Raman spectroscopy of gerhardtite at 298 and 77 K

Raman spectroscopy has been used to study the nature of gerhardtite, a naturally occurring basic copper(II) nitrate of formula Cu₂NO₃(OH)₃. Raman spectra at 77 K show four bands at 3557, 3548, 3467 and 3409 cm⁻¹ in accordance with the crystal structure with three non-equivalent hydroxyl groups. Estimations of hydrogen bond distances of 0.3019, 0.2987, 0.2855 and 0.2804 pm are obtained. Cooling to liquid nitrogen temperatures did not affect these values. Two bands are observed at 1048 and 1052 cm⁻¹ in the NO₃ stretching region. Cooling to liquid nitrogen temperature altered the ratio of these nitrates from 3/1 to 1/3. Two sets of bands in the antisymmetric stretching region of the nitrate in the 77 K spectra suggest that there are two non-equivalent nitrate anions in the gerhardtite structure at liquid nitrogen temperature. X-ray crystallography suggests three independent oxygens in the one nitrate group. The shift in band positions on obtaining Raman spectra at 298 at 77 K indicates a phase change is occurring for the mineral upon cooling to 77 K.