Chemical and crystal controlled magnetization of rocks

Abstract

In large part this review is intended to show the role of chemical magnetization among the principal processes by which rocks become magnetized. Chemical magnetization can occur at any time during the geological history of a formation. The high coercive force of hematite makes for a stable magnetization; thus, red beds have been investigated extensively. However, the extent to which red beds are chemically magnetized has not been established. Since the hematite crystal shows a strong magnetic anisotropy, being ferromagnetic in the basal plane and only slightly, if at all, ferromagnetic along the c-axis, preferred crystallographic orientation can exert a control as to the direction of magnetization. In the Allard Lake hemo-ilmenite ore of Quebec, the preferred orientation seems to exert a controlling influence on the direction of magnetization. Also, such an influence may exist in the Clinton Fe ore of Alabama. On the other hand, chemically magnetized hematite from concretions in the Wilcox beds of Texas shows no appreciable preferred crystallographic orientation as found by both X-ray and magnetic susceptibility measurements. Thus, in this case at least, hematite crystals could grow without control of orientation by the Earth9s magnetic field.