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Gibbsite Formation From Aluminum-Interlayers in Montmorillonite¹

ABSTRACT

Gibbsite crystallized from hydroxy-Al which previously had been precipitated on the interlayer surfaces of the 2 to 0.2µ fraction of montmorillonite. Hydroxy-Al interlayers were prepared with OH/Al molar ratios of 0.38, 0.75, 1.50, 2.25, and 3.00 and were aged 1, 3, and 6 months. In the samples that were not aged, the CEC decreased from 78 to 2 me. per 100 g. as the OH/Al molar ratio increased. In addition, the basal spacing increased from 10.2 to 14.7Å. (K-saturated, 300°C.) as the OH/Al molar ratio increased.

X-ray diffraction, thermo-gravimetric, and differential thermal analysis studies indicated that the hydroxy-Al interlayers formed with OH/Al molar ratios of 0.38, 0.75, and 1.50 were stable when aged 6 months; whereas, samples with OH/Al molar ratios of 2.25 and 3.00 were not. Gibbsite formed after 1 month in the sample with an OH/Al ratio of 3.00 and after 3 months in the sample with an OH/Al ratio of 2.25, and the amount of gibbsite increased with time. Gibbsite formed in the presence of montmorillonite at a pH of < 4.3. After the Al-interlayers were removed through gibbsite formation, the montmorillonite regained its cation-exchange capacity. The crystallization of gibbsite from Al-interlayers may be one mechanism for the formation of this mineral in some acid soils.

¹ Contribution from the Agronomy Department, Virginia Polytechnic Institute, Blacksburg. Presented before Div. II, Soil Science Society of America, Ithaca, N. Y., Aug. 20, 1962. Financial assistance from the National Science Foundation. Grant No. G-11402 is gratefully acknowledged.

² Research Assistant and Professor of Agronomy, respectively.

Received for publication December 26, 1962. Accepted for publication April 9, 1963.