Concentration of Iron Oxides from Soil Clays by 5 M Naoh Treatment: the Complete Removal of Sodalite and Kaolin

A B S T R A C T: The K~mpf & Schwertmann (1982) procedure for concentrating iron oxides in soil clays by dissolution of kaolin and gibbsite by boiling for 1 h in 5 M NaOH may not dissolve all kaolin, and also results in the precipitation of sodalite. For the complete dissolution of kaolin in kaolin-rich soil clays a boiling time of 2 h in 5 M NaOH was required. The large amounts of sodalite produced were not removed by the prescribed single wash in 0-5 M HC1. Oxalate soluble AI contents of iron oxide concentrates were sometimes very high and dithionite Fe contents were very low both in concentrates containing sodalite, and in those for which sodalite was not detected by XRD, but where a previously unsuspected amorphous sodalite-like phase may have been present. Complete removal of precipitated sodalite was achieved by two extractions with 0.5 M HCI at 25~ for 20 rain. This modified procedure does not alter the Al-substitution and crystal size of goethite, hematite and maghemite as determined by XRD measurements. Iron oxides are often poorly crystalline and present at low concentrations in soils (K~mpf & Schwertmann, 1982). For the characterization of iron oxides by X-ray diffraction, MOssbauer spectroscopy, electron microscopy, chemical analysis and other techniques, an iron oxide concentrate may be required. The method proposed by Norrish & Taylor (1961) for the concentration of iron oxides by boiling soil clays for 30-60 min in 5 M NaOH has been used for many studies of iron oxides in soils (Davey et al. A modification to the above method by adding-0.2 M Si to 5 M NaOH to inhibit the dissolution and recrystallization of Al-substituted goethites (K~impf & Schwertmann, 1982) has been widely adopted in recent years (Schwertmann & Latham, 1986; Anand & Gilkes, 1987). Sodalite (sodium aluminium silicate) may be precipitated during the procedure and the presence of 0.2 N Si may increase the amount of sodalite that forms. One washing with 0.5 M HC1 is used to remove the sodalite precipitated during the above procedure. Boiling kaolin-rich clays for 1 h in 5 M NaOH does not always completely remove kaolin as was observed by Davey et al. (1975)~ Bigham et al. (1978) doubled the extraction time in order to achieve complete dissolution of kaolin. We have observed that when iron oxides are concentrated from kaolin-rich soil clays using the procedure proposed by K …