COVER: Wire silver with calcite from Kongsberg, Norway (7 cm tall). Wire

silver is the first known example of mineral growth by solid-state ion conduction in

nature. See #openaccess paper “Natural solid-state ion conduction induces metal

isotope fractionation” by Anderson et al.,

https://pubs.geoscienceworld.org/gsa/geology/article/47/7/617/570312/natural-

solid-state-ion-conduction-induces-metal

Photo by: Jeff Scovil; specimen from Dave Bunk Minerals

Cover design by: Eric Christensen

Geological Society of America | GEOLOGY | Volume 47 | Number 7 | www.gsapubs.org 617

Natural solid-state ion conduction induces metal isotope fractionationCalvin J. Anderson1, Ryan Mathur2, John Rakovan1, and Anton S. Tremsin3

1Department of Geology and Environmental Earth Science, Miami University, 118 Shideler Hall, 250 S. Patterson Avenue, Oxford, Ohio 45056, USA

2Geology Department, Juniata College, 1700 Moore Street, Huntingdon, Pennsylvania 16652, USA3Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, California 94720, USA

ABSTRACTSolid-state ion conduction (SSIC) occurs in the formation of natural and synthetic wire

silver and causes consistent isotope fractionation of the mobile ion, favoring the heavy isotope. Textural analysis of natural and synthetic samples revealed that wire silver is a mosaic-like polycrystalline aggregate with superimposed striations, consistent with very rapid basal ad-dition of Ag atoms constrained within a lateral growth footprint at the Ag-Ag2S interface. Growth experiments demonstrate that this process is fundamentally dependent not on the chemical environment, but only on the SSIC ability of the substrate, readily provided in this case by argentite (Ag2S), a superionic-conducting material. Stable Ag isotope analysis of wire silvers provides a means to observe the geochemical effects of SSIC in Ag2S. Natural samples were found to be enriched in the heavy isotope with a median (interquartile range) of +0.283‰ (+0.145‰ to +0.453‰). Furthermore, 109Ag enrichment was amplified by an order of magnitude in synthetic samples grown at high temperature (>450 °C), which had a median δ109Ag of +2.788‰ (+1.829‰ to +3.689‰). Known isotope fractionation mechanisms would indicate that SSIC products should have negative δ isotope values because normal reaction kinetics are more likely to mobilize the lighter isotope. This indicates a previously unrecog-nized isotope fractionation mechanism associated with SSIC in nature, and has important implications for the geochemistry of ore deposits where SSIC phases are present.

INTRODUCTIONOur understanding of the geochemistry and

transport of metals in nature is mostly built on liquid-phase interactions. The possible roles of solid-state processes, however, have remained relatively unexplored. This is surprising given that a number of important sulfide, halide, and sulfosalt minerals are documented solid-state ion conductors (Hull, 2004; Bindi et al., 2006; Bindi and Menchetti, 2011). Solid-state ion con-duction (SSIC) describes the process by which metal ions can be rapidly transported through a crystal lattice in response to a thermoelectric potential, in contrast to diffusion, which is com-paratively much slower and whose driving force is a concentration, stress, or thermal gradient. However, evidence of SSIC by natural geologic processes has not been previously identified.

To investigate the possibility of SSIC in natural systems, we chose to study the mineral

acanthite (Ag2S) for several reasons: (1) its high-temperature polymorph argentite possesses the highest ionic conductivity of any known mate-rial (Hull, 2004; Kato et al., 2016), (2) it is an important and relatively common ore mineral (commonly found as paramorphs after argen-tite) (Ercker, 1951; Fontboté et al., 2017), and (3) it is sometimes intimately associated with an unusual and poorly understood habit of native silver, commonly called “wire silver” (see Fig. DR1 in the GSA Data Repository1; Anderson and Rakovan, 2017; Böllinghaus et al., 2018).

Observations from previous experimental studies strongly suggest that solid-state pro-cesses are involved in wire silver formation. Wire silver can be grown by simply heating acanthite (Ercker, 1951), and if carried out in contact with solid Ag, the native Ag dissolves into the sulfide resulting in excessive wire growth (Beutell, 1919). Provided that the ther-

mal gradient is maintained (Beutell, 1919), this reaction can proceed in air (Edwards, 2001), water (Jensen, 1939), and even a vacuum with no other reagents (Kohlschütter and Eydmann, 1912), which implies that Ag2S and its physi-cal properties alone are sufficient for wire silver growth to occur.

We report the first direct evidence that not only does SSIC occur in natural systems, but can also result in geochemically significant metal mobilization and isotope fractionation. Two lines of inquiry were followed to identify SSIC in native wire silver formation through comparison of natural and synthetic samples. First, textural-mineralogical analysis served to evaluate the role of SSIC in wire silver growth. Then, stable Ag isotope analysis of wire silver provided a means to determine the geochemical effects of SSIC on the mobile ions.

METHODS

Wire Silver SynthesisSynthetic wire silvers were grown at high

temperature with open-flame methods inspired by the literature (Bischof, 1843; Kohlschütter and Eydmann, 1912; Beutell, 1919; Edwards, 2001; Ercker, 1951). Wires 3–8 mm long could be grown in as little as a half-hour (Figs. DR2 and DR3). In some experiments, plates of solid silver were placed in contact with the acanthite, which dramatically increased the volume of wire growth. 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 minimum and maximum threshold tempera-tures of ~450 °C and 700 °C (melting point of Ag2S; Frueh, 1961) respectively. In order to bet-ter imitate the conditions of wire silver–bearing

1GSA Data Repository item 2019221, additional images (Figures DR1–DR13), Movie DR1, and tabular isotope data (Tables DR1 and DR2), is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org.

