Near-infrared and mid-IR spectroscopy of selected humite minerals

Spectral reflectance properties of a number of iron-bearing humite minerals have been examined to draw quantitative relationships between composition, structure, and reflectance spectra. The measured knowledge yields information on the geologic history of surface minerals on Mars through the identification of types, abundance, and composition of iron-bearing silicate minerals. A prominent feature in the spectra of humite minerals is a complex band system in the NIR that arises in the range 11250–7850 cm−1 due to Fe2+. The broad band profile resolved into four bands are assigned to dd-transitions of Fe2+(M16), bands around 10,200 and 9000 cm−1, and of Fe2+(M26), bands centred at 9800 and 9400 cm−1. The fundamental overtones of OH-stretching modes are observed in the range 7400–7100 cm−1. The sharp band centered around 5075 cm−1 with extended bands on either side of 5200–4810 cm−1 is characteristic of OH– and Si–OH combination modes. The complexity of OH spectrum increases with number of silicate layers in humites. Infrared spectroscopy is able to differentiate the mineral group into two-, three- or four-layered silicates by the main OH band observed at 3573, 3568, 3564 and 3563 cm−1 for chondrodite(1), chondrodite(2), humite and clinohumite. An anomaly for Ti-clinohumite is observed at 3557 cm−1 which is accounted for by the presence of titanium.