CRYSTAL CHEMISTRY OF CLAYS FORMED WITHOUT AIR: OXIDATIVE RECALCITRANCE AND TRACE METAL RETENTION OF TRIOCTAHEDRAL FERROUS SMECTITES (Invited Presentation)

The career of Georges Calas has demonstrated the invaluable role of crystal chemistry in understanding mineral reactions. In this presentation, we apply crystal chemistry to explore the properties and behavior of ferrous trioctahedral smectites. These clays have seen limited study to date because of their instability in air and thus rare occurrence on Earth’s land surface. However, ferrous smectites have wide importance to an array of anoxic modern and ancient earth, planetary, and environmental systems. To advance our understanding of these unexplored clay minerals, we synthesized and characterized a series of Fe(II)-Mg trioctahedral smectites that span the range of clay compositions observed in nature. Kinetic experiments reveal that 15 to 40% of Fe(II) in these clays is recalcitrant to oxidation by dissolved oxygen over multi-week reaction periods, producing a mixed-valent phase that retains all iron in the clay structure. We hypothesize that residual Fe(II) occupies structural sites that stabilize it against oxidation, such as in clusters surrounded by Fe(III) where additional oxidation would create excess positive charge. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than ferric smectites or a mixture of new phases. Natural ferrous smectites often contain various trace metals inherited during mafic rock alteration but their fate during oxidation is unclear. We have synthesized additional Fe(II)-smectites containing Mn(II), Co(II), Ni(II), or Zn(II), all of which occupy octahedral sites, as well as containing a mixture of V(III) and V(IV) distributed between octahedral and tetrahedral sites. Mn, Co, Ni, and Zn are fully retained in the clay structure during oxidation by dissolved oxygen and hydrogen peroxide but nearly half of V is solubilized as vanadate. During these reactions, up to 3% of Co, 43% of Mn, and 76% of V oxidized, with each showing distinct reactivity with specific oxidants. These findings demonstrate that redox-active trace metals in trioctahedral smectites may serve as proxies for past exposure to oxygen and reactive oxygen species, complementing the partial recalcitrance of Fe(II).