Nitrogen stable isotope composition and the origins of cupric nitrate mineralization

Cupric nitrate minerals are rare, but when present may record evidence of past biological activity. The cupric nitrate minerals we examined from localities around the world are broadly divided into four main groups based upon their inferred conditions of formation and δ15N values. These groups, named for their probable source of nitrogen, include: 1) termite, 2) bat, 3) atmospheric and 4) coal mine fires and natural gas. It is clear that in three of these categories, the role of past or present biological activity is critical for formation of cupric nitrate minerals. For cupric nitrate minerals associated with an atmospheric source of nitrogen the link is not clear, though a microbial source is possible. Nitrogen isotope analyses of gerhardtite and buttgenbachite mineral pairs from two localities suggest fractionation between these two minerals at ambient temperatures is 1.1‰ (Australia) to 0.8‰ (Democratic Republic of Congo). Similarly, differences between δ15N values of sal ammoniac and gerhardtite pairs suggest fractionation between these minerals at higher temperatures in coal bed fires is 1.9‰ (Pennsylvania, USA) to 1.7‰ (Germany).