Simulation of Soil Iron Oxide Production via Alteration of Ferrihydrite Confirms Direct Formation of Goethite, Maghemite and Hematite: Implication for Magnetic Enhancement Models
Abstract
In spite of the empirical success of the correlation between rainfall and magnetic mineral enhancement in soils, a generally acceptable model of enhancement has not been established. 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 --> maghemite --> hematite pathway is particularly intriguing. Here, we report experimental ferrihydrite alteration in pH=6.56 at 90 ºC without the presence of any strongly adsorbing organic ligand. Aging of an initially pure 2-line ferrihydrite over a 32-hour period leads to constant production and growth of goethite crystals, together with small amounts of maghemite and hematite aggregates, which evolve to fully crystalline hematite particles. The nanoparticles were sampled at regular time-intervals and the specimens were freeze dried to prevent further alteration before characterization. The details of the precise pathways of the initial, intermediate and final products and their relative amounts are difficult to estimate in mixtures; however, XRD, Mössbauer, TEM and magnetic observations indicate that both the strongly and weakly magnetic products were formed from the same ferrihydrite starting material, remained in coexistence during an intermediate "aggregate" stage, and evolved to produce fully crystalline hematite and goethite. The alteration pathway observed is not too dissimilar from recently reported experimental results under similar laboratory conditions; however, fundamental differences are the rapidity of the conversion to "final" hematite crystals observed here (32 hours) and the continuous production and coexistence of large amounts of goethite throughout.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP31B0710B
- Keywords:
-
- 0419 Biomineralization;
- BIOGEOSCIENCES;
- 1512 Environmental magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM