Raman spectroscopy of the polyanionic copper(II) minerals buttgenbachite and connellite: Implications for studies of ancient copper objects and bronzes

The minerals connellite and buttgenbachite, complex hydroxy sulfate and nitrate, respectively, of Cu(II), are rare products formed during the corrosion of bronze and brass objects. Infrared and Raman spectra of buttgenbachite and connellite were obtained at 298 and 77 K using a Raman microprobe in combination with a thermal stage. The Raman spectra show the presence of nitrate, sulfate and chloride in the mineral. Spectra of the hydroxyl-stretching region are complex with multiple bands being observed. These observations are in agreement with the structure of the minerals in that the nitrate ion occupies two different sites and that six hydroxyl groups are crystallographically independent. Various OH stretching bands are attributed to independent hydroxyl units in the crystal structure and zeolitic water OH stretching modes. Raman spectroscopy is an excellent technique for the identification of these complex minerals and for the determination of the distribution of anions in their structure