Hydrodynamic effects produced by a high speed rotating (Ultra-Turrax) generator accelerate the transformation of y-FeOOH to a-FeOOH in aqueous media and cause an increase in the proportion of a-FeOOH in the products of amorphous iron(iii) gel ageing.It is well known1 that y-FeOOH (lepidocrocite) is transformed to y-Fe203 when heated in air at 200-300 "C. A recent report2 described mechanical dehydration of y-FeOOH at substantially lower temperatures to yield a-Fe203. Grinding in cyclohexane under analogous conditions, on the other hand, results in the formation of y-Fez03.3When heated in water at 16&180"C, y-FeOOH is transformed into a mixture of a-FeOOH and a-Fe2O3. Boiling in distilled water causes no phase transformation of y-FeOOH even when continued for 50 h.4 The amorphous iron(m) gel Fe203.nH20, when boiled in water for several hours, is
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