High temperature stability of natural maghemite: a magnetic and spectroscopic study
- chair: Gehring, A.U. / Fischer, H. / Louvel, M. / Kunze, K. / Weidler, P.G. (2009)
Geopys. J. Int. 179 (2009), 3, 1361-1371
- Date: 2009
Gehring, A.U. / Fischer, H. / Louvel, M. / Kunze, K. / Weidler, P.G. (2009): „High temperature stability of natural maghemite: a magnetic and spectroscopic study“. In: Geopys. J. Int. 179 (2009), 3, 1361-1371
Micro-Raman spectroscopy showed that the magnetic grains in the lithic fragments form oxidative solid solution series with magnetite and maghemite as end-members. The five active Raman modes at 712, 665, 507, 380 and 344 cm−1 indicate that maghemite in the magnetic grains has well-defined structural properties.
The FMR spectral analysis provides evidence for long-range dipolar coupling, which suggests intergrowth of the magnetic phases of the oxidative solid solution series. Thermomagnetic experiments and hysteresis measurements reveal a Curie temperature of about 890 K for this maghemite. Upon heating to 970 K part of the maghemite is altered to thermodynamically more stable hematite.
After selective thermal decomposition of the maghemite in a protected atmosphere, the remaining magnetic phase has a Curie temperature of 850 K, characteristic for magnetite. The unique thermal stability of this natural maghemite above its Curie temperature is explained by the well-defined mineral structure, which formed during slow oxidative alteration of magnetite under arid climate conditions.