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Analytical and Bioanalytical Chemistry

, Volume 398, Issue 7–8, pp 2915–2928 | Cite as

Authenticity and provenance studies of copper-bearing andesines using Cu isotope ratios and element analysis by fs-LA-MC-ICPMS and ns-LA-ICPMS

  • Gisela H. Fontaine
  • Kathrin Hametner
  • Adolf Peretti
  • Detlef Günther
Paper in Forefront

Abstract

Whereas colored andesine/labradorite had been thought unique to the North American continent, red andesine supposedly coming from the Democratic Republic of the Congo (DR Congo), Mongolia, and Tibet has been on the market for the last 10 years. After red Mongolian andesine was proven to be Cu-diffused by heat treatment from colorless andesine starting material, efforts were taken to distinguish minerals sold as Tibetan and Mongolian andesine. Using nanosecond laser ablation–inductively coupled plasma mass spectrometry (ICPMS), the main and trace element composition of andesines from different origins was determined. Mexican, Oregon, and Asian samples were clearly distinguishable by their main element content (CaO, SiO2 Na2O, and K2O), whereas the composition of Mongolian, Tibetan, and DR Congo material was within the same range. Since the Li concentration was shown to be correlated with the Cu concentration, the formerly proposed differentiation by the Ba/Sr vs. Ba/Li ratio does not distinguish between samples from Tibet and Mongolia, but only between red and colorless material. Using femtosecond laser ablation multi-collector ICPMS in high-resolution mode, laboratory diffused samples showed variations up to 3‰ for 65Cu/63Cu within one mineral due to the diffusion process. Ar isotope ratio measurements proved that heat treatment will reduce the amount of radiogenic 40Ar in the samples significantly. Only low levels of radiogenic Ar were found in samples collected on-site in both mine locations in Tibet. Together with a high intra-sample variability of the Cu isotope ratio, andesine samples labeled as coming from Tibet are most probably Cu-diffused, using initially colorless Mongolian andesines as starting material. Therefore, at the moment, the only reliable source of colored andesine/labradorite remains the state of Oregon.

Figure

Cu diffusion can be used to turn a plain, colorless andesine into a red gemstone. Cu and Ar isotope ratios in combination with main and trace elemental analysis can appoint andesine to thrie origin in Oregon, Mexico and Asia. In this study, red andesines sold as coming from Tibet and Mongolia are revealed to be most probably Cu-diffusion treated.

Keywords

Authenticity Isotope ratios Laser ablation Gemstone analysis Multi-collector ICPMS Cu 

Notes

Acknowledgments

Prof. Igor Villa from the Institute of Geology of the University in Bern is warmly thanked for access to and substantial help with the use of the noble gas spectrometer and argon data evaluation. In addition, his comments, suggestions, and critical remarks to general and special aspects of the topic were greatly appreciated. Prof. George Rossman’s personal description of their Ar analysis procedure was very helpful in carrying out the experiments. We thank Ken Scarratt of GIA (Thailand) for providing diffusion-treated andesine samples for this research project. Dr. John Emmet’s diffusion treatment of our samples from Mongolia—carried out at his own cost and a key experiment for the interpretation of our data—is highly valued. We thank those companies that helped by supplying samples, organizing the expedition to Tibet, and being interested in the outcome of the research, whatever it may be. It was a very important, clarifying field expedition and an experience unique of its kind. Dr. Joachim Koch, Dr. Helmar Wiltsche, and Reto Glaus are thanked for software and hardware support with the laser system.

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Gisela H. Fontaine
    • 1
  • Kathrin Hametner
    • 1
  • Adolf Peretti
    • 2
  • Detlef Günther
    • 1
  1. 1.Laboratory of Inorganic ChemistryETH ZurichZürichSwitzerland
  2. 2.GRS Gemresearch Swisslab AGLucerneSwitzerland

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