FORMATION OF FERRIC OXIDES FROM AQUEOUS-SOLUTIONS - A POLYHEDRAL APPROACH BY X-RAY ABSORPTION-SPECTROSCOPY .2. HEMATITE FORMATION FROM FERRIC GELS

The evolution of the local structure around ferric ions has been followed through X-ray Absorption Spectroscopy (XAS) during the formation of hematite from ferric gels at 92°C. Fe K-edge structures and Extended X-ray Absorption Fine Structure (EXAFS) results confirm that ferric ions remain 6-fold coordinated during this process. A detailed study of the structural modifications involved has been conducted based on the analysis of the nearest and next-nearest Fe distances, which give access to the interpolyhedral relations at all stages of the evolution. Non-aged, freshly precipitated, hydrous gels possess a goethite/akaganeite-like local structure. During gel → hematite transformation, an intermediate stage is indicated by a local structure which cannot be interpreted as a mixture of the initial gel and final hematite. This intermediate stage is stable between 1 and 6 hours ageing and is characterized by Feoctahedra joined by corners (d(Fe-Fe) = 3.43 A ̊), edges (d(Fe-Fe) = 3.05 A ̊), and faces (d(Fe-Fe) = 2.89 A ̊). The local structure of this transient phase is similar to that of ferrihydrite and feroxyhite, but unlike these two oxides, it remains long-range disordered as XRD patterns display no hkl reflection. At 6 hours ageing, hematite is detected by EXAFS and XRD. Hematite crystallites develop at the expense of the gel, and after 130 hours at 92°C the transformation is completed. The transient configuration between the freshly precipitated gel and final hematite is an important step during the formation of Fe oxides in continental and marine environments. The appearance of facesharing octahedra within hydrous gels is thought to reduce the activation energy barrier of the ferric gel → hematite transformation. The metastability of the natural gels, which possess a local structure similar to that of the transient phase, is explained by the role played by the impurities in inhibiting the dioctahedral reorganization of the framework and hence the formation of hematite. © 1990.