CITATION: Anderson, C.J., Mathur, R., Rakovan, J., and Tremsin, A.S., 2019, Natural solid-state ion conduction induces metal isotope fractionation: Geology, v. 47, p. 617–621, https:// doi .org /10 .1130 /G45999.1

Manuscript received 11 January 2019 Revised manuscript received 12 April 2019

Manuscript accepted 15 April 2019

https://doi.org/10.1130/G45999.1

© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 2 May 2019

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ore deposits, several wires were also grown from hydrothermal solutions by following procedures from Jensen (1939).

Texture AnalysisBecause Ag has a fairly short X-ray attenu-

ation length (~4.6 µm), the bulk crystallinity of natural and synthetic wire silver was probed with energy resolved neutron imaging (ERNI) with a spatial resolution of ~200 µm (Tremsin et al., 2017). ERNI was conducted at the Japan Spalla tion Neutron Source (JSNS; Tokai, Japan). Surface textures were analyzed with scanning electron microscopy (SEM) (Anderson and Rako van, 2017). Neutron transmission data (Figs. DR8–DR10), SEM images of various specimens (Figs. DR3–DR7), and real-time observation of silver “blooming” from Ag2S substrates (Figs. DR11and DR12) can be found in the Data Repos-itory. SEM analysis was conducted on the Zeiss Supra 35 VP FEG SEM equipped with electron backscatter diffraction (EBSD) at Miami Uni-versity’s Center for Advanced Microscopy and Imaging (Oxford, Ohio, USA).

Stable Ag Isotope AnalysisStable Ag isotope chemistry (109Ag/107Ag) of

wire silvers and acanthites was measured with multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) on the Thermo Sci-entific Neptune Plus at Penn State University (University Park, Pennsylvania, USA). In refin-ing our sampling procedure, we determined that a sample size of at least 1 mg was required for consistent results. For this reason, only three hydrothermal synthetic wires were suitable for analysis. Wires were dissolved in 4 ml of 8N HNO3 and then purified using ion exchange chromatography (Mathur et al., 2018). The U.S. National Institute of Standards and Technology (NIST) standard used had a 107Ag/109Ag ratio of 1.07638. Sample preparation was carried out according to published methods, and mass bias was corrected using Pd (Mathur et al., 2018). Recently, concerns were raised about the reli-ability of 109Ag/107Ag ratios when the dissolved system contains Cl resulting in incomplete re-covery of Ag (Fujii and Albarede, 2018). There-fore, great caution was taken during preparation to eliminate possible contamination, and our samples were essentially Cl free. Data plots and sample statistics were generated with R software (https:// www .r -project .org; R Core Team, 2018). Tabular data (Table DR1) and sample statistics (Table DR2) are included in the Data Repository.

RESULTS

Textural Evidence for Solid-State Ion Conduction in Nature

Wire silver, both natural and synthetic, was found to be a mosaic-like polycrystalline ag-gregate composed of elongate crystallites,

commonly with a low aspect ratio (Fig. 1A). This texture is in stark contrast to that of metal whiskers (e.g., Sn, Cd, Li), which are generally low-defect single crystals that form by pressure gradients or screw dislocations (Compton et al., 1951; Morris and Bonfield, 1974; Galyon, 2004; Li et al., 2011). The longitudinally parallel stria-tions, which give wire silver its characteristic bundle-like morphology, are commonly cross-cut by crystallites, indicating that the striations are not controlled by planes of coherent grain boundaries, but rather by an external constraint during growth. These morphological-textural relationships are consistent across a wide range of growth conditions and scales (Figs. DR5–DR8) and with published EBSD pole figure maps of wire silver crystal sections (Bölling-haus et al., 2018). It bears repeating that the morphological and textural characteristics of natural and synthetic wire silvers are virtually indistinguishable.

Only two processes, namely, mechanical extrusion or laterally constrained base growth, could produce this combination of unique mor-phology and internal texture. However, mechan-ical extrusion can be ruled out because (1) the crystal growth steps would have been erased, but instead were commonly observed on wire sur-faces, (2) Ag2S is too malleable to provide a rigid mechanical counterpressure (Liversidge, 1877), and (3) we observed no conduits or apertures in the host acanthite through which extrusion would have occurred. In contrast, laterally con-strained base growth at the Ag-Ag2S interface can simultaneously explain the unique striated morphology and the polycrystalline texture of wire silver.

Scanning electron microscopy (SEM) not only yielded information about the surface texture of wire silver, but also provided direct observation of the initial stages of growth in real time. Upon prolonged exposure of Ag2S to the electron beam, horn-like growths of Ag spontaneously “bloomed” from the surface (see Fig. 1B). As growth progressed, patches of these blooms coalesced and became raised up on Ag2S pedestals (Fig. DR9) similar to pedestals ob-served on many natural and synthetic wire silver specimens (Fig. 1C; Fig. DR11). This phenom-enon has been reported before by Sadovnikov et al. (2015), who showed it to be accompanied by a transformation of the acanthite substrate (space group P21/n; Hahn, 2005) into argentite (space group Im3m; Hahn, 2005) caused by “radiation heating.” Although that study mis-identified the blooms themselves as Ag2S, we confirmed with EBSD that the blooms are in-deed native Ag (Fig. DR12).

