The accurate crystal chemistry of ferric smectites from the lateritic nickel ore of Murrin Murrin (Western Australia). II. Spectroscopic (IR and EXAFS) approaches
Résumé
Fe-rich smectites from lateritic weathering profiles have previously been studied by XRD, ICP-AES, SEM-EDX and TEM-EDX analyses (Gaudin et al., 2004). These smectites exhibit intermediate chemistries between five end-members: Al-Fe beidellites, Al-Fe montmorillonites and Mg+Ni-saponite. The spectroscopic study by FTIR and XAS of these smectites reveals that: (1) tetrahedral Fe3+ is near or below the detection limit (0.05 cation for 4Si); (2) the large chemical variability is due to substitution of the three major cations (Fe, Al, Mg) within adjacent octahedra; (3) Ni is not concentrated in another clay phase such as Ni-kerolite and is located in the octahedral sheets of smectite; (4) octahedral cations are not randomly distributed but ordered in separated Fe, Al Mg, Ni clusters; (5) the Mg-Ni saponite end-member actually appears as small trioctahedral clusters of Mg and Ni distributed within the dioctahedral smectite.