Raman spectroscopic study of the basic copper sulphates : implications for copper corrosion and bronze disease

The basic copper sulphates are of interest because of their appearance in many environmental situations such as copper pipe corrosion, restoration of brass and bronze objects, leaching from waste mineral dumps and the restoration of frescoes. The Raman spectra of the basic copper sulphate minerals antlerite, brochantite, posnjakite and langite are reported using a diode laser operating at an excitation wavelength of 785 nm. In line with their crystal structures, each basic copper sulphate mineral has its own characteristic Raman spectrum, which allows their identification. Except for brochantite, multiple bands are observed for the SO stretching vibration. Similarly, multiple bands are observed for the antisymmetric SO and OSO bending regions. Hydroxyl deformation modes in the 730-790 cm⁻¹ region are observed. The use of an He-Ne laser operating at an excitation wavelength of 633 nm allowed the hydroxyl stretching bands of the minerals to be obtained. Antlerite and brochantite are characterized by hydroxyl stretching bands at 3580 and 3488 cm⁻¹. The minerals posnjakite and langite display Raman hydroxyl stretching vibrations at 3588 and 3564 cm⁻¹. The Raman spectra of these two minerals show water OH stretching bands at 3405, 3372 and 3260 cm⁻¹. Raman spectroscopy allows the ready identification of these minerals.