The importance of NIR spectroscopy has been successfully demonstrated in the present study of smithsonite minerals. The fundamental observations in the NIR spectra, in addition to the anions of OH^– and CO₃^2–, are Fe and Cu in terms of cation content. These ions exhibit broad absorption bands ranging from 13000 cm^–1 to 7000 cm^–1 (770 nm to 1430 nm). One broad diagnostic absorption feature centred at 9000 cm^–1 (1110 nm ) is the result of ferrous ion spin allowed transition, (⁵T_2g → ⁵E_g). The splitting of this band (> 1200 cm^–1) is a common feature in all the spectra of the studied samples. The light green coloured sample from Namibia show two Cu(II) bands in NIR at 8050 cm^–1 and 10,310 cm^–1 (1240 nm and 970 nm ) assigned to ²B_1g → ²A_1g and ²B_1g → ²B_1g transitions. The effects of structural cation substitution (Ca²⁺, Fe²⁺, Cu²⁺, Cd²⁺ and Zn²⁺) on band shifts in the electronic spectra1 region of 11,000 cm^–1 to 7500 cm^–1 (910 nm to 1330 nm) and vibrational modes of OH^– and CO₃^2– anions in the 7300 cm^–1 to 4000 cm^–1 (1370 nm to 2500 nm ) region were used to distinguish between the smithsonites.

Keywords: smithsonite, cadmium smithsonite, hydroxy carbonates, NIR spectroscopy, divalent copper, ferrous ion