nontronite

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the
Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO2, NH3, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.

A comparison is made between the Raman and infrared spectra of ferruginous smectite and a nontronite using both absorption and emission techniques. Raman spectra show hydroxyl stretching bands at 3572, 3434, 3362, 3220 and 3102 cm−1. The infrared emission spectra of the hydroxyl stretching region are significantly different to the absorption spectrum. These differences are attributed to the loss of water, absent in the emission spectrum, the reduction of the samples in the spectrometer and possible phase changes. Dehydroxylation of the two minerals may be followed by the loss of intensity of the hydroxyl stretching and hydroxyl deformation frequencies. Hydroxyl deformation modes are observed at 873 and 801 cm−1 for the ferruginous smectite, and at 776 and 792 cm−1 for the nontronite. Raman hydroxyl deformation vibrations are found at 879 cm−1. Other Raman bands are observed at 1092 and 1032 cm−1, assigned to the SiO stretching vibrations, at 675 and 587 cm−1, assigned to the hydroxyl translation vibrations, at 487 and 450 cm−1, attributed to OSiO bending type vibrations, and at 363, 287 and 239 cm−1. The differences in the molecular structure of the two minerals are attributed to the Al/Fe ratio in the minerals.

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 to 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.

FTIR and Infrared emission spectroscopy (IES) has been used to characterise the Uley (Australian) and Garfield nontronites. These clay minerals are characterised by a strong emission band at 3570 cm -1 attributed to the FeFeOH unit. Dehydroxylation is followed by the loss of intensity of this band as a function of temperature. Dehydroxylation is also followed by the loss of intensity of the FeFeOH deformation vibration at 843 cm -1. IES shows that the dehydroxylation occurs as a continuous process in comparison to DTA/TGA studies where the dehydroxylation occurs abruptly at 425 °C. Water in these high iron bearing smectites have been observed through the stretching mode at 3430 cm -1 and the bending mode at 1630 cm -1. Different types of water are identified in the nontronite structure by the analysis of the band profile in the 1590-1680 cm -1 region. Low frequency vibrations show that the Uley green nontronite is similar to the Garfield nontronite. The brown Uley nontronite is closer to the Hohen-Hagen nontronite. The Uley nontronites may, therefore, be used spectroscopically to replace other nontronites as a reference clay mineral.

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolis...

Differential thermal analysis and thermogravimetry have been used to compare the dehydration and dehydroxylation behaviour of a series of nontronites including two newly discovered nontronites from Uley, South Australia. A comparison is made with the thermal behaviour of ferruginous smectite. Three dehydration endotherms were found at around 60°C, 100°C and 160°C. A strong dehydroxylation endotherm for all samples was found at ∼430°C. The nontronites from Hohen-Hagen and Garfield show well-defined steps for dehydroxylation whereas the two Uley nontronites showed continuous dehydroxylation. The intensities of the infrared spectra of the hydroxyl-stretching region showed excellent correlation with the nontronite dehydroxylation endotherms. The infrared spectra of the water bending region showed two bands centred at ∼1630 and 1670 cm−1. These bands are attributed to adsorbed water and coordinated water.

Smectites including the iron-bearing smectites have proved difficult to measure using the techniques of Raman spectroscopy. The use of Fourier transform (FT)-Raman spectrometers using indium-gallium-arsenide detectors has enabled the spectra of a selection of high iron-bearing smectites and nontronites to be measured. Low-intensity hydroxyl stretching Raman bands were found at 3436 and 3355 cm−1 and are attributed to the Fe–FeOH unit. Low-frequency bands were observed at around 201, 163, 128, and 90 cm−1. The 90 cm−1 band is ascribed to the stretching vibration of the hydrated cation. Bands observed at around 780 and 880 cm−1 are ascribed to the hydroxyl deformation modes of the Fe–FeOH and Fe–AlOH.