b'Don Emersons best of Exploration GeophysicsFeatureFigure 5.Range of magnetic susceptibilities for important magnetic minerals and major rock types. Stippled portions of bars indicate common susceptibility ranges for various lithologies. Note the bimodal susceptibility distributions for many rock types.Puranen (1989) presented results from very large petrophysicalVery highly oxidised rocks, however, tend to contain sampling programs in Finland. His data confirmed that allhaematite rather than magnetite and are therefore also broad field names, such as granite, gabbro, mica schist,weakly magnetic. Within each of the subpopulations, the amphibolite etc., exhibit distinctly bimodal susceptibilitymodal and mean values of susceptibility are much more distributions. Figure 7 shows frequency distributions ofclosely related to rock type than for the total susceptibility susceptibility for major intrusive rock types, based on Puranensdistribution. For the paramagnetic subpopulation, in data. The two modes of the frequency distribution correspondparticular, the susceptibility is directly related to the to distinct paramagnetic and ferromagnetic populations, withchemical composition, which tends to have a restricted a pronounced intervening gap. Iron in the weakly magneticrange for each lithology. Clark and Emerson (1991) give subpopulation is incorporated into paramagnetic silicatethe relationship between iron content and susceptibility minerals, predominantly as Fe2+, whereas similar rocks that arefor paramagnetic rocks and between magnetite content moderately to strongly magnetic contain significant Fe3+, whichand susceptibility for rocks that contain more than ~ 0.1% is incorporated into magnetite. magnetite by volume.49 PREVIEW APRIL 2020'