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Gibbsite, Bayerite, and Nordstrandite Formation as Affected by Anions, pH, and Mineral Surfaces¹

ABSTRACT

X-ray diffraction, differential thermal, and chemical analysis indicate that the pH, presence of anions or salt, and mineral surfaces have important roles in the formation of crystalline Al(OH)₃ polymorphs from Al-interlayers and/or hydroxy-Al polymers.

Hydroxy-Al was introduced into kaolinite, quartz, and montmorillonite systems by slowly adding appropriate amounts of AlC₃ and NaOH (or OH-resin) into rapidly stirred suspensions. Systems having various OH/Al molar ratios were prepared, and they were aged 0, 1, 3, and 6 months.

The pH or OH/Al molar ratio had the greatest influence on controlling the relative amounts of gibbsite and bayerite that crystallized in the various systems. Gibbsite crystallized better in acid environments, nordstrandite in slightly acid to neutral, and bayerite in basic environments. Nordstrandite also crystallized under acid conditions in systems having relatively low amounts of Na⁺ and Cl^- ions.

Mineral surfaces and/or interlayer water structures, in addition to anions or salt, control the degree of crystallinity of the various Al(OH)₃ polymorphs.

¹ Contribution from the Agronomy Department, Virginia Polytechnic Institute, Blacksburg. Presented before Div. II, Soil Science Society of America, Kansas City, Mo., Nov. 19, 1964. Financial assistance from the National Science Foundation, Grant No. G-11402, is gratefully acknowledged.

² Former Research Assistant, now Assistant Professor of Agronomy, University of Kentucky, and Professor of Agronomy, respectively.

Received for publication February 8, 1965. Accepted for publication May 10, 1965.