Crystal chemistry of cobalt and nickel in lithiophorite and asbolane from New Caledonia

The crystal chemistry of Ni- and Co-bearing manganese oxides (lithiophorite and asbolane) has been investigated by X-ray Absorption Spectroscopy (XAS). The Mn oxides come from the lateritic weathering profiles of the ultrabasites of New Caledonia. The distinct behaviours of Ni and Co concern both oxidation states and local structures.The electronic structure and short range order around Co atoms do not depend on the nature of the Cocontaining phase. Co atoms are trivalent and 6-fold coordinated. Co-(O, OH) and Co-(Co, Mn) interatomic distances derived from EXAFS are equal to those found around Mn atoms which rules out the possibility of an adsorption of Co atoms directly above and below vacancies of MnO₂ layers. The high structural order around Co contrasts with the structural disorder observed around Mn. Cobalt atoms do not occupy specific Mn sites and are not randomly distributed within the octahedral Mn layers.Unlike Co, Ni exhibits distinct surroundings in both phases. In asbolane, Ni atoms build partial Ni(OH)₂ layers. Ni-OH distances are lower as compared with the free Ni hydroxide because of the formation of hydrogen bonds between Ni(OH)₂ and MnO₂ layers. In lithiophorite Ni atoms are located in the hydrargillite layer (Al(OH)₃). Both chemical composition and structural considerations militate for a Ni for Li substitution in lithiophorite. Finally, evidence is given for the existence of a mixed-layering between lithiophorite and asbolane and the chemical variations generally observed in these Mn oxides are interpreted as a variable proportion of (Mn, Co)(O, OH₂, Ni(OH)₂ and (Al, Li, Ni)(OH)₃ layers.