Open AccessMultimodal investigation of thermally induced changes in magnetic fabric and magnetic mineralogy
Author(s) -
Li Z. X.,
Dobson J.,
Chen Z.,
Chang W. J.,
St. Pierre T. G.
Publication year - 1998
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 - 10.1046/j.1365-246x.1998.00691.x
Subject(s) - maghemite , magnetite , demagnetizing field , magnetic mineralogy , magnetic susceptibility , rock magnetism , anisotropy , mineralogy , geology , kaolinite , materials science , magnetic anisotropy , magnetic field , remanence , magnetization , metallurgy , chemistry , crystallography , optics , physics , quantum mechanics
Low‐field magnetic susceptibility and its anisotropy (AMS) were measured for a suite of sandstone and siltstone samples. AMS orientations measured on two systems (Bartington and Digico) differed before thermal treatment of the samples but became the same after thermal demagnetization in air to 600 °C. Six position measurement schemes for the Bartington system do not eliminate the effects of specimen inhomogeneity and other errors, whereas 12‐ and 24‐position measurements give good agreement with the Digico anisotropy meter and with the observed petrofabric. Thermal demagnetization from temperatures between 400 and 650 °C had the effect of enhancing both the magnetic susceptibility and AMS. Although the most profound mineralogical change due to heating was the conversion of kaolinite into metakaolin, IRM, XRD, DTA and Mössbauer spectroscopic analysis demonstrate that the changes in magnetic properties were due to the transformation upon heating of trace amounts of sulphides into magnetite and/or maghemite and haematite. Both magnetic susceptibility and the degree of anisotropy decrease with higher‐temperature thermal demagnetization due to the oxidation of the newly formed magnetite and/or maghemite into haematite. The magnetic foliation of the newly formed magnetite/maghemite and haematite is parallel to the bedding, possibly following the orientation of the original sulphides.