Abstract

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 ⁴T_1g(F) → ⁴T_2g(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.

© 2006 NIR Publications

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