Rhodium-Plated Iron Meteorites
The annual Tucson Gem and Mineral Show has been a reliable source for the niche and the unusual, as seen with the rhodium-plated iron meteorites (figure 1) acquired by author BL at the 22nd Street Show in February 2020. The 28.53 ct mounted pendant and 36.69 ct loose slab were initially suspected to be imitations due to the abnormally high metallic luster and bright white color compared to typical polished and etched iron meteorites. However, chemical analysis of elements in the mounted sample using laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) identified the specimens as authentic etched iron meteorites with layers of plated metals including nickel, copper, and rhodium.
Meteorites can be classified into three broad categories: stone, iron, and stony iron. The latter two groups are utilized within the gem and jewelry industry. The iron group consists of metallic meteorites composed primarily of iron with varying nickel content, a mineral intergrowth of low-nickel kamacite and high-nickel taenite. The iron group can be further categorized as hexahedrites (containing less than 6% nickel), octahedrites (6–13% nickel), and ataxites (more than 13% nickel) (O.R. Norton, The Cambridge Encyclopedia of Meteorites, 2002, pp. 184–198). When cut, polished, and etched with nital (a mixture of nitric acid and ethanol), the acid dissolves the isometric kamacite and taenite crystals at different rates, revealing a unique pattern dependent on the crystal’s grain size. Octahedrites reveal a coarse pattern with a range of crystal widths known as a Widmanstätten structure (again, see figure 1). These lines and structures act like a fingerprint, as no two etched meteorites reveal the exact same pattern. Etched iron meteorites can also be easily classified by the size of their kamacite bands. Octahedrites show a width of banding ranging from 50 mm to < 0.2 mm. The pendant and slab showed an average band width of 0.50 mm and 2.68 mm, respectively, both well within the octahedrite threshold.
The pendant sample was analyzed with LA-ICP-MS spectrometry, and results were plotted and compared to a confirmed natural, non-plated octahedrite meteorite (figure 2). The x-axis shows time lapsed as the laser began drilling from the surface (starting at 40 seconds), therefore also representing depth. The y-axis shows the weight percent of the elements present. These include iron (Fe), nickel (Ni), copper (Cu), and rhodium (Rh) and represent the multiple layers of plating. The non-plated meteorite showed a steady amount of Fe (approximately 93 wt.%) and Ni (approximately 6 wt.%) throughout the total elapsed time from the beginning of analysis. The plated meteorite showed an initial thin peak of rhodium, quickly followed by a slightly wider peak of nickel. As these peaks began to flatten, a broad band of copper became visible. Near the 96 second mark, the graph of the plated meteorite began to resemble the non-plated meteorite, with approximately 92 wt.% Fe and 8 wt.% Ni. This analysis proved the material to be a genuine octahedrite meteorite with platings of Cu, Ni, and Rh. In order to achieve a final surface plating of rhodium to a metal, various under-platings are usually required. Through this technique of measuring the chemistry of the plated layers by LA-ICP-MS, gemologists were able to determine the identity of the subsurface material, as well as the composition of the various platings through a minimally destructive process. The pits or craters created by the laser are generally small (50 μm in diameter) and therefore impossible to visibly detect without magnification (L.A. Groat et al., “A review of analytical methods used in geographic origin determination of gemstones,” Winter 2019 G&G, pp. 512–535). Utilizing LA-ICP-MS to analyze metallic platings and subsurface materials is a method not previously explored by the authors. This study could offer insight into using LA-ICP-MS technology in new ways in the gem and jewelry industry.
Iron meteorites are prone to rusting due to their high iron content, as even slight humidity within air causes rapid corrosion. To slow this process, the stones are commonly kept in humidity-controlled environments, moisture absorbent silica gel, or coated with oil or hard clear lacquer. Rhodium plating offers an attractive luster and color to the surface; however, its essential function in this instance is to prevent moisture from altering the iron. Unfortunately, rust spots can be seen on the edges of both samples where the plating failed to adhere. Rhodium is a member of the platinum metal group, which is characterized by high stability, good resistance to corrosion, and a high melting point. While it is too brittle to be used for solid jewelry, rhodium has become a popular plating material, as it has a brighter color than silver or platinum.
Meteorites are popular collectibles with wide usage as jewelry. Etched iron meteorites can be found as watch dials and bezels, beads, hololith rings, and pendants. As this specialized material becomes more common, consumers should be aware of potential imitations or treatments such as these rhodium-plated examples.
