Open AccessHigh temperature stability of natural maghemite: a magnetic and spectroscopic study
Author(s) -
Gehring A. U.,
Fischer H.,
Louvel M.,
Kunze K.,
Weidler P. G.
Publication year - 2009
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2009.04348.x
Subject(s) - maghemite , magnetite , curie temperature , magnetic mineralogy , materials science , mineralogy , rock magnetism , thermomagnetic convection , raman spectroscopy , nuclear magnetic resonance , magnetization , analytical chemistry (journal) , ferromagnetism , geology , condensed matter physics , chemistry , remanence , metallurgy , magnetic field , physics , quantum mechanics , chromatography , optics
SUMMARY A combined magneto‐mineralogical approach is used to diagnose maghemitization in magnetic grains of basaltic rock fragments from sand dunes in the Namibian desert in SW Africa. Data were obtained from static magnetic analysis, ferromagnetic resonance (FMR) spectroscopy, micro‐Raman spectroscopy and electron microscopy. 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.