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Abstraet--Hydrazine and dithionite, both of which are strong reducing agents, react differently with various dioctahedral smectites. Both the nature of the reducing agent and the structure of the clay affect the course of the reaction. Hydrazine reduces octahedral Fe ~ § efficiently if the mineral has a low tetrahedral charge, The reducing action of dithionite does not depend upon the charge.The results obtained by different physical methods of investigation suggest that reduction of iron is associated with protonation of an adjacent OH group. The Fe 2+ formed is readily re-oxidised but the structural change~ occurring on reduction are reversible only when AI-OH-Fe, but not when Fe-OH-Fe associations are involved. Reaction mechanisms are proposed and changes in the distribution of iron in the octahedral sites are discussed. INTRODUCTIQN

Abstrac~M6ssbauer spectra of 15 smectites were investigated. In these samples, ferric iron occupies both M(1) and M(2) octahedral sites, the distribution being partly determined by the relative covalency of the bonds formed.The quadrupole splittings are linearly related to b -3. They show that Fe 3+ octahedra are much more distorted in montmorillonite and beidellite than in nontronite and volkonskoite and that M(I) sites are more prone to change than M(2). Ferrous iron occurs in relatively undistorted octahedra in some otherwise distorted octahedral sheets and vice versa.

A~tract---Structural Fe 2+ in montmorillonite is readily oxidized by contact with water, salt solutions or on mild heating. This is shown clearly by the Mtssbauer spectra and is associated with a sharpening of the infrared absorption near 880 cm 1. It was inferred that this band comprises the Fe 2+ OH--AI and Fe 3 § OH AI deformations. The rate at which oxidation occurs depends on the exchangeable cations. High acidity of the interlayers is conducive to oxidation, as is contact with Cu2+-containing solutions or concentrated H~O2 solutions.The results show clearly that any chemical treatment of montmorillonite causes changes in the oxidation state of structural iron.

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