Domain Segregation in Ni-Fe-Mg-Smectites

Abstract

Abstract--The first stage of lateritic weathering of pyroxenes in the Niquelandia area, Brazil, leads either to Fe-rich products or to a phyllosilicate clay. In relatively unfractured parent rock the phyllosilicate clay contains Ni-rich smectites, the atomic ratio of Ni : octahedral cations ranging from 0.3 to 0.5. These smectites were studied by polarized light microscopy, X-ray powder diffraction (XRD), transmission electron microscopy, and electron microprobe, and infrared, optical absorption, M6ssbauer, and extended X-ray absorption fine-structure (EXAFS) spectroscopy. The chemical composition of the smectite is constant on the optical microscope scale even to the smallest analyzed particles (3000 ~, in diameter and about 75 ,~ thick). From XRD data the mineral is principally a swelling, trioctahedral smectite; however, some kerolite-pimelite-like layers are present, and a weak 06,33 reflection indicates the presence of a small amount of a dioctahedral phase. M6ssbauer results show that all Fe cations are Fe 3 § in octahedral sites. The structural formula of the smectite is: (Cao.ol Ko.os)(Alo.~ 7Feo. sMgo.4sNit .47Cro.o2)(Si 3.92Alo.o8)O 1 o(OH)2The results obtained from all the above methods suggest that in the smectites Ni, and, perhaps, a small amount of Mg are clustered in pimelite-like domains (or layers), whereas Fe and some A1 are clustered in nontronite-like domains (or layers). Most selected-area electron diffraction patterns exhibit continuous or punctuated (hk) rings, indicating that particles contain several stacked layers. The patterns of some thin particles, however, suggest dioctahedral layers having trans-octahedral vacancies, such as in the Garfield, Washington, nontronite. Thus, the Ni-Fe-Mg-smectite, which seemingly is homogeneous, actually consists of mixed trioctahedral and dioctahedral layers or domains.

Crystallochemical Study of a Population of Particles in Smectites from a Lateritic Weathering Profile

“…The macroscopic chemical heterogeneity of the sample can be explained by a mixing of clay layers which exhibit a continuous chemical change along a solid-solution chemical line. These results contrast with those obtained for other lateritic Ni-Fe-Mg smectites (Decarreau et al, 1987) which exhibit strong chemical segregations within clay particles.…”

“…In a previous paper, such an approach was applied to a Ni-rich smectite from a lateritic weathering profile and with a strong clustering of octahedral cations in the mixed trioctahedral and dioctahedral layers or domains (Decarreau et aL, 1987).…”

The Fe-Rich Clay Microsystems in Basalt-Komatiite Lavas: Importance of Fe-Smectites for Pre-Biotic Molecule Catalysis During the Hadean Eon

“…Table 2 summarizes the occurrences of the different Fe-Mg clay species in the principal geological sites offered by modern analogues. Segregation is commonly observed in trioctahedral Fe 3+ -Mg-Ni smectites where Fe 3+ -Al ions on one hand, and Mg 2+ -Ni 2+ ones on the other hand generally form di-and trioctahedral layers (Decarreau et al 1987;Manceau 1990;Vieira Coelho et al 2000;Gaudin et al 2004).…”

Layer Charge and Charge Distribution of Smectites: A Parameter Which Controls Important