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

Abstract Solution activities of aluminum (Al 3+ ) 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‐⅓pAl 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‐⅓pAl 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 ) [SiO 2 ] x } R rather than gibbsite and kaolinite.