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The influence of phosphate anion on the interaction of hydroxy-Al ions and monomelic silicic acid was studied in systems having an initial Si concentration of 1.54 × 10−3 M, a Si/Al molar ratio of 0.5, an OH/Al molar ratio of 2.0, and P/Al molar ratios of 0–0.04. Parent solutions were heated to 95°-100°C for 110 hr (1 atm), and the precipitation (>0.01 Mm) and soluble products (<0.01 μm) formed were examined by X-ray powder diffraction, infrared spectroscopic, electron optical, and chemical analyses. The amount of imogolite in the precipitates decreased with increasing the P/Al molar ratio of the parent solution from 0 to 0.005. The tube length and the degree of order of imogolite formed decreased as the P/Al molar ratio of the parent solution increased. Imogolite was not found in the precipitates formed in the solution at a P/Al molar ratio of 0.01. As the P/Al molar ratio of the solution rose, the amount of “proto-imogolite” (soluble complexes of hydroxy-Al ions and orthosilicic acid) in freeze-dried soluble products increased; bayerite and/or boehmite also were present in the precipitates. No material precipitated from the solution having a P/Al molar ratio of 0.04. The phosphate anion apparently strongly inhibits the growth of “proto-imogolite” nuclei and the subsequent formation of imogolite tube structure. The present study indicates that phosphate anion, common in soil solutions and natural waters, affects the genesis of imogolite in nature.

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