Structure of selected basic copper(II) sulphate minerals based upon spectroscopy – Implications for hydrogen bonding

The application of near-infrared and infrared spectroscopy has been used for identification and distinction of basic Cu-sulphates that include devilline, chalcoalumite and caledonite. Near-infrared spectra of copper sulphate minerals confirm copper in divalent state. Jahn–Teller effect is more significant in chalcoalumite where 2B1g → 2B2g transition band shows a larger splitting (490 cm−1) confirming more distorted octahedral coordination of Cu2+ ion. One symmetrical band at 5145 cm−1 with shoulder band 5715 cm−1 result from the absorbed molecular water in the copper complexes are the combinations of OH vibrations of H2O. One sharp band at around 3400 cm−1 in IR common to the three complexes is evidenced by CuOH vibrations. The strong absorptions observed at 1685 and 1620 cm−1 for water bending modes in two species confirm strong hydrogen bonding in devilline and chalcoalumite. The multiple bands in ν3 and ν4(SO4)2− stretching regions are attributed to the reduction of symmetry to the sulphate ion from Td to C2V. Chalcoalumite, the excellent IR absorber over the range 3800–500 cm−1 is treated as most efficient heat insulator among the Cu–sulphate complexes.