Fe(II)-Fe(III) green rust identified in soil as a natural mineral, for which the name "fougerite" has been proposed, is responsible for the blue - green colour of gley horizons, and exerts the main control on Fe dynamics (Trolard et al., 1997; Bourrié et al., 1999). An EXAFS study of the structure of the mineral, using preparation techniques that avoid any contact with oxygen from the atmosphere (Refait et al., 2001), confirms that the mineral belongs to the group of Green Rusts, in which brucite-like layers alternate with interlayers of anions and water molecules. According to the rhomboedral R3m crystal structure, the Fe - Fe distance corresponds to the parameter a of the conventional hexagonal lattice, i.e. 0.31-0.32 nm. The second peak of the pseudo radial distribution fonction (PRDF) P2 peak is found at that distance, and the series of peaks is in accordance to the hexagonal array of successive neighbouring cations at aV3, 2a, aV7, 3a... However, the intensity of the P2 peak is much less intense in the mineral than in synthetic green rusts. The comparison with synthetic pyroaurites shows that this is due to a partial substitution of FeII by MgII in the brucite-like layer, which leads to the general formula of the mineral : [FeII (1-x)MgII yFeIII x (OH)2+2y]+x . [x/n An- . (1-x+y) H2O]-x, where An- is the intercalated anion, and where one water molecule is supposed intercalated over each Mg-occupied site as for Fe. From this finding, the regular solid solution model proposed previously (Génin et al., 2001) must be modified, with provision for Mg incorporation in the mineral. Accordingly, the Ionic Activity Products must be written for each of the three end-members: Mg(OH)2, Fe(OH)2 and Fe(OH)3, the pure end-members keeping the crystal structure of the green rust. Soil solutions from hydromorphic soils are used to check the model and to constrain the ranges of variation of the mole ratios x = FeIII/Fetotal and y = Mg/Fetotal .