Removal of Iron Oxides and Amorphous Material From Soil Clays—Treatment Sequence Effect

In the great majority of soil samples, more iron, alumina, and silica were found when iron removal preceded extraction of alkali‐soluble amorphous material. Iron oxyhydroxide is postulated to be present as a protective surficial coating whose removal makes accessible more alkali soluble silica and alumina. Iron may also be part of an amorphous component whose solubility in alkali is increased after removal of the contained iron by a dithionite treatment. Prior alkali treatment reduces the susceptibility of iron oxyhydroxides to the dithionite treatment.