Weathering products formed on surfaces of both potassium and plagioclase feldspar (An70), which were continuously leached in a Soxhlet extraction apparatus for 140 days with 7·21 of distilled water per day at a temperature of approximately 78°C, are morphologically identical to natural products developed on potassium feldspars weathered under conditions of good drainage in the humid tropics. The new products, which first appear as tiny bumps on the feldspar surface, start to develop mainly at exposed edges but also at apparently random sites on flat cleavage surfaces. As weathering continues, the bumps grow outward from the feldspar surface to form tapered projections, which then develop into wide-based thin films or sheets. The thin sheets of many projections merge laterally to form one continuous flame-shaped sheet. The sheets formed on potassium feldspars may then roll to form tubes that are inclined at a high angle to the feldspar surface. Etch pits of triangular outline on the artificially weathered potassium feldspars serve as sites for development of continuous, non-rolled, hollow tubes. It is inferred from its morphology that this weathering product is halloysite or its primitive form. The product of naturally weathered potassium feldspars is halloysite · 4H2O.

Formation of halloysite from feldspar:low temperature, artificial weathering versus natural weathering.

Hydrothermal syntheses were performed at various pH values and temperatures to induce variability in kaolinite defect density. Temperature of synthesis ranged from 200 to 240~ for 21 d. Initial pH at room temperature ranged from 0.5 to 14. The starting material was a hyd~othermally treated gel, with an atomic Si/A1 ratio of 0.93, partly transformed into kaolinite. Kaolinite was obtained for a wide range of pH. Although no influence of temperature on "crystallinity" (i.e., defect density) was observed, the effect of pH was important. A continuous series was obtained from a low-defect kaolinite, with high thermal stability and a hexagonal morphology for the most acidic final pH, to a high-defect kaolinite, with low thermal stability and lath shape for the most basic final pH. These variations of kaolinite properties appear related to the pH dependence of kaolinite surface specia- tion. Increasing pH value results in increased cation adsorption on the kaolinite external surfaces and increases in the elongation of particles.

Influence of Synthesis pH on Kaolinite ``Crystallinity'' and Surface Properties

To obtain kaolinite at low temperature and pressure from the system Si(OH)4-Al3+-H2O, the sixfold coordination of aluminum is a necessary prerequisite. Kaolinite was synthesized at pH values from 2 to 9 and with a ratio of SiO2 to Al2O3 in solution from 1 to 10 by means of the complexation of Al3+ and fulvic acid.

http://science.sciencemag.org/content/sci/171/3974/896.full.pdf

Kaolinite: synthesis at room temperature.

A structure model for amorphous hydrated or dehydrated silico-aluminas with composition varying between 0 < Al: Al + Si < 1 is presented. A central core made from a tetrahedral network in which silicon is partially substituted by aluminium carries a net negative electrical charge. This charge is balanced by more or less polymerized hydroxyaluminium cations forming a coating around the core.

Structural organization in amorphous silico-aluminas

Experimental study of the hydrothermal formation of kaolinite

Low-temperature syntheses of kaolin minerals

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Low-temperature syntheses of kaolin minerals