Abstract: A series of mixed iron and titanium oxide coprecipitates ranging in composition between 0 < Ti/Ti + Fe < 1 was synthesized and aged under varying conditions of pH, temperature and time in order to establish a working model for pedogenic titanium and titano-ferric oxides. X-ray powder diffraction (XRD), selective chemical dissolution, magnetic susceptibility, charge distribution and electron optical data indicate that the freshly prepared Fe-Ti oxides consist of an Fe-rich (Ti-ferrihydrite) phase (Ti/Ti + Fe ≤ 0.70) having pH-dependent positive charge and a Ti-rich phase (Ti/Ti + Fe ≥ 0.7) with permanent and pH dependent negative charge.
Synthetic Ti-ferrihydrite and amorphous TiO2 were completely soluble in acid ammonium oxalate (2 hr extraction in the dark) whereas poorly crystalline anatase (width at half height, WHH > 2.0°2θ) was partly oxalate soluble. NH4-oxalate soluble Ti was particularly high in soils developed under a cool montane climate (afro-alpine) and lower in soils of warmer subtropical climate, which contain anatase and rutile.
Several mixed Fe-Ti crystalline phases were identified after aging NH3 coprecipitates of Fe and Ti nitrate at 70°C and pH 5.5 for 70 days: (1) goethite and hematite in the composition range 0 < Ti/Ti + Fe ≤ 0.20; at low Ti concentrations (<5 mole %) goethite was favored and/or hematite inhibited; (2) microcrystalline pseudorutile in the composition range 0.20 ≤ Ti/Ti + Fe ≤ 0.70; (3) anatase and ferriferous anatase in the range 0.70 ≤ Ti/Ti + Fe < 1.0; with decreasing proportion of Ti the crystallinity of anatase decreased.
The results suggest that secondary or pedogenic Ti-Fe oxides can form by coprecipitation and crystallization in the weathering solution, and emphasize the essential role of water (as opposed to dry oxidation) in the alteration of primary titaniferous minerals.