Spectroscopic (ESR, EXAFS) evidence of Cu for (Al-Mg) substitution in octahedral sheets of smectites

Abstract

ESR has been used to obtain information on the octahedral or interlamellar position of Cu(II) in natural smectites from Burkina Faso (West Africa). On the basis of 060 XRD reflections and chemical data, these smectites were found to be Al-rich and dioctahedral. After both Mehra & Jackson, and De Endredy deferrification treatments, the Cu contents remained high (4500 and 22000 p.p.m., respectively). The Cu(II) ESR spectra of these deferrated smectites were compared to those of two reference smectites for which the structural position of Cu(II) was precisely known. The interlayer Cu(II) signal was obtained on a Cu-saturated Camp Berteau montmorillonite, while the octahedral Cu(II) signal was obtained on a synthetic Cu-rich smectite. For this latter reference sample, EXAFS spectroscopy provided evidence that Cu was in six-fold coordination in the octahedral sheet only, and was not exchangeable. In agreement with the experiments by Clementz, Pinnavaia and Mortland, a shift in the g⊥ ESR signal was observed when the air-dried Cu-saturated Camp Berteau montmorillonite (g⊥ = 2·05) was soaked in water for 48 h (g⊥ = 2·13). A small shift in the opposite sense was observed for the synthetic Cu smectite (g⊥ = 2·05 for the air-dried sample, g⊥ = 2·02 for the water-soaked sample). For the two natural smectites a small shift similar to that for the synthetic Cu-smectite was observed. These results indicate that up to 10% of the Cu atoms substitute for Al-Mg-Fe atoms in the octahedral sheets of the smectites studied.

Natural and Synthetic Copper Phyllosilicates Studied by XPS

“…Nevertheless, Cu in all phyllosilicates studied here was in the (II) oxidation state, as checked by ESR spectra (Creach, 1988;Mosser et al, 1990aMosser et al, , 1990b. Koppelman and Dillard (1977) mentioned the reduction of Cu(II) adsorbed on chlorite by the photoreduction effect; this effect increased with exposure time to the X-ray beam.…”

“…The Cu(II) crystallographic positions in the different clays were determined by ESR and EXAFS spectroscopies, and by X-ray microanalysis (Creach, 1989;Mosser et aL, 1990aMosser et aL, , 1990b.…”

“…In the phyllosilicates, Cu is in the center of a distorted octahedron (Creach, 1988;Mosser et aL, 1990aMosser et aL, , 1990b, with the same interatomic distance of 1.95 • between Cu and the 4 planar O, and a greater distance between Cu and the 2 apical O (2.41 A for chrysocolla and 2.32 A for synthetic Cu-smectite). For Cut the coordination of oxygen and copper is more properly described as 4-planar rather than as 4 + 2 distorted octahedral coordination, with Cu-O bond length of 1.95 ,~ to 1.96 ~ for the four planar oxygens and 2.78 ,~ for the two distant O atoms (Asbrink and Norrby, 1970).…”

Preparation, Characterization and Electrochemistry of Synthetic Copper Clays

“…It has been proposed that this property of copper is not compatible with a regular clay structure Petit et al 1995). Octahedral sites occupied by Cu(II) in phyllosilicates are distorted, with Cu-O distances of 1.95 ,~ for the 4 in plan oxygens, and 2.32 .~ (in smectites) or 2.41 ,~ (in chrysocolla) for the 2 apical oxygens (Mosser et al 1990(Mosser et al , 1992. The Cu content of natural smectites is limited to about 2% (Mosser et al 1990;.…”

“…Octahedral sites occupied by Cu(II) in phyllosilicates are distorted, with Cu-O distances of 1.95 ,~ for the 4 in plan oxygens, and 2.32 .~ (in smectites) or 2.41 ,~ (in chrysocolla) for the 2 apical oxygens (Mosser et al 1990(Mosser et al , 1992. The Cu content of natural smectites is limited to about 2% (Mosser et al 1990;. Copper in Cu-rich clays is usually preferentially found in the interlayer spaces (Ildefonse et al 1986).…”