The effect of Al on Fe oxides. XIX. Formation of Al-substituted hematite from ferrihydrite at 25 degrees C and pH 4 to 7

Iron oxides in surface environments generally form at temperatures of 25 +/ - 10 degrees C, but synthesis experiments are usually done at higher temper atures to increase the rate of crystallization. To more closely simulate na tural environments, the transformation of 2-line ferrihydrite to hematite a nd goethite at 25 degrees C in the presence of different Al concentrations and at pH values from 4 to 7 was studied in a long-term (16-20 y) experimen t. Aluminum affects the hydrolysis and charging behavior of 2-line ferrihyd rite and retards crystallization. Al also promotes the formation of hematit e over goethite and leads to multidomainic discoidal and framboidal crystal s instead of rhombohedral crystals. The strong hematite-promoting effect of Al appears to be the result of a lower solubility of the Al-containing fer rihydrite precursor relative to pure ferrihydrite. Hematite incorporates Al into its structure, as is shown by a decrease in the a and c-cell lengths and a decrease in magnetic hyperfine fields (Mossbauer spectroscopy). With hematite formed at low-temperature, these decreases were, however, smaller for the cell length and greater for the magnetic field than for hematite pr oduced at higher temperatures. Both phenomena are removed by heating the he matite at 200 degrees C. They are attributed to structural OH and/or struct ural defects. The relative content of Al in the structure is lower for hema tite formed at 25 degrees C than for hematites synthesized at higher temper atures (80 and 500 degrees C). The maximum possible substitution of one six th of the Fe positions was not achieved, similar to soil hematites. These r esults show that properties of widely distributed soil Al-containing hemati tes can reflect formation environment.