Anthropic acceleration of a natural clay mineral reaction in marshland soils (Atlantic Coast, France)

Soil clay minerals in recent natural polders react on a human timescale in response to local environmental conditions. With increasing age, the mineral reaction leads to the dissolution of the chlorite component and a composition change of the different illite-smectite mixed-layer minerals (I-S MLMs): i.e. smectite layer content decreases and illite content increases. The process of oxidation, which is proven by magnetic susceptibility to trigger clay mineral reaction, changes the mineralogical composition of the sediment above the redox front. The mineral changes appear to be a non-linear function of time. In natural conditions the process lasts >1000 y. However, anthropoic forcing such as artificial drainage accelerates the oxidation reaction to complete the whole process in a few tens of years.

Synthesis and crystallogenesis of ferric smectite by evolution of Si-Fe coprecipitates in oxidizing conditions

Silico-ferric coprecipitates, with chemical formula, were aged in suspension at 75°, 100° and 150°C and the structural evolution of solids with time studied by XRD, TEM, and IR, Mössbauer and EXAFS spectroscopy. The initial Si-Fe coprecipitate was found to be amorphous but showed local order similar to that of a smectite layer. At 75°C only a weak structural evolution of the silico-ferric product towards a smectite-like structure was observed. Experiments performed at 100° and 150°C led to synthesis of a ferric smectite with structural formula. During syntheses a highly soluble silico-ferric complex appeared; the Si/Fe atomic ratio of this complex was 3, and the apparent concentration of Fe3+in solution reached 27 mm/l. These syntheses prove that the crystallization of a dioctahedral smectite, containing only Fe3+atoms in the octahedral sheet, is possible under strictly oxidizing conditions. However, crystal growth of a ferric smectite under these conditions is slow and only syntheses carried out at sufficiently high temperatures give convincing results.