Griffithite is a trioctahedral smectite with dioctahedral domains, found in the <2 μm fraction of weathered basic rock from Griffith Park, California, USA. Crystalline admixtures (albite, calcite, quartz and maghemite) are concentrated in the 0.2–2 μm fraction, while the <0.06 μm fraction contains only trace amounts of other minerals. Griffithite is primarily an Fe-rich saponite with negative charge located in the tetrahedral sheets. The octahedral occupancy is ~91%, and ~26% of the octahedra contain trivalent atoms imparting a net positive charge to the octahedral sheet. Medium levels of Fe(III) reduction in griffithite, up to 60% of total Fe, can be achieved by adding solid sodium dithionite to clay dispersions in a citrate-bicarbonate buffer. By contrast >90% reduction of Fe(III) to Fe(II) is achieved in nontronites using the same method. The lower reducibility of Fe(III) in griffithite relative to nontronites may be due to structural differences between griffithite and nontronites, such as a more negative tetrahedral charge and a positive octahedral charge.