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Solution Activities of Aluminum and Silicon in Highly Weathered Soils that contain Gibbsite and Kaolinite¹

ABSTRACT

Solution activities of aluminum (Al³⁺) and monosilicic acid (Si(OH)₄) in dilute acid suspensions of six Brazilian Oxisols were measured over a period of 145 days. The steady state activity function of pH-pAl ranged from 1.86 to 2.69, while that of pSi(OH)₄ ranged from 3.69 to 4.22. Although all of the soils contained kaolinite and gibbsite, the observed solute activity functions of pH-pAl and pSi(OH)₄ deviated considerably from those expected for a thermodynamic equilibrium between gibbsite and kaolinite. It was found that the solution activities of Al and Si(OH)₄ in these soils appeared to result from the dissolution of a surface aluminosilicate component (R) of variable composition-{[Al(OH)₃]_(1-x)[SiO₂]_x}_R rather than gibbsite and kaolinite.

¹ Agronomy pap. no. 1159. Contribution from the Dep. of Agronomy, New York State College of Agriculture and Life Sciences, Cornell Univ., Ithaca, NY 14853. This study is part of the work supported by the U.S. Agency for International Development under research contract csd-2490. Presented before Div. S-9, Soil Sci. Soc. of Am., Knoxville, Tenn, 27 Aug. 1975.

² Currently Group Leader, Research, Clay Div., J. M. Huber, Corp., Huber, GA and Graduate Research Assistant, Dep. of Agronomy, Cornell Univ., Ithaca.

Received for publication March 8, 1976. Accepted for publication March 9, 1977.