Rates and Products of Oxidation of Ferrous Iron in Trioctahedral Smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt. % Fe) and one high (27 wt. % Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L-1 nitrite was negligible for the moderate-iron smectite and yielded only ~17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. All oxidized smectites maintained a trioctahedral structure and no crystalline secondary minerals formed based on powder X-ray diffraction and variable temperature Mössbauer spectroscopy. This study demonstrates that trioctahedral smectites exhibit distinct oxidation behaviors from dioctahedral smectites and retain their structure and bulk composition following oxidation. Because trioctahedral ferrous smectites accommodate ferric iron in their structure and are resistant to complete oxidation, transitions from anoxic to oxic conditions likely generate mixed-valence smectites rather than a mixture of new phases. The fate of mixed-valent smectites during diagenesis, and thus the signature of trioctahedral smectite oxidation preserved in the rock record, is unclear. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics, indicating that these clay minerals represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.