Previous studies have shown that during wire silver growth, Ag+ ions migrate through the bulk solid to the growth interface, even in a vacuum (Kohlschütter and Eydmann, 1912; Beutell, 1919). There are only two mechanisms

by which this entirely solid-state migration could occur: diffusion or SSIC. However, the extremely rapid growth rate of wire silver in our experiments (millimeters and centimeters in seconds and minutes) indicates extremely rapid migration of Ag, much too great to be explained by simple diffusion.

C

20 µm

Ag2S

Ag0

A

10 µm

B

2 µm

Ag0

Arg

Figure 1. Scanning electron microscopy im-ages of wire silver showing various aspects of characteristic morphology and growth. A: Synthetic wire silver showing represen-tative morphological and textural charac-teristics associated with wire apex. Note similarity of horn-like features at head to appearance of blooms in B. Dashed red lines highlight two different crystallites, one of which cross-cuts several striations. B: Patches of native Ag (Ag0) “blooms” on developing argentite (Arg) pedestals cap-tured after 30 min of exposure at 10 kV. C: Synthetic wire silver showing base and near-vertical acanthite (Ag2S) pedestal rep-resentative of both synthetic and natural wires on millimeter scale; acanthite (rather than argentite) pedestal is expected in hand specimens because argentite reverts to acanthite under ambient conditions. Loca-tion of pedestal is shown by red arrow.

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Therefore, we propose a new growth mecha-nism for wire silver that involves base growth by SSIC of Ag+ ions via a local displacive phase transformation of Ag2S from acanthite to argen-tite, where variable but high ion flux leads to a high number of defects, and in turn, generates many crystallites, which are constrained later-ally by the argentite footprint (Fig. 2). Given that the morphological and textural characteristics of natural and synthetic wire silver are virtually indistinguishable, this appears to represent the first mineralogical evidence for SSIC in natu-rally occurring samples.

Stable Ag Isotope AnalysisStable Ag isotope analysis (Fig. 3) revealed

that natural wire silvers are generally enriched in the heavy isotope 109Ag, with a δ109Ag of me-dian (interquartile range) +0.283‰ (+0.145‰ to +0.453‰). Low-temperature (LT) synthetic wires were generally consistent with the range for natural wire silver. High-temperature (HT) synthetic wires were significantly more enriched in 109Ag, with a median δ109Ag of +2.788‰ (+1.829‰ to +3.689‰), and values as large as +6.218‰ were observed. This is an order of magnitude more fractionation than that of any natural silver system. The disparity between LT and HT wire silvers mirrors the fact that the rate of SSIC in argentite is greater at high tempera-tures (Hull, 2004). Thus, the large spread in the HT data is likely due to variations in the Ag+ ion conduction rate, possibly caused by variable hindrance of SSIC by minor and/or trace ele-ment impurities, or by temperature fluctuations inherent in the growth method (see Methods). There was no apparent correlation between the δ109Ag values of synthetic wires and the initial δ109Ag of their respective acanthites.

DISCUSSIONThe most interesting result is that the heavy-

isotope enrichment we observed, especially from the HT wire syntheses, is opposite of that predicted by known fractionation mechanisms. These include mass-dependent, kinetic, equilib-rium, and redox isotope effects. Mass-dependent isotope effects driven by passive diffusion would produce enrichment in 107Ag because it is lighter and would have a higher diffusion rate, and al-though this should increase with temperature, it would still be several orders of magnitude too small to explain the huge increase in Ag fractionation in our HT wires (Criss, 1999). A kinetic isotope effect would lead to an expecta-tion that 109Ag-S bonds would have a lower zero-point energy than 107Ag-S bonds and be harder to break, and thus 109Ag would be less mobile in the lattice, leading to wire enrichment in the more mobile 107Ag (Criss, 1999). In equilibrium fractionation, 107Ag-S bonds would have a higher vibrational energy because 107Ag is lighter, and so the isotope exchange reaction would favor

the formation of 109Ag-S bonds, freeing more 107Ag to enrich the wire silver; moreover, the magnitude of this effect would decrease with increasing temperature (Criss, 1999). Recently, a redox isotope effect was observed by Mathur et al. (2018) where precipitation (involving re-duction) of native Ag resulted in a shift toward more negative δ109Ag values in the newly de-posited silver. Because wire silver growth also involves a reduction of Ag+, enrichment in 107Ag would be expected.

Other, less-common mechanisms also fall short of explaining the inverse isotope values. The superconductor isotope effect has been known for a long time (Maxwell, 1952), but is a fundamentally different phenomenon from that observed in wire silver growth and does not apply to the case of SSIC (see note in the Data Repository). Magnetic isotope effects typically have strong temperature dependence (Bucha-chenko, 2013), and it has been predicted that superionic conduction could lead to significant fractionation of isotopes with different nuclear spins (I ) (Kimball and Eswaran, 1976). How-ever, this cannot explain fractionation of Ag

because both isotopes have identical spin (I = 1/2). The probability of spin-selective interac-tions also depends on the nuclear magnetic mo-ment (µ) (Buchachenko, 2001), which is ~15% larger for 109Ag than for 107Ag (Haynes et al., 2017); it is currently unclear whether SSIC is truly spin-selective.

