Wyoming-type montmorillonites

It is considered likely that the catalytic Na-rich, Wyoming-type montmorillonites are distinctive not because of the composition of the original volcanic ash, but rather due to the saline-rich waters into which the volcanic ash was deposited and subsequently altered during shallow burial. 

The catalytically active, Wyoming-type montmorillonites are inferred to have developed their distinctive, high-Na smectite-rich compositions following deposition of granitic volcanic ash into a shallow, saline ocean during the Late Cretaceous. Oxygen isotopic data from Hadean zircons have been interpreted as indicating that oceans may have been present on the Earth by as early as 4,300-4,400 million years ago (62-63). While current evidence provides no information on the composition of the hypothesized Hadean seawater, the existence of oceans on the Hadean Earth is a necessary environment on the ancient Earth if catalytically active montmorillonites formed. 

Although uncertainty remains, the environments required for the formation of Wyoming-type montmorillonites (i.e., explosive eruptions of volcanic ash granitic compositions deposition of volcanic ash into saline water shallow burial) may conceivably have been present on the prebiotic Earth. Formation of RNA oligomers would have proceeded fi-om activated monomers if the aqueous environment had a pH in the neutral to mildly basic range (40). 

Wyoming-type montmorillonite [layer charge x = 0.75, with 1/3 being tetrahedral charge sites (4)] bearing one monolayer of adsorbed water has a water/counterion molar ratio of 5 1/3, equivalent to a 10.4 m solution. For two or three water monolayers, the H20/counterion ratio increases proportionately and the equivalent solution molalities are 5.2 m or 3.46 m, respectively. Thus, from the perspective of counterion solvation, interlayer water on montmorillonite should be similar to a very concentrated aqueous solution. 

The restricted interlayer mobility of the cations can be seen qualitatively in MD trajectory plots projected onto the basal planes (YX plane) and onto the plane of the clay mineral c-axis (ZX plane). These are shown in Figures 5 and 6 for trajectories in the interlayer of Wyoming-type montmorillonite (18). Figure 5, for the one-layer hydrate, indicates sporadic (over 200 ps) entrapment of in the siloxane... 

Table I. Self-Diffusion Coefficients for Monovalent Adsorbed Cations on Wyoming-Type Montmorillonite...

Reynolds and Hower [1970] have studied a range of illite-montmorillonites (from Colorado, Montana, and Wyoming), using Reynold s method for calculating the diffracted intensity. They find that three main types of interstratification are present (1) random, (2) allevarditelike, (3) four-layer units of type IMII. (1) occurs generally with 40 to 100% ex-pandibility, (2) for <40%, and (3) only for < 10%. 

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