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.

Abstract. Polarized Fe K-edge EXAFS spectra of biotite and chlorite monocrystals have been recorded using the Synchrotron radiation at LURE, Orsay. The anisotropic contribution of the nearest cation shell is clearly brought to evidence. In the limit case in which the electric field vector would be disposed normal to the layer plane, the contribution of cations belonging to octahedral sheets would not be sampled. At this orientation, the Fe-(Si, A1) contribution is selected and it is then possible by extrapolation of spectra taken at lower angles to extract phase and amplitude functions associated with this atomic pair. Two applications of this experiment are presented. (i) In chlorite, the amplitude of the Fe-(Si, A1) contribution is directly proportional to the distribution of Fe atoms between the TOT layer and the interlayer. It is found that about 25 percent of the iron is located in the interlayer. (ii) In the biotite structure, the contribution of the (Si, A1) shell is almost constant for a wide range of Fe-containing phyllosilicates and can be subtracted from their EXAFS spectrum. Such an operation will permit a more accurate analysis of the reality of solid solutions.

Spectra of an oriented sample of RbC8 were recorded for a set of angles from 0° up to 40°. For each peak in the Fourier transform, the amplitude ratio permits to calculate the angle of the corresponding [MATH] vector with respect to the plane. The thermal study from 300 K down to 35 K shows a high anisotropy of the Debye-Waller factor, the amplitude of the vibration being larger in the plane. The second stage RbC24 shows a higher static disorder than the first stage RbC8