Natural variations in the abundance ratio and the atomic weight of copper

Abstract

The absolute ratio Cu63/Cu65 was determined for 106 samples that include a variety of copper minerals. Relative to a reference copper sample with a previously determined absolute ratio of 2.2440±0.0021, variations range from −9.0 to +3.3‰ (1.5‰ = 95% confidence limit of a single determination). No significant variations were found for chalcopyrite, tetrahedrite, enargite, or in the Canyon Diablo and Bruderheim meteorites. Some samples of bornite and of chalcocite exhibit variations with enrichments in either Cu63 or in Cu65. Secondary oxides of copper show significant enrichment only in the heavy isotope. The relative variations previously reported by Walker, Cuttitta, and Senftle for samples from the Cougar mine, Colorado, are confirmed. A mean value of 2.2431 for 82 mineral samples probably is unduly weighted with the rarer secondary minerals that are enriched in Cu65. The ratio Cu63/Cu65 for bulk or commercially processed copper is very near the value of 2.2440, and the atomic weight of such copper is 63.5455±0.0010. The causes of the natural variations in copper abundance ratios are not known. Unless otherwise stated the indicated uncertainties are over-all limits of error based on 95% confidence limits for the mean and on allowances for effects of known sources of possible systematic error.