There remains one possible fractionation mechanism, but which has not yet been found in nature. Electron excitation of molecules can cause isotope fractionation based on preferen-tial predissociation of one isotope’s bonds (Kato and Baba, 1995; Jensen et al., 1995). During SSIC in argentite, the Ag+ current is accompa-nied by a much greater free-electron current, some of which no doubt collides with Ag+ ions, possibly pumping them into an excited elec-tronic state. Which Ag isotope, if either, could undergo predissociation would intimately de-pend on the isotope-specific vibronic structure of the Ag-S bonds (Fig. DR13). If, hypotheti-cally, predissociation were an additional path-way by which 109Ag could preferentially break its bonds, then it would be the more mobile isotope, leading to the observed positive δ109Ag

Figure 2. Hypothesized wire silver growth by solid-state ion conduction (SSIC). Large circles represent Ag and small yellow circles represent S. 1: Acanthite in its initial state has distorted bcc (body-cen-tered cubic) sulfur sublattice. 2: Thermoelectric gradient in-duces local phase transfor-mation to argentite, driven by down-gradient buildup of free electrons and subsequent at-traction of Ag+ ions, which, upon reaching surface, are reduced to Ag0 by free elec-trons. Variable ion flux leads to high number of defects, forming crystallites in differ-ent orientations (illustrated by black lines—lattice planes). 3: Acanthite to argentite phase transition results in 3% volume increase and formation of ar-gentite pedestal. Basal crys-tallite growth results in poly-crystalline aggregate “blooms” with cross-sectional morphol-ogy constrained by expanding pedestal footprint. Several ad-jacent growth pedestals may coalesce and grow as single unit. 4: Because Ag is supplied by SSIC through argentite “por-tal” (i.e., pedestal), crystallites only form within register of that portal. As basal crystallite addition continues, aggregate extends up from surface, its ex-ternal morphology constrained by irregular footprint of coalesced pedestals, which propagates as coherent striations. As silver wire increases in size, margins of argentite pedestal increase in steepness until it becomes normal to surface. Wire growth ceases once thermal gradient or electric potential is removed, or source of Ag+ becomes depleted.

4

3

2

1argentite

acanthite

native Ag

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values in wire silver. Because thermal vibra-tions (phonons) are the largest contribution to vibrational energy at high temperatures, we would also anticipate a significant increase with temperature. Unfortunately, available data are currently insufficient to determine whether this mechanism can account for our observed iso-tope values. If it is later confirmed, it would open up new lines of inquiry regarding the pos-sible roles of excited states in native and sul-fide mineral-forming and mineral-transforming reactions.

Finally, we note that acanthite, although not statistically different from non-wire silver, tends to be negatively skewed toward isotopi-cally heavy compositions. This can be explained if some of the analyzed acanthites acted as con-duits of SSIC at some point in their geologic history. This may help explain other cases where SSIC minerals have been associated with anomalously heavy metal isotope values ( Wilson et al., 2016).

CONCLUSIONSTextural analysis of wire silver, combined

with data from historical experiments and real-time observations, strongly indicates that nat-ural (and synthetic) occurrences are products of SSIC. This is the first recognition of SSIC by natural processes. Furthermore, SSIC dur-ing wire silver growth leads to significant tem-perature-enhanced fractionation of stable Ag isotopes that cannot be explained by known naturally occurring isotope fractionation mech-anisms. In deposits where wire silver occurs, stable Ag isotope analysis of wire silvers could be used to assess the influence of SSIC and its resulting effect on isotope geochemistry. If SSIC can transport metals under reasonable geologic

conditions, then it may constitute an important and previously overlooked process in materials that are capable of ion conduction, including multiple metal-bearing sulfides, sulfosalts, and native metals.

ACKNOWLEDGEMENTSWe thank Takenao Shinohara, Kenichi Oikawa, Matthew Gonzales, Richard Edelmann, and Mat-thew Duley for assistance with instrumentation and imaging; and 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, Terry Wallace, Jack and Pete Heckscher, John Jaszczak, Dan Weinrich, and Mark Mauthner. We are grateful to Peter Megaw for his insightful help leading to this manuscript, and to two anonymous reviewers. This study was funded by the Mineralogical Research Initiative.

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Figure 3. Stable Ag iso-tope analysis of natural and synthetic wire sil-ver compared with other forms of silver and sil-ver ores. Positive values indicate enrichment in heavier isotope. When notches in boxes for two populations do not over-lap, difference between their medians is “statis-tically significant at the 0.05 level” (Chambers et al., 1983, p. 62). Vertical gray bar shows projection of native non-wire silver notch for comparison with other populations. Includes data from: †—Mathur et al. (2018); ‡—Chugaev and Cherny-shev (2012); §—Desaulty et al. (2011); ∥—Hauri et al. (2000); ◊—Voisey et al. (2017). For tabulated data and sample statistics, see Tables DR1 and DR2 respectively (see footnote 1). *—Hydrothermal (low-temperature) synthetic wire silver data have been corrected for isotopic composition of source silver solution. max—maximum; min—minimum; IQR—inter-quartile range; CI—confidence interval.

