Formation of Maghemites From Oh-free and Oh- Containing Precursors
Abstract
Two pathways for forming magnetite-maghemite are known: Biological mineralisation of magnetite and thermal transformation of precursor phases. Maghemites can be derived from low temperature oxidation of magnetite (OH-free) or from high temperature dehydroxylation of OH-containing precursors. A distinction between thes e kind of maghemites is indispensable in archeology to determine if settling place have been affected by fire or not. The aim of this study was to check whether the system magnetite-maghemite formed by thermal formation pathway inherit some OH from the precursors. Synthetic iron oxides were heated with and without addition of reducing agent at 180 - 300 and 400°C. Natural samples were taken from Stanjek (2001) and Schwertmann &Taylor (1974). Synthetic magnetite was used as OH-free precursor and ferrihydrite, lepidocrocite and goethite were used as OH-containing precursors. To investigate maghemite structural properties, namely cell edge length "a", site occupancy fraction and mean coherence length we used the Rietveld analysis of X-ray diagrams. Fe2+ and Fe+3 were determined from chemical analysis and the amount of OH resulted from balancing the negative charge of 32 oxygens per unit cell. Some structural properties and the oxidation parameters show a considerable distinction between maghemites formed fro m dehydroxylation-oxidation of OH- precursors and maghemites derived from magnetite oxidation at low-temperature.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.6883S