A B S T R A C T : Silico-ferric coprecipitates, with chemical formula S i4Fe23 + O.. nHzO, were aged in suspension at 75 ~ 100 ~ and 150~ and the structural evolution of solids with time studied by XRD, TEM, and IR, Mfssbauer 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~ only a weak structural evolution of the silico-ferric product towards a smectite-like structure was observed. Experiments performed at 100 ~ and 150~ led to synthesis of a ferric smectite with 9 e 3+ C structural formula Sl4F j.s30~0(OH)z %.26. During syntheses a highly soluble silico-ferric complex appeared; the Si/Fe atomic ratio of this complex was 3, and the apparent concentration of Fe 3+ in solution reached 27 mM/l. These syntheses prove that the crystallization of a dioctahedral smectite, containing only Fe 3+ 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.

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The upper Messinian succession recovered at Ocean Drilling Program Hole 968A on the Cyprus lower slope was generated under brackish conditions (Lago Mare), as deduced from sedimentological and paleontological data. Gypsum that displays marine signatures (stable isotope composition of gypsum and monospecific nannoplankton included in gypsum crystals) is interbedded within this series and interpreted as being reworked from earlier Messinian evaporite deposits, such as those present onshore in Cyprus.