A Raman spectroscopic study of selected minerals of the rosasite group

Abstract

Minerals in the rosasite mineral group namely rosasite, glaucosphaerite, kolwezite, mcguinnessite, nullaginite and pokrovskite have been studied by Raman spectroscopy at 298 and 77 K and complimented with infrared spectroscopy. The spectral patterns for the minerals rosasite, glaucosphaerite, kolwezite and mcguinnessite are similar to that of malachite implying the structure is the same as malachite i.e. monoclinic. A comparison is made with the spectra of malachite. The symmetry of the carbonate anion in the rosasite mineral group is C2v or Cs and is composition dependent. Two (CO3)2- symmetric stretching modes are observed for the rosasite minerals at 1060 and 1090 cm-1. Two hydroxyl stretching modes are observed for the rosasite mineral group. The position of these bands is determined to be a function of the hydrogen bond lengths. Hydrogen bond distances for rosasite are calculated as 2.867, 2.799 and 2.780 Å whereas for pokrovskite the distances are 3.280 and 2.999 Å. The effect of lowering the temperature from ambient to 77 K results in a decrease of the hydrogen bond distances by 5%. Multiple Raman bands are observed in the 800 to 850 cm-1 and the 720 to 750 cm-1 regions and are attributed to ν2 and ν4 bending modes confirming the reduction of the carbonate anion in the rosasite structure.