A variety of electrochemical methods have been used to characterize the redox properties of minerals, but few have used mineral particles (as found in the natural environment) and even fewer have used electrochemical impedance spectroscopy (EIS). We have developed protocols for forming mineral particles into packed powder electrodes (PDEs), using agarose-stabilized electrolyte in the pore space. Using this approach, PDEs prepared with various iron oxides have been characterized using a range of electrochemical methods: starting with chronopotentiometry to establish a stable open-circuit potential, interspersed with small (+/-10 mV) perturbance linear polarization resistance measurements, EIS, and linear-sweep voltammetry (LSV). The results obtained with magnetite (Fe3O4) are of particular interest because of their relevance (ranging from nanomedicine to biogeochemistry) and implications regarding redox reactions (including passivation) at mineral surfaces. In general, the results obtained are consistent with prior work on Fe3O4 suspensions (e.g., Eocp ≈ 280 mV vs. SHE) or by other methods (e.g., polarization resistance). EIS has been performed on PDEs containing Fe3O4 from a variety of sources and after a variety of pretreatments. Extensive equivalent circuit modeling of the EIS data has been performed to identify the most appropriate models, sensitivity of model parameters, and how operational variables influence the model parameters. An example of the latter is the addition of chemical oxidants (e.g., H2O2) to the electrolyte to provide controlled passivation of the Fe3O3 surface, which causes the expected changes in the EIS model: such as porosity, charge transfer resistance, or double-layer capacitance. Further application of this approach should provide additional insight in the effects of passive film formation on redox active minerals in the environment.