Near-infrared spectroscopic study of nontronites and ferruginous smectite

Abstract

The existence of life on planets such as Mars depend upon the presence of water. This water may not necessarily be as liquid or crystalline water but may be as interlayer water such as is found in smectitic clays. One group of smectites, relevant to the search for interplanetary life are those which have a high iron content, known as nontronites. Near-IR reflectance spectroscopy has been used to show the presence of water and hydroxyl units in these minerals. Three near-IR spectral regions are identified, (a) the high frequency region between 6400 and 7400 cm −1 attributed to the first overtone of the hydroxyl stretching mode; (b) the 4800–5400 cm −1 region attributed to water combination modes; and (c) the 4000–4800 cm −1 region attributed to the combination of the stretching and deformation modes of the FeFeOH units of nontronite. Two types of hydroxyl groups were identified using near-IR spectroscopy, hydroxyl units coordinated to the iron; and hydroxyl groups from water in the nontronite structure. The first hydroxyls are characterised by several bands, firstly in the 7055–7098 cm −1 region assigned to the first overtone of the AlFeOH stretching unit, secondly in the 6958–6878 cm −1 region attributed to the FeFeOH unit. The overtone of the hydroxyl stretching frequency of water was observed at around 6800 cm −1. These observations show that nontronites can be a source of water that may support life.