The Retention of Amorphous, Colloidal ‘Ferric Hydroxide’ by Kaolinites

Summary The sorption and distribution of amorphous colloidal ‘ferric hydroxide’ particles on the surfaces of kaolinite flakes were studied by electron microscopy. The sorption reaction was crystallographically specific with particles observed only on basal plane surfaces and was unaffected by the presence of excess NH 4 +, Ca ++ , or Al +++ ions. Cationic surface‐active agents prevented the reaction. Differences observed in colloid distribution and density on the introduction of a competing surface indicated that only one surface of a flake was attracting colloid particles and similar experiments with quartz and gibbsite suggested the silicatetrahedra surface of kaolinites as the probable site of colloid fixation. The distribution of colloid was unaffected by pH variation, saturation with ammonium acetate, prolonged washing, or ultrasonic vibrations. The colloid was removed only by extraction with sodium dithionite.