Oxidation-reduction mechanism of iron in dioctahedral smectites: I. Crystal chemistry of oxidized reference nontronites

The crystal chemistry of Fe in four nontronites (Garfield, Panamint Valley, SWa-1, and NG-1) was investigated by chemical analysis, X-ray goniometry, X-ray absorption pre-edge spectroscopy, powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) spectroscopy, and X-ray diffrac tion. The four reference nontronites have Fe/(Fe + Al + Mg) ratios ranging from 0.58 to 0.78, and are therefore representative of the different chemic al compositions of dioctahedral ferruginous smectites. Pre-edge and powder EXAFS spectroscopy indicate that NG-1 contains 14 to 20% of tetrahedrally c oordinated Fe3+, whereas the other three samples have no detectable Fe-IV(3 +). The partitioning of Fe-VI(3+) between cis (M2) and trans (M1) sites wit hin the octahedral sheet was determined from the simulation of X-ray diffra ction patterns for turbostratic nontronite crystallites by varying the site occupancy of Fe. Based on this analysis, the four nontronite samples are s hown to be trans-vacant within the detection limit of 5% of total iron. The in-plane and out-of-plane local structure around Fe atoms was probed by an gular P-EXAFS measurements performed on highly oriented, self-supporting fi lms of each nontronite. The degree of parallel orientation of the clay laye rs in these films was determined by texture goniometry, in which the half w idth at half maximum of the deviation of the c* axis of individual crystall ites from the film plane normal, was found to be 9.9 degrees for Garfield a nd 19 degrees for SWa-1. These narrow distributions of orientation allowed us to treat the self-supporting films as single crystals during the quantit ative analysis of polarized EXAFS spectra. The results from P-EXAFS, and fr om infrared spectroscopy (Madejova et al. 1994), were used to build a two-d imensional model for the distribution of Fe, and (Al, Mg) in sample SWa-1. In this nontronite, Fe, Al, and Mg atoms are statistically distributed with in the octahedral sheet, but they exhibit some tendency toward local orderi ng. Fe-Fe and (Al, Mg)-(Al,Mg) pairs are preferentially aligned along the [ 010] direction and Fe-(Al, Mg) pairs along [3 (1) over bar 0], and [<(31)ov er bar>0] directions This distribution is compatible with the existence of small Fe domains separated by (Al,Mg), and empty octahedra, which segregati on may account for the lack of magnetic ordering observed for this sample a t low temperature (5 K) (Lear and Stucki 1990).