ABSTRACT The precipitation of carbonate into alkalinc silicate solutions results in the formation of self-assembled crystal aggregates with noncrystallographic morphologies. These precipitates emulate biologically induced mineral textures as well as display forms typical of primitive microfossils. The precipitation behavior varies with pH, i.e., as a function of the species created by dissociation of the silicic acid under alkaline conditions. Calcite single crystals and crystal aggregates precipitated in these media display complex forms derived From the specific inhibition of some crystal faces, and eventually, noncrystallographic shapes such as sheaf-of-wheat with self-organized banding develop. When strontianite and witherite precipitate in these environments at pH higher than 10, their crystal aggregates display in addition very specific morphologies, such as target patterns, scrolls, twisted ribbons, spirals, fingers, etc., with typical sizes ranging from microns to millimeters. The crystallites of the metal carbonate are embedded in a silicate matrix and are co-oriented and parallel to each other, suggesting that both the loci for nucleation and the orientation of the carbonate groups are controlled by the silica phase. The silica concentration (>250 ppm SiO 2 ), ionic force, and pH values (>8.5) required for the phenomenon to be observed are well within the range of values measured in contemporary alkaline lakes. A number of geological scenarios where the phenomenon could occur have been identified, among which are: a) contemporary lakes and thermal springs associated with alkaline magmatism such as those in the African rift valley; b) Precambrian (particularly Archean) terranes where cherts formed as a result of direct precipitation of silica; and c) a scenario on Earthlike planets where the existence of a silica-rich environment derived From hydrolysis of alkaline rocks is predicted.

Abstract Smectite clays have been modified with organic onium ions for more than 60 y and are important in a diverse range of industries including oil-well drilling, paint, grease, ink, cosmetics, environmental clean-up, polymer nanocomposites, and pharmaceuticals. The number of commercially available onium ions limits the variety of surface modifications that can be made. A substantial amount of research has been conducted on other methods of organic modification of smectite clays in order to remedy this situation. One method that has seen increased attention is the use of ion-dipole-bonding of organic molecules, oligomers, or polymers to the exchangeable cation on the clay surface. Utilizing this method, a unique class of surface-modified clays can be produced. In some cases self-assembly of certain organic molecules has been discovered in which the molecules form rigid posts around each cation on the surface. The intercalation/exfoliation behavior of smectite clays modified via ion-dipole bonding utilizing various small organic molecules and selected polar polymers is discussed here. The types of organic molecules that can be utilized to surface modify smectites is discussed in detail as are the characteristics of the complexes that are formed.