In a lecture
delivered in 1947, John Bernal proposed that clay minerals may have served as catalyst for the formation of biologically important molecules in the processes that led to the origin of life (Bernal, 1949
). Montmorillonite, a member of the phyllosilicate group of minerals, is one of the most abundant clay minerals on Earth. Model studies have demonstrated that montmorillonite catalyzes the formation of RNA-like oligomers (Ferris and Ertem, 1992a
; Ertem et al
and references therein). The extent of catalysis, that is, the yield of oligomers, and the length of the longest oligomer formed in these reactions vary with the source of montmorillonite (Ferris et al., 1990
; Kawamura and Ferris, 1994
Phyllosilicates have also been identified on Mars by orbiters, telescopes, and robotic rovers. Data acquired by OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) on the Mars Express Orbiter reveal the presence of hydrated phyllosilicates, along with iron-bearing silicates and sulfates in the southern hemisphere (Bibring et al., 2005
). They are thought to have formed early in the planet's history, 4.6 billion to 4 billion years ago when, it is believed, Mars was warmer and wet (Bibring et al., 2006
). More recent data obtained by CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) (Ehlmann et al., 2008
) and HRSC (High Resolution Stereo Camera) (Chevrier, 2008
) identified the sites of clay deposits on Mars.
Montmorillonites are hydrous aluminum silicates arranged into a layered structure that contains small amounts of sodium or calcium, or both. It may also contain other alkali and alkaline earth cations between the layers. Each layer is composed of one octahedrally coordinated alumina sheet held between two tetrahedrally coordinated silica sheets.*
Theoretically, the structural formula of montmorillonite can be derived from pyrophyllite, which has the structure shown below:
and [ ]VI
indicate the tetrahedrally and octahedrally coordinated cations, respectively. A small fraction of Si4+
in the tetrahedra and a larger fraction of Al3+
in the octahedra are replaced by cations with lower valency states, such as Al3+
, respectively. The charge deficiency that arises from these isomorphous substitutions is counterbalanced by the interlayer cations, mostly Na+
, held between the layers. The structural formula of montmorillonite after these substitutions may be represented by
The extent of substitutions is defined as layer charge ξ
) with xy
0.6 (Emmerich et al., 2009
). Cation exchange capacity is the number of interlayer cations expressed in milliequivalent of cation per 100 grams of clay mineral.
Since the composition of montmorillonites of different sources is comparable (), the differences in catalytic activity may arise from the variations of the extent and distribution of isomorphous substitutions among them (Ertem, 2004
, p 561). Isomorphous substitutions in montmorillonites are discussed in detail by Grim and Kulbicki (1961
), Schultz (1969
), and Wolters et al.
Oxide Composition of Montmorillonites: Data Are Normalized to Weight-Ignited Dry Material to Ensure Comparability
The present study was designed to establish whether a correlation exists between the extent of catalytic activity of montmorillonites collected from different localities, which exhibit varying catalytic activities, and their layer charge density. Layer charge density of clay minerals is determined by saturating them with alkyl ammonium cations of increasing chain length and measuring the d(001) spacing by X-ray diffractometry (XRD). From the change in the d(001) spacing of montmorillonite-alkyl ammonium complexes with the length of alkyl ammonium chains, the layer charge is calculated.
Alkyl ammonium complexes of clay minerals
The interlayer cations of phyllosilicates and other layer minerals can quantitatively be exchanged with n
-alkyl ammonium cations (Lagaly and Weiss, 1970a
; Ertem and Lagaly, 1978
). In the mineral-alkyl ammonium complexes thus formed, the hydrophilic [−NH3
end of the cations are oriented close to the mineral layer, while the hydrophobic alkyl chains stand away from the mineral surface (Lagaly and Weiss, 1970a
). The structure of the resulting orderly complex that contains alternate layers of (mineral : n
-cation : mineral) is shown in . The distance between the clay mineral layers [basal spacing, d(001)
] can be precisely measured by XRD. Since the distance between the layers before saturating the clay mineral with n
-alkyl ammonium cations is known, the thickness of the organic layer can easily be determined.
Orientation of alkyl ammonium cations in the interlayer is a function of the layer charge density (Lagaly and Weiss, 1971
). As the distance between the charge-deficient sites increases, the area available for each cation also increases. As a result, cations may orient themselves either parallel (monolayer or bilayer), at an angle, or perpendicular (with increasing layer charge) to the clay surface, depending upon the charge density and the size of the alkyl ammonium cation. Therefore, by measuring the interlayer spacing, the exact orientation of the alkyl ammonium cations in the interlayer and the distance between the charge-deficient sites can be calculated.