Simulation of soil iron oxide production via alteration of ferrihydrite confirms direct formation of maghemite and partially oxidized magnetite—Implication for magnetic enhancement models
Abstract
In spite of the empirical success of the correlation between rainfall and magnetic mineral enhancement in soils across China, Russia and elsewhere, a generally acceptable model of enhancement has eluded our community. Recent field and laboratory studies demonstrate the importance of both strongly magnetic (magnetite, maghemite) as well as weakly magnetic (goethite, hematite) nano-phase minerals forming in response to rainfall and temperature. In particular, the ferrihydrite -> (hydro) maghemite -> hematite pathway of Torrent et al. (2003, et seq.) and formation of magnetite or hematite from nano-goethite under reducing or oxidizing atmosphere by Till et al. (2014) are particularly instructive. Here we report ferrihydrite alteration in constant pH=6.8 at 90°C even without the presence of any strongly adsorbing organic ligand. Aging of an initially pure 2-line ferrihydrite over 4 hours, and freeze-drying the specimens to prevent further alteration, produces small amounts of a mixture of maghemite, hematite and a small amount of partially oxidized magnetite, as identified by its isotropic point, detected by cooling an SIRM imparted at 300K. The details of the precise pathways of initial, intermediate and final products and their relative amounts are difficult to estimate in mixtures, but in future experiments we will attempt to do just that. However, since both the strongly and weakly magnetic products were formed from the same ferrihydrite starting material, it may not be necessary to assume that magnetite -> maghemite, or maghemite -> hematite, or hematite -> magnetite are unique pathways for production of magnetic enhancement in soils. Instead, it appears that multiple, simultaneously active pathways may allow ferrihydrite to directly produce weakly and strongly magnetic iron oxides in soil at the same near normal pH.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMGP43D..06B
- Keywords:
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- 1505 Biogenic magnetic minerals;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1512 Environmental magnetism;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1518 Magnetic fabrics and anisotropy;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM