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Abstract The Henkel plot (logarithm of magnetic susceptibility versus density of rock samples) reveals that most rocks fall on either a “magnetite trend” or a “paramagnetic trend.” Interpretation of gravity and magnetic surveys is improved when the mineralogical and lithological basis of these trends is understood. We present a quantitative mineralogical mixing model, involving the components QFC (quartz‐feldspar‐calcite), FM (ferromagnesian silicates), and M (magnetite) and discuss the geological processes which produce or modify these mixtures. Igneous rocks mostly plot on the magnetite trend, where the FM/M ratio is about 10. The density‐susceptibility mineralogical mixing model is compatible with the CIPW mineral calculation for igneous classification from chemical analyses. Sedimentary and metamorphic processes usually involve oxidation, reduction, and/or iron loss, all which are magnetite‐destructive and lead to petrophysical measurements along the paramagnetic trend where FM/M > 1,000. Mineralization, with the introduction of sulfides and oxides, leads to dense rocks which do not plot along the magnetite nor paramagnetic trends. This quantitative analysis provides a method to integrate geological processes in the interpretation of geophysical surveys.