Preparation and Measurement of Cassiterite for Sn Isotope Analysis

摘要

> Increased interest in the fractionation of Sn isotopes has led to the development of several techniques for preparing cassiterite (SnO ₂ , the primary ore of Sn) for isotopic analysis. Two distinct methods have been applied in recent isotopic studies of cassiterite: (a) reduction to tin metal with potassium cyanide ( KCN ) at high temperature (800?°C), with subsequent dissolution in HC l, and (b) reduction to a Sn solution with hydriodic acid ( HI ) at low temperature (100?°C). This study compares the effectiveness and accuracy of these two methods and contributes additional methodological details. The KCN method consistently yielded more Sn (?70% in comparison with ?5%), does not appear to fractionate Sn isotopes at high temperatures over a 2‐hour period and produced consistent Sn isotope values at flux mass ratios of ≥?4:1 (flux to mineral) with a minimum reduction time of 40?min. By means of a distillation experiment, it was demonstrated that HI could volatilise Sn, explaining the consistently low yields by this method. Furthermore, the distillation generated Sn vapour, which is up to 0.38‰ per mass unit different from the starting material, the largest induced Sn fractionation reported to date. Accordingly, the HI method is not recommended for cassiterite preparation for Sn isotopic analysis.