During the oxidation of iron, poorly crystallized phases are firstly formed: 2 line-ferrihydrite and 6-line ferrihydrite, which present for the last phase, a similarity with wustite FeO but also with hematite o-FezO3. Crystallization increases with time and the solid phase obtained is dependent on temperature and pH. Obviously, high temperature favours the formation of the oxide hematite cr-FezO:. As for the pH factor, it is more complicated. Low and high values of pH (2 to 5 and 10 to14) favour the formation of goethite cr-FeOOH, while obtaining hematite is favoured at neutral pH (values around 7). Goethite or hematite are obtained either through a dissolution-crystallisation process or in the solid state, through a topotactic transformation. Given the structural relationships observed between fenihydrite and wustite and hematite, it is allowed to think that a structural continuity could exist between wustite Felr-ryO and hematite via fenihydrite.

Copper hydroxide Cu(Off2 is metastable. ft transforms easily into copper oxide CuO more stable, either in the solid state by a thermal dehydration or at room temperature, in aqueous basic solutions. In the solid state the transformation is performed at a relatively low temperature,423 K. It is a topotactic or a pseudo-morphic transformation for the reason that a clear relationship exists between axes of the two solids, in the three directions. The reacting process is described and corresponding vectorial relations between crystal parameters are proposed. It is not the same case in aqueous basic solutions. Copper hydroxide gives rise to oxide through the formation of a complex anion, Cu(OH)+2-, by a reconstructive transformation involving a dissolution reaction followed by a precipitation.

Goethite ( cr-FeOOH ), lepidocrocite ( y-FeOOH ), hematite ( o-FezO: ) aod maghemite ( T-FezO: ) constitute the main natural occrrrences with magnetite ( Fe:Oq ), for ferric oxides and oxyhydroxides.By thermal dehydrations goethite and lepidocrocite respectively give rise to hematite and maghemite by topotactic transformations. In fact, this kind of transformation permits alone to explain why hematite, the most stable oxide in a thermodynamical point of view, is not obtained directly by dehydration of lepidocrocite" Topotactic transformatians can only exist between solids displaying structural relationships. In these conditions, atom displacements are reduced so that dehydration processes are performed at a relatively low temperature and then clear vectorial relations can be established between crystal parameters of the two structures.Therefore, \Àre can consider that trivalent iron oxyhydroxides and oxides, belong to two different structural types: goethite/hematite type and lepidocrocite/maghemite type and conclude that, in this case, kinetic phenomena are more important than laws of thermodynamic for explaining the experimental results found. Key words: iron oxides, iron oxyhydroxides, topotactic transformations, thermal dehydrations, thermodynamic studies, crystallographic studies. doesn't present a cubic close-packed (ccp) stacking in contrast with results published in previous works. Nevertheless, after the loss of water, sheets of iron octahedra collapse to give rise to maghemite which presents, this time, ccp alTays of oxygen anions.On the other hand, the study of the evolution of iron surroundings during the dehydration process of hydroxides is rather surprising. Condensation of iron octahedra after the loss of water is performed mainly by vertices instead of edges which number decreases. Hematite is the only phase which presents a face shared between two octahedra ( see fig. 6 ). Moreover, it seems that octahedra linked by vertices give rise to structures more compact as it can be seen in table 5 .

Synthesis of single crystals of Cd(OH)Cl and Cu(OH)Cl allowed us to revise the structure of these hydroxychlorides and con5rm their space group: P6 3 mc (R ‫؍‬ 0.041; R w ‫؍‬ 0.050) for Cd(OH)Cl and P2 1 /c (R ‫؍‬ 0.051; R w ‫؍‬ 0.069) for Cu(OH)Cl. Positions of hydrogen atoms were determined and an hypothesis of hydrogen bonds is discussed in relation to the infrared spectra.A comparison of the structures shows that an isomorphism is not possible between the two compounds, due to the important Jahn+Teller e4ect of divalent copper. On the other hand, a structural relationship exists between Cd(OH)Cl and -Cd(OH) 2 , which belongs to the brucite-type family. Academic Press

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