Thermodynamic Evaluation of Amorphous Aluminosilicate Binding Agents in Fragipans of Western Kentucky1
- A. D. Karathanasis2
Composition of soil solutions in contact with fragipan and nonfragipan horizons of Fragiudalfs in western Kentucky were related to the composition of soil amorphous constituents removed by various extractants. Increased amorphous Si/Al ratios in fragipan horizons were correlated well with reduced pH4SiO4 solute functions. A Si-rich amorphous aluminosilicate precipitate of the general formula [Al(OH)3](1 − x) [SiO2]x and with a molar fraction ranging from 0.55 to 1.00 is proposed to account for the rigidity of the fragipan horizons. Similar free Al-rich amorphous aluminosilicate precipitates are suggested to control Al and Si activities in solutions of nonfragipan horizons with pH >4.5. Silica and Al activities of soil solutions with pH <4.5 are consistent with the solubility of crystalline soil minerals. The amorphous aluminosilicate precipitates are considered to be weathering products of feldspar dissolution reactions. The composition, solubility, and rigidity of the amorphous aluminosilicate precipitates are believed to be controlled by conditions of loess deposition and drainage. Iron, Mg, and Na appear to make only minor contributions to the binding amorphous aluminosilicate agent, with Na having the most active involvement in the cementation process.
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