δ109Ag (‰) −2 0 1 2 3 4 5 6

Acanthite (Ag2S) n=27

Native (non-wire) silver n=105

Silver in native gold n=26

Silver coins (smelted) n=91

Natural wire silver n=57

Synthetic wire silver (>450°C) n=49

Synthetic wire silver (130-180°C) n=3 ● ● ●

median } 95% CIof median

1st quartile

3rd quartile

max in 1.5 IQR

min in 1.5 IQR

possible outlier

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SUPPLEMENTAL MATERIALS FOR

Natural solid-state ion conduction induces metal isotope fractionation

Calvin J. Anderson1, Ryan Mathur2, John Rakovan1, and Anton S. Tremsin3

1Miami University, Department of Geology and Environmental Earth Science, 118 Shideler Hall,

250 S. Patterson Ave. Oxford, OH 45056

2Juniata College, Geology Department, 1700 Moore St. Huntingdon, PA 16652

3Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA

94720, USA

Note on applicability of superconductor isotope effect

The superconductor isotope effect describes the influence of immobile isotopes on the mobility

and quantum relationship of mobile electrons (i.e. the probability of Cooper pair formation), and

thereby the change in the critical transition temperature in superconducting materials (Maxwell,

1952). By contrast, in the growth of wire silver, the isotopic cations are themselves mobile, and

travel in the same net direction as the free electrons (Wagner, 1933a,b,c). To our knowledge,

Bardeen–Cooper–Schrieffer (BCS) theory has not yet been applied to the case of parallel motion

of free electrons and cations through an anionic sublattice. While it is possible that unknown high

temperature quantum effects (>>293 K) could play a role in superionic solids, this is unlikely

given that such effects generally manifest only at very low temperatures (<<293 K).

GSA Data Repository 2019221

Figure DR1: Wire silver on a matrix of calcite and acanthite, Freiberg, Saxony, Germany.

9.4 cm tall. Irv Brown specimen. Jeff Scovil photograph.

Figure DR2: Apparatus used for rapid wire silver growth. The inner cone of a Bunsen burner

flame is composed of uncombusted gas and air, which is much cooler than the cone of

oxidation (bright blue cone). Acanthite was heated to above 450°C but below its melting

temperature (700°C) inside the inner cone of the flame. Sample chunks were supported

by nichrome wire, in order to reduce contamination by iron and formation of iron oxides

which inhibited growth. For larger pieces of acanthite (>0.5 cm3), three Bunsen burners

were arranged such that their individual flames coalesced into a single one. The

temperature and thermal gradient strength experienced by the sample could be adjusted

by changing the sample’s vertical position inside the flame. Plates of Ag were added to

some experiments by balancing them on top of the acanthite. The wire silver shown in

this image grew in less than an hour.

Figure DR3: SEM image (5 keV) of synthetic wire silver grown by the open flame method

showing characteristic longitudinally parallel surface striations.

Figure DR4: SEM image (5 keV) of natural wire silver from Freiberg, Germany, showing

the same textural characteristics as synthetic wires (Fig. DR5-DR7). In the left of the

image, a crystallite boundary can be seen cross-cutting several striations.

Figure DR5: SEM image (5 keV) of a flame-grown synthetic wire silver showing

characteristic polycrystalline texture with superimposed striations (Anderson and

Rakovan, 2017). This texture is also representative of natural wire silver, but the lack of

tarnish in freshly-synthesized wires makes them ideal for imaging. Note crystallites that

cross-cut multiple striations, especially in the lower right (see Fig. 1.A in Main Text for

outlined version). In addition, the horn-like shapes at the tip of the wire appear very

similar to silver “blooms” in Figure DR11.

Figure DR6: SEM image (5 keV) of the base of a flame-grown synthetic wire silver. Growth

steps on individual crystallites can be seen, particularly in the center of the image. In the

bottom left is the acanthite substrate, and between the substrate and the wire is the

vertical Ag2S pedestal. Image is focus-stacked from 3 different focal points to improve

depth of field.

Figure DR7: SEM image (5 keV) of hydrothermal synthetic wire silver shows the same

textural characteristics as flame grown and natural wires, although more etched. Note

growth steps preserved on individual crystallites.

Figure DR8: Neutron transmission spectrum measured over the area of an entire wire shows

typical results for natural and synthetic wire silver. Range I corresponds to neutron

wavelengths where neutron resonance absorption can be used for the elemental/isotopic

analysis of the sample (Tremsin et al., 2017); Only Ag resonances are clearly resolved,

and no inclusions of other materials were detected. Range II corresponds to neutron

wavelengths where neutron diffraction can be used for the study of microstructure

(wavelengths are comparable to distance between the lattice planes). The absence of

both Bragg dips (present for single-crystal material) or Bragg edges (indicative of

polycrystalline material) in this range indicates the samples are polycrystalline

aggregates with crystallites below the resolution limit (≈ 200µm). Neutron transmission

decays rapidly at wavelengths above ≈ 1 Å due to the increase of the absorption cross

section. Additional data for this particular sample are shown in Figs. DR9, DR10.

Figure DR9: Relative neutron transmission spectrum of single grain areas (spots shown in

Fig. DR10). The transmission of small areas (≈ 0.5 x 1mm2) corresponding to single

grains are normalized by the transmission of the entire sample. The dips observed in the

spectrum indicate the presence of discrete single crystal domains, which diffract

neutrons at a specific wavelength meeting the Bragg equation. The gradual shift among

the dip wavelengths indicates the gradual rotation of single crystal grains relative to each

other along the sample, a trend only observed in this sample; the other samples analyzed

did not exhibit single-grain dips within the limit of spatial resolution of our technique,

which is currently on the scale of ≈ 200µm. This suggests the other samples are also

aggregates, but with grains smaller than 200µm.

Figure DR10: (Top) Neutron transmission image of natural wire silver from Freiberg,

Germany, with full spectrum of the beam. (Bottom) Narrow-energy transmission images

of the same specimen revealing the presence of large grains within the sample (dark

areas). The dark areas in these images correspond to the areas where neutrons are

diffracted away from the transmitted beam. In this sample, the grain orientations change

gradually along the length (i.e., non-random orientation of grains) as demonstrated by

the highlighted areas (white dashed boxes) and the transmission spectrum of spots 1-5

shown in Fig. DR9. Other samples did not show single grain inclusions within the limit

of our spatial resolution. See Movie DR1 for real-time scan of this specimen.

