Daria L. Wain

Near infrared spectroscopy of the smithsonite minerals

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 CO32–, are Fe and Cu in terms of cation content. These ions exhibit broad absorption bands ranging from 13,000 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, (5T2g → 5Eg). 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 2B1g → 2A1g and 2B1g → 2B2g transitions. The effects of structural cations substitution (Ca2+, Fe2+, Cu2+, Cd2+ and Zn2+) on band shifts in the electronic spectral region of 11,000 cm−1 to 7500 cm−1 (910 nm to 1330 nm) and vibrational modes of OH− and CO32– anions in the 7300 cm−1 to 4000 cm−1 (1370 nm to 2500 nm) region were used to distinguish between the smithsonites.

An application of near infrared spectroscopy to the study of carbonate minerals–smithsonite, rhodochrosite, sphaerocobaltite and cadmium smithsonite

Near infrared (NIR) spectroscopy has been applied to the study of selected calcite group minerals including smithsonite, rhodochrosite, sphaerocobaltite and cadmium smithsonite. The isomorphic substitution of calcium in calcite group minerals by divalent cations such as Fe, Cu, Cd, Mn, Mg, Co, Zn is in agreement with variable spectral properties observed through NIR spectroscopy. This substitution results in highly-coloured minerals. The NIR spectra of calcite group minerals that contain absorption features due to the divalent cations, Fe2+, Cu2+ and Co2+ act as an aid to mineral identification. The main indicator is the strength of ferrous ion bands in the NIR spectra supporting spectral classification of calcite group minerals. The observation of Cu2+ bands in smithsonite and cadmium smithsonite with different band positions around 12400 and 8500 cm−1 (0.81–1.18 μm) confirms Cu2+ substitution for Zn2+. Co2+ is the major cation in sphaerocobaltite and exhibits a strong feature from 12000 to 7000 cm−1 (0.83–1.43 μm) which is distinctly different from other minerals of the group. The broad band centred at 9835 cm−1 (1.02 μm) with a split component at 8186 cm−1 (1.22 μm) is assigned to 4T1g(F) → 4T2g(F) spin-allowed transition of Co2+ ion. Significant shifts are observed for carbonate ion due to the wide range of cation substitutions in the mineral structure in conjunction with the ferrous iron which is a common impurity in calcite minerals. This NIR spectral features enable mineral identification. The implication is that NIR spectroscopy can be used to remotely detect carbonate minerals and assess their isomorphic substitution.

Near infrared spectroscopy of natural alunites

Near-infrared spectroscopy (NIR) has been used to analyse alunites of formula K(Al3+)6(SO4)4(OH)12. Whilst the spectra of the alunites shows a common pattern differences in the spectra are observed which enable the minerals to be distinguished. These differences are attributed to subtle variations in alunite composition. The NIR bands in the 6300-7000 cm(-1) region are attributed to the first fundamental overtone of both the infrared and Raman hydroxyl stretching vibrations. A set of bands are observed in the 4700-5500 cm(-1) region which are assigned to combination bands of the hydroxyl stretching and deformation vibrations. NIR spectroscopy has the ability to distinguish between the alunite minerals even when the formula of the minerals is closely related. The NIR spectroscopic technique has great potential as a mineral exploratory tool on planets and in particular Mars.