Influence of Relative Humidity on the Crystallization of Fe(III) Oxides from Ferrihydrite

Abstract Ferrihydrite prepared in different manners was kept under relative humidities ranging from 75 to 100% and at temperatures of 45° and 55°C for 180 days. Ferrihydrite transformed to hematite and goethite at relative humidities close to 100%, but at lower relative humidities the transformation was less pronounced and hematite was highly favored over goethite. Increasing temperature also favored hematite over goethite, and Al substitution completely prevented goethite formation. These results suggest that hematite can form in relatively dry, warm soils or sediments, although more slowly than in moister environments.