An Application of near Infrared Spectroscopy to the Study of the Selenite Minerals: Chalcomenite, Clinochalcomenite and Cobaltomenite

The selection of five naturally occurring selenite minerals that contain two different transition metal ions, Cu 2+ and Co 2+ could be distinguished by near infrared spectroscopy. Dependence of composition on spectral properties is a key to mineral identification and differentiation of the members of the selenite group. The nature of the band positions and splitting of band components in the electronic spectra of Cu 2+ selenites in the region 12,400–8000 cm −1 are in conformity with octahedral geometry distortion. The two split components which are observed for the Co 2+ band near 9000 cm −1 in cobaltomenites are considered as the vibrational satellites of spin-allowed transition 4 T 1g (F)→ 4 T 2g (F). Bands observed at 6950 cm −1 , 6810 cm −1 and 6700 cm −1 are the overtones of OH stretches of structural water in selenites and a strong absorption feature near 6700 cm −1 is the result of hydrogen bonding between (SeO 3 ) 2– and H 2 O. These bands are shifted in cobaltomenites. A sharp absorption band at 5170 cm −1 is a common feature in all the spectra of selenite minerals and is the contribution by the combinations of the OH vibrations of water molecules, ν 3 and ν 1 . A series of overlapping bands around 4500 and 4100 cm −1 is the result of the combination of the vibrational modes of (SeO 3 ) 2– ion in the minerals.