ISOTOPE FRACTIONATION IN WIRE SILVER FORMATION: HISTORIC SPECIMENS FROM KONGSBERG NORWAY

Recent studies (Anderson et al. 2019; Geology) have shown that the formation of the wire habit in native silver on average leads to the fractionation of silver isotopes with an increase in the heavy stable isotope ¹⁰⁹Ag. Our sample population of over 150 historic specimens was only achievable through the utilization of mineral museum collections around the world. One of the most important sources of wire silver specimens has been the silver deposits of Kongsberg, Norway. Analysis of 17 samples from Kongsberg yields a range in δ¹⁰⁹Ag from -0.184‰ to 0.407‰ with a mean of 0.123‰ (s=0.2) and median of 0.099‰. This is a slightly broader range than that found for other localities, yet, measures of central tendency among specimen-groups from the 15 localities represented are statistically equivalent. Thus, while stable silver isotopes help illuminate the growth history of wire silver specimens, they are not sufficient for provenance determination.

One of the historically most interesting specimens we have analyzed from Kongsberg, informally referred to as "The Ring", is in the collection of the Muséum National d’Histoire Naturelle (National Museum of Natural History) in Paris France. The specimen is unique in its morphology and size, with the main wire forming an integral loop that measures 9 x 10.5 cm. This and numerous other wires sit on a calcite matrix. The specimen was given as a gift in 1770 to King Louis XV of France by the King of Denmark, Christian VII. At the end of his reign in 1774, Louis XV donated a number of fine mineral specimens from Norway and Germany to Georges-Louis Leclerc, Comte de Buffon for the Royal Cabinet of Natural History. This is how the National Museum of Natural History became the owner of this spectacular silver, and it is currently part of the Treasures of the Earth exhibit in the Gallery of Mineralogy and Geology. Analysis of a single wire fragment from this specimen yielded a δ¹⁰⁹Ag value of -0.135. This is within the range of δ¹⁰⁹Ag values of other Kongsberg wire silvers measured, and based on extensive analysis of other specimens, heterogeneities in silver isotope values from this locality were expected.