Open AccessThe Effect of Heat Treatment on Magnetic Minerals in Red Sandstones, Studied using the Technique of Rotational Hysteresis
Author(s) -
Cowan B. K.,
O'Reilly W.
Publication year - 1972
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1972.tb06159.x
Subject(s) - magnetite , pigment , mineralogy , hysteresis , annealing (glass) , magnetic hysteresis , materials science , analytical chemistry (journal) , chemistry , nuclear magnetic resonance , geology , metallurgy , magnetic field , condensed matter physics , magnetization , environmental chemistry , physics , organic chemistry , quantum mechanics
Summary The effect of heat treatment on rotational hysteresis ( W R ‐ H ) characteristics has been observed for samples of red sandstone, haematites and clay minerals. On heating the sandstones up to 930°C in air a steady rise in W R levels was observed. This process, which may be due to the production of α‐Fe 2 O 3 . pigment or an annealing effect (which is more probable), has an activation energy of about 0.2 eV/molecule. No magnetite was produced below 950 °C but, after heating to 1200 °C, a loss in α‐Fe 2 O 3 , pigment was observed together with a corresponding production of Fe 3 O 4 . W R ‐ H analysis of heated chamosite showed the production of magnetite at moderate temperatures (∼ 500 °C). An increasing magnetic hardness was observed as specific surface increased for a series of pigment samples. This effect and the surprising observation that magnetic hardness increased after annealing may be consistent with the competing anisotropy model for haematite. The pigment and specularite content of the sandstones was estimated by comparison with W R , ‐ H curves for synthetic samples. Although these are not necessarily exactly like those occurring naturally, it is still possible to make comparative estimates for rock samples within a given formation. Although the number of samples in the present study is too few to draw positive conclusions, the results are not inconsistent with a correlation between W R ‐ H and palaeomagnetic characteristics. The total amount of haematite (specularite plus pigment) is in moderately good agreement with values obtained by another method.