Movie DR1: Sequence of narrow-energy (≈ 5.5 mÅ) neutron transmission images of the

sample shown in Fig. DR10, normalized by the white spectrum image. Movie is

provided as a separate file available for download. The black areas correspond to the

areas of single crystal Ag grains within the sample, which diffract neutrons away from

the transmitted beam. The gradual motion of these grains from one frame to another

(scan of neutron wavelength) indicate the gradual change in the orientation of these

single crystal grains along the length of this sample.

Figure DR11: SEM images of native Ag “blooming” from Ag2S in an electron beam. (Left)

Ag nanoparticles (tiny white dots, cf. Mikhlin et al., 2011) formed almost immediately

across the entire rastered surface. Shortly after, isolated patches began to grow much

faster than the surrounding areas, forming clusters of little horn-like growths. (Right)

After 30 minutes, the patches approximately doubled in volume, and became raised up

on small argentite pedestals (See Fig. DR12). EBSD data indicate that the horns are

native Ag. See Fig. DR12 for EBSD pattern from the spot indicated by the blue cross. It

was very difficult to get clear EBSD signal from pedestals, however their signal was

virtually identical to that of the substrate, which Sadovnikov et al. (2015) determined

was argentite. Accelerating voltage was 10 keV with 10.0 mm working distance and

60° aperture.

Figure DR12: (Left) The acanthite (blue) / argentite (red) interface during Ag blooming,

where a 3% volume increase leads to a small displacement of the surface and the

formation of an argentite pedestal. (Right) EBSD pattern from the spot marked by a blue

cross in Fig. DR11. The pattern matches the EBSD pattern of native Ag. The black

region in the lower part of the image is due to shadowing of the detector by the irregular

surface of the acanthite specimen used. In some cases, we observed the diffraction

patterns rotate as the blooms gradually changed orientation (growing in real-time).

Figure DR13: Hypothetical Jablonski diagrams of (A) diatomic Ag–S molecule and (B)

periodic Ag2S showing possible mechanisms of isotope fractionation by preferential

predissociation. In a diatomic molecule (A), preferential predissociation can occur when

a bound excited state and unbound (repulsive) state intersect at an energy very close to

an isotopically distinct vibrational level. In a periodic solid (B), predissociation can

occur when two of more potential energy surfaces very nearly approach each other, such

that the non-radiative electronic relaxation imparts the excess energy directly to the

nuclear kinetic energy; a heavier isotope, which has more momentum, is more likely to

be able to achieve the hop into the neighboring potential well (crystallographic site) with

the aid of the additional kinetic energy. Both situations (A and B) result from the

tunneling of electrons from an excited state onto a potential energy surface with which

it is strongly coupled. The pink and blue lines correspond to 107Ag and 109Ag

respectively. Dashed lines indicate the dissociation limits, and arrows show electronic

transitions. (A) modified from (Jensen et al., 1995).

Table DR1: Stable Ag isotope data for natural and synthetic wire silver, and associated

acanthite. Table is provided as a separate spreadsheet available for download. The letters

appended to labels are necessary to provide unique identifiers. For example, both

acanthite and a natural wire were taken from CA0032, and then later a synthetic wire

was grown from acanthite from the same specimen. Hydrothermal synthetic wires (LT)

are reported here in raw values, and in Figure 3 and Table DR2 are corrected for the

initial isotopic composition of the AgSO4 solution found in the last row of this table. The

entity that provided each sample is listed in the last column, where FMI = Fine Minerals

International, MUC = Miami University Collection, MBF = Mineralogical Collection of

the Bergakademie Freiberg, NHMM = Natural History Museum of Milan, SM = Spirifer

Minerals, CEM = The Collector’s Edge Minerals Inc., HM = Humboldt Museum, OGM

= Our Gangue Minerals, CMNH = Carnegie Museum of Natural History, JRC = John

Rakovan Collection, THC = Terry Huizing Collection, SFC = Sigfried Flach Collection,

CC = The Crystal Circle, WM = Weinrich Minerals, MM = Mark Mauthner, TW = Terry

Wallace, PM = Peter Megaw, VL = Volker Lüders, JJ = John Jaszczak, WW = Wolfgang

Wendel, AO = Acros Organics.

Label δ109Ag Material Locality Source CA032A −0.320‰ Acanthite Alberoda, Germany MBF CA035A +0.137‰ Acanthite Annaberg, Germany MBF CA041A −0.130‰ Acanthite Aue, Germany MBF CA126A +0.213‰ Acanthite Aue, Germany SFC CA030A +0.267‰ Acanthite Brand-Erbisdorf, Germany MBF CA024A +0.074‰ Acanthite Freiberg, Germany MBF CA028A +0.406‰ Acanthite Freiberg, Germany MBF CA029A +0.236‰ Acanthite Freiberg, Germany MBF CA077A +0.109‰ Acanthite Fresnillo, Mexico PM CA065A +0.259‰ Acanthite Fresnillo, Mexico PM CA033A +0.048‰ Acanthite Ehrenfriedersdorf, Germany MBF CA034A +0.225‰ Acanthite Ehrenfriedersdorf, Germany MBF

