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Abstract--The transformation of ferrihydrite to goethite and/or hematite in alkaline media is strongly retarded by the presence of silicate species. These species probably stabilize ferrihydrite by adsorbing on the particles of ferrihydrite and linking them into an immobile network.At concentrations low enough for the transformation to proceed, silicate species promote the formation of hematite and hinder the nucleation ofgoethite. The presence of silicate species modifies the morphology of both reaction products. Hematite forms ellipsoidal single crystals, commonly displaying outgrowths of goethite. Silicate species in solution appear to enhance the development of the (021) faces ofgoethite, probably by preferential adsorption on these faces; at high levels of silicate species, goethite crystals adopt a pseudohexagonal habit. This morphology has not been observed previously for goethite.

Abstract--The dissolution of synthetic magnetite, maghemite, hematite, goethite, lepidocrocite, and akaganeite was faster in HCI than in HCIO4. In the presence of H § the C1-ion increased the dissolution rate, but the 004-ion had no effect, suggesting that the formation of Fe-Cl surface complexes assists dissolution. The effect of temperature on the initial dissolution rate can be described by the Arrhenius equation, with dissolution rates in the order: lepidocrocite > magnetite > akaganeite > maghemite > hematite > goethite. Activation energies and frequency factors for these minerals are 20.0, 19.0, 16.0, 20.3, 20.9, 22.5 kcal/ mole and 5.8 • 1011, 1.8 • 101~ 7.4 • 10 7, 5.1 • 101~ 2.1 • 101~ 3.0 x 1011 g Fe dissolved/mZ/hr, respectively. The complete dissolution of magnetite, maghemite, hematite, and goethite is well described by the cube-root law, whereas that of lepidocrocite is not.

Abstract--Hydroxy-carboxylic acids inhibit the crystallization of ferrihydrite in the pH range 9-11 in the order citric > meso tartaric > L-tartaric > lactic and favor hematite formation relative to goethite in the order L-tartaric > citric > meso tartaric > lactic.The crystal shape of hematite can change from hexagonal plates to acicular in the presence of these acids. The influence of the acids on the crystallization rises with increasing concentration and with falling pH.The effectiveness in suppressing crystallization depends on whether and how strongly the acid adsorbs on ferrihydrite and how strongly it complexes with Fe z+ in solution. Inhibition of crystallization of hematite is believed to be due to the di-and tricarboxylic acid linking ferrihydrite particles in an immobile network. Goethite formation is suppressed by the acid complexing with Fe in solution and hindering nucleation; strongly adsorbing acids also adsorb on the nuclei and hinder further growth. Certain acids can induce hematite formation because they contain a group which acts as a template for nucleation of hematite.