Reduction of Fe(III) in griffithite

Griffithite is a trioctahedral smectite with dioctahedral domains, found in the <2 mu m Fraction of weathered basic rock from Griffith Park, Californi a, USA. Crystalline admixtures (albite, calcite, quartz and maghemite) are concentrated in the 0.2-2 mu m fraction, while the <0.06 mu m fraction cont ains only trace amounts of other minerals. Griffithite is primarily an Fe-r ich saponite with negative charge located in the tetrahedral sheers. The oc tahedral occupancy is similar to 91%, and similar to 26% of the octahedra c ontain trivalent atoms imparting a net positive charge to the octahedral sh eet. Medium levels of Fe(III) reduction in griffithite, up to 60% of total Fe, can be achieved by adding solid sodium diihionite to clay dispersions i n a citrate-bicarbonate buffer. By contrast >90% reduction of Fr(III) to Fe (Il) is achieved in nontronites using the same method. The lower reducibili ty of Fe(LII) in griffithite relative to nontronites may be due to structur al differences between griffithite and nontronites, such as a more negative tetrahedral charge and a positive octahedral charge.