Label δ109Ag Material Locality Source CA074A +0.420‰ Acanthite Brand-Erbisdorf, Germany MM CA007A −0.329‰ Acanthite Lingqui, China FMI CA051A +0.255‰ Acanthite Lingqui, China FMI CA054A +0.187‰ Acanthite Lingqui, China FMI CA006A −0.144‰ Acanthite Lingqui, China CE CA132A −0.118‰ Acanthite Imiter, Morocco SM CA009A −0.303‰ Acanthite Imiter, Morocco SM CA045A −0.333‰ Acanthite Imiter, Morocco SM CA052A −0.143‰ Acanthite Imiter, Morocco FMI CA031A +0.355‰ Acanthite Niederschlema, Germany MBF CA023A +0.538‰ Acanthite Pobershau, Germany MBF CA038A −0.026‰ Acanthite Pöhla, Germany MBF CA026A −0.438‰ Acanthite Schneeberg, Germany MBF CA025A +0.353‰ Acanthite St. Andreasberg, Germany MBF CA059A +0.647‰ Acanthite Stone Cabin Portal, Idaho, U.S.A. MUC CA055N +0.367‰ Nat Ag Batopilas, Mexico FMI CA104N +0.259‰ Nat Ag Dobrá Voda, Czech Republic HM CA068N +0.115‰ Nat Ag Eiselben, Germany VL CA069N +0.141‰ Nat Ag Hettstedt, Germany VL CA093N +0.307‰ Nat Ag Brand-Erbisdorf, Germany HM CA102N +0.256‰ Nat Ag Brand-Erbisdorf, Germany HM CA056N +0.257‰ Nat Ag Imiter, Morocco FMI CA082N +0.405‰ Nat Ag Imiter, Morocco SM CA058N +0.221‰ Nat Ag Kongsberg, Norway FMI CA089N +0.237‰ Nat Ag Kongsberg, Norway HM CA088N1 +0.193‰ Nat Ag Kongsberg, Norway HM CA088N2 +0.200‰ Nat Ag Kongsberg, Norway HM CA099N +0.186‰ Nat Ag Kongsberg, Norway HM CA103N +0.435‰ Nat Ag Freiberg, Germany HM CA094N +0.281‰ Nat Ag Pöhla, Germany HM CA084N +0.138‰ Nat Ag Roehrigschacht, Germany VL CA011N −0.111‰ Nat Ag Oyon, Peru OGM CA032W +0.460‰ Nat Wire Alberoda, Germany MBF CA041W +0.485‰ Nat Wire Aue, Germany MBF CA105W +0.301‰ Nat Wire Dobrá Voda, Czech Republic HM CA079W +0.418‰ Nat Wire Freiberg, Germany CMNH CA095W +0.055‰ Nat Wire Freiberg, Germany HM CA039W +0.430‰ Nat Wire Freiberg, Germany MBF CA114W +0.255‰ Nat Wire Freiberg, Germany CE CA008W +0.247‰ Nat Wire Freiberg, Germany MUC CA004W +0.145‰ Nat Wire Guanjuato, Mexico CC CA002W +0.474‰ Nat Wire Brand-Erbisdorf, Germany THC CA054W +0.511‰ Nat Wire Lingqui, China FMI CA006W +0.180‰ Nat Wire Lingqui, China CE CA131W1 +0.593‰ Nat Wire Imiter, Morocco SM

Label δ109Ag Material Locality Source CA131W2 +0.629‰ Nat Wire Imiter, Morocco SM CA131W3 +0.651‰ Nat Wire Imiter, Morocco SM CA040W +0.259‰ Nat Wire Johanngeorgenstadt, Germany MBF CA053W +0.173‰ Nat Wire Kongsberg, Norway FMI CA061W +0.265‰ Nat Wire Kongsberg, Norway FMI CA003W +0.358‰ Nat Wire Kongsberg, Norway JRC CA042W +0.477‰ Nat Wire Lauter, Germany MBF CA092W +0.509‰ Nat Wire Niederschlema, Germany HM CA044W +0.325‰ Nat Wire Niederschlema, Germany MBF CA117W +0.241‰ Nat Wire Niederschlema, Germany WW CA043W +0.088‰ Nat Wire Oberschlema, Germany MBF CA116W +0.451‰ Nat Wire Oberschlema, Germany WW CA038W +0.329‰ Nat Wire Pöhla, Germany MBF CA078W +0.413‰ Nat Wire Příbram, Czech Republic CMNH CA106W +0.143‰ Nat Wire Příbram, Czech Republic HM CA090W +0.309‰ Nat Wire Rudny, Kazakhstan HM CA080W +0.376‰ Nat Wire Rudny, Kazakhstan CMNH CA123W +0.561‰ Nat Wire Sardinia, Italy NHMM CA124W +0.210‰ Nat Wire Sardinia, Italy NHMM CA125W +0.240‰ Nat Wire Sardinia, Italy NHMM CA098W +0.012‰ Nat Wire Schlema, Germany HM CA100W +0.384‰ Nat Wire Schlema, Germany HM CA101W +0.560‰ Nat Wire Schlema, Germany HM CA096W +0.647‰ Nat Wire Schlema, Germany HM CA118W −0.031‰ Nat Wire Schlema, Germany WW CA119W +0.704‰ Nat Wire Schlema, Germany WW CA097W +0.306‰ Nat Wire Schneeberg, Germany HM CA036W +0.394‰ Nat Wire Schneeberg, Germany MBF CA037W +0.581‰ Nat Wire Schneeberg, Germany MBF CA130W +0.389‰ Nat Wire Schneeberg, Germany VL CA025W +0.181‰ Nat Wire St. Andreasberg, Germany MBF CA109W +0.614‰ Nat Wire Xiaoqinggou, China CE CA005W +0.226‰ Nat Wire Zacatecas, Mexico JJ CA081S +1.002‰ Syn Wire (HT) Synthetic CMNH CA054S +2.241‰ Syn Wire (HT) Synthetic FMI CA052S +1.166‰ Syn Wire (HT) Synthetic FMI CA023S +2.977‰ Syn Wire (HT) Synthetic MBF CA026S +1.742‰ Syn Wire (HT) Synthetic MBF CA029S +3.367‰ Syn Wire (HT) Synthetic MBF CA030S +3.696‰ Syn Wire (HT) Synthetic MBF CA032S +1.563‰ Syn Wire (HT) Synthetic MBF CA034S +5.151‰ Syn Wire (HT) Synthetic MBF CA038S +2.813‰ Syn Wire (HT) Synthetic MBF CA146S +2.454‰ Syn Wire (HT) Synthetic MUC CA009/145S +2.013‰ Syn Wire (HT) Synthetic MUC

Label δ109Ag Material Locality Source CA132/145S +2.329‰ Syn Wire (HT) Synthetic MUC CA065/145S +1.932‰ Syn Wire (HT) Synthetic MUC CA077/145S +2.025‰ Syn Wire (HT) Synthetic MUC CA006/145S +1.368‰ Syn Wire (HT) Synthetic MUC CA028/145S +2.603‰ Syn Wire (HT) Synthetic MUC CA029/145S +5.146‰ Syn Wire (HT) Synthetic MUC CA009/082S +5.776‰ Syn Wire (HT) Synthetic MUC CA132/082S +2.956‰ Syn Wire (HT) Synthetic MUC CA052/082S +4.377‰ Syn Wire (HT) Synthetic MUC CA009/056S +3.669‰ Syn Wire (HT) Synthetic MUC CA132/056S +3.130‰ Syn Wire (HT) Synthetic MUC CA052/056S +1.450‰ Syn Wire (HT) Synthetic MUC CA006/056S +4.021‰ Syn Wire (HT) Synthetic MUC CA009/133S +1.824‰ Syn Wire (HT) Synthetic MUC CA065/133S +2.527‰ Syn Wire (HT) Synthetic MUC CA006/133S +1.448‰ Syn Wire (HT) Synthetic MUC CA132/058S +2.971‰ Syn Wire (HT) Synthetic MUC CA065/058S +3.919‰ Syn Wire (HT) Synthetic MUC CA006/058S +4.676‰ Syn Wire (HT) Synthetic MUC CA009/121S +4.507‰ Syn Wire (HT) Synthetic MUC CA009/134S +4.444‰ Syn Wire (HT) Synthetic MUC CA006/134S +3.015‰ Syn Wire (HT) Synthetic MUC CA132/115S +2.372‰ Syn Wire (HT) Synthetic MUC CA160/009S1 +1.249‰ Syn Wire (HT) Synthetic MUC CA161/009S1 +1.197‰ Syn Wire (HT) Synthetic MUC CA160/065S1 +2.762‰ Syn Wire (HT) Synthetic MUC CA161/065S1 +3.658‰ Syn Wire (HT) Synthetic MUC CA009S1 +6.218‰ Syn Wire (HT) Synthetic MUC CA065S +5.772‰ Syn Wire (HT) Synthetic MUC CA160/009S2 +1.587‰ Syn Wire (HT) Synthetic MUC CA160/065S2 +1.175‰ Syn Wire (HT) Synthetic MUC CA161/009S2 +2.993‰ Syn Wire (HT) Synthetic MUC CA161/065S2 +3.334‰ Syn Wire (HT) Synthetic MUC CA009S2 +3.584‰ Syn Wire (HT) Synthetic SM CA045S +1.844‰ Syn Wire (HT) Synthetic SM CA147S +1.124‰ Syn Wire (HT) Synthetic TW CA006S +2.496‰ Syn Wire (HT) Synthetic CE CA115S +4.593‰ Syn Wire (HT) Synthetic WM CA010H +0.194‰ Syn Wire (LT) Synthetic MUC CA006H −0.476‰ Syn Wire (LT) Synthetic CE CA006H2 −0.119‰ Syn Wire (LT) Synthetic CE AgSO4 −0.291‰ Correction (LT) Reagent AO

Table DR2: Sample statistics of δ109Ag analysis, including data from (Mathur et al., 2018;

Chugaev and Chernyshev, 2012; Desaulty et al., 2011; Hauri et al., 2000; Voisey et al.,

2017). The upper notch for native silver and the lower notch for natural wire silver do

not overlap, indicating the difference between their medians is statistically significant at

the 95% confidence level (Chambers et al., 1983). *Low temperature (hydrothermal)

wire silver data have been corrected for the isotopic composition of the source silver.

Category n Median Q1 Q3 −95% CI +95% CI Acanthite 27 +0.137‰ −0.136‰ +0.263‰ +0.016‰ +0.257‰ Native silver 105 +0.065‰ −0.103‰ +0.180‰ +0.022‰ +0.108‰ Silver in gold 26 +0.190‰ +0.044‰ +0.320‰ +0.105‰ +0.275‰ Silver coins 91 +0.012‰ −0.029‰ +0.067‰ −0.004‰ +0.028‰ Natural wire silver 57 +0.283‰ +0.145‰ +0.453‰ +0.220‰ +0.346‰ Synthetic wire silver (HT) 49 +2.788‰ +1.829‰ +3.689‰ +2.375‰ +3.201‰ Synthetic wire silver (LT)* 3 +0.410‰ N/A N/A N/A N/A

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