TY - JOUR
T1 - Multimodal investigation of thermally induced changes in magnetic fabric and magnetic mineralogy
AU - Li, Zheng-Xiang
AU - Dobson, J.
AU - Chen, Z.
AU - Chang, W.J.
AU - St.pierre, Tim
PY - 1998
Y1 - 1998
N2 - 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 degrees 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 degrees 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 Mossbauer 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.
AB - 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 degrees 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 degrees 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 Mossbauer 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.
U2 - 10.1046/j.1365-246X.1998.00691.x
DO - 10.1046/j.1365-246X.1998.00691.x
M3 - Article
VL - 135
SP - 988
EP - 998
JO - Geophysical Journal International
JF - Geophysical Journal International
SN - 0956-540X
ER -