample within the cone; only the min and max threshold temperatures of about 450°C (boiling point of S) and 700°C (melting point of Ag2S, ref. 12) respectively. 1 Because natural wire silver occurs only in hydrothermal deposits that have never exceeded ~300°C since the primary ore formation, it must form under a different set of low-temperature conditions in nature. Therefore, in order to better imitate the conditions of wire-silver-bearing ore deposits, several wires were grown by little-known hydrothermal methods at low temperature 7 (~130°C) . Typically, each growth experiment consisted of a chunk of acanthite in a Ag2SO4 + potassium antimony tartrate solution, sealed in an acid digestion bomb and heated to various temperatures from 130°-180°C. See refs. 7,13 for more details (also cf. refs. 14–17). Texture Analysis. SEM analysis was conducted on the Zeiss Supra 35 VP FEG SEM equipped with electron backscatter diffraction (EBSD) at Miami University’s Center for Advanced Microscopy and Imaging. SEM images of various specimens (Figs. S1, S3-S6) and real-time observation of silver “blooming” from Ag2S substrates (Fig. S7) can be found in the Supplementary Materials. Energy resolved neutron imaging (ERNI) experiments were conducted at the Materials and Life Science Experimental Facility at Japan Spallation Neutron Source (JSNS). Spatial resolution for this technique is approximately 200µm. Isotope Analysis. Stable silver isotope chemistry (109Ag/107Ag) of wire silvers and acanthites were measured with inductively-coupled plasma mass spectrometry (ICP-MS) on the Thermo Scientific Neptune Plus at Penn State University. In refining our sampling procedure, we determined that a sample size of at least 1 mg was required for consistent results. For this reason, only 3 hydrothermal synthetic wires were suitable for analysis. Sample preparation was carried out according to published methods, and mass bias was corrected using Pd18. Recently, concerns were raised about the reliability of 109Ag/107Ag ratios when the dissolved system contains Cl (accompanied by incomplete recovery of Ag)19, but our samples were essentially Cl-free and great caution was taken in preparation procedures to eliminate possible contamination. The isotope data is plotted in Figure 3. Tabular data (Table S1) and sample statistics (Table S2) are included in the Supplementary Materials. δ109Ag values are reported in per mil (‰), according to the formula: "#$Ag/"#)Ag *+,-./ δ"#$Ag = – 1 ×108 "#$Ag/"#)Ag *0+12+32 2 Results Texture Analysis. Because Ag has a fairly short X-ray attenuation length (about 4.6µm), the bulk crystallinity

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