THE EFFECT OF DRY HEAT ING ON THE CHEMISTRY, SURFACE AREA, AND OXALATE SOLUBIL ITY OF SYNTHETIC 2-LINE AND 6-LINE FERRIHYDRITES

under dry conditions. Heating to 400 K lowered the ratios of structural OH to Fe203 without aphase transformation. The lowest ratios observed at 400 K were 1.23 (2-line) and 0.85 (6-line ferrihydrite) after 1150 h. For 6-line ferrihydrites heated to 400 K, the solubility in oxalate and the BET surface areas decreased with increasing heating time, whereas the unit-cell parameters, the relative peak intensities and the mean coherence lengths did not change significantly. Heating the 2-line ferrihydrites to400 K produced microporosity. Ferrihydrite is a poorly-ordered iron oxyhydroxide (Towe & Bradley, 1967; Chukhrov et al., 1973) which seems to be an indicator of pedogenic environments in cool to temperate, moist climates (cf. Schwertmann, 1988). In soils of warm and drier areas, i.e. where relative humidity is markedly lower than 100 % for prolonged periods, hematite and goethite are common, but not ferrihydrite. The mutual exclusion of ferrihydrite and hematite in soil may be explained by the internal dehydration of ferrihydrite to hematite (Fischer & Schwertmann, 1975), although the rate of this transformation process is considerably higher in the presence of liquid water than at relative humidities down to 75 % (Torrent et al., 1982). Numerous tudies have focused on the transformation process and its products eithe