AMORPHOUS FERRI-ALUMINOSIL ICATES IN SOME TROPICAL FERRUGINOUS SOILS

ABSTRACT: The amorphous mineral selectively dissolved from the clay (<2 ~m) fractions of twenty-six ferruginous soils contains considerable iron in addition to silicon and aluminium. SIO2/A1203 and SIO2/R203 molar ratios are between 2"03-3.52 and 1"72-2"95 respectively. The model of the amorphous ferri-aluminosilicate (AFAS) consists mainly of negatively charged tetrahedrally coordinated silica-alumina phase Si3A106(OH)4 containing domains of neutral FeOOH, with an outer positively charged hydroxyaluminium polymeric component [Al(OH)2.s],.The calculated hydroxyl water content of the AFAS averages 17.8~; cation exchange capacity varies from 48.6 to 112-0 mEq/100 g and shows a negative correlation with the outer hydroxy-aluminium octahedral component and a positive correlation with the ratio of the tetrahedral Si-AI component to the octahedral hydroxyaluminium component. The K-fixation capacity (1 "9-6.1 mEq/100 g) of the AFAS does not appear to be related to the chemical composition. The genesis of the amorphous mineral is discussed