We measured the self-potential (SP) response to advective transport of KCl concentration fronts through two laboratory-scale sand-bodies with different mineralogical composition. In pure quartz sand, the amplitude and polarity of the SP signals agreed with a previously published model. In sand containing 3% potassic feldspars and 1% micas and clays, the shape of the SP response differed significantly: the amplitude was much larger than the model prediction and a change in polarity occurred after the passage of the front. Furthermore, the KCl concentration in the effluent was strongly reduced. We suggest that the minor mineral phases reacted with the K⁺ ions, trapping them during the front passage and releasing them later. Even though the reactions involved are not fully identified to date, this study demonstrates that even small amounts of chemically active mineral phases, such as micas or clays, can significantly influence the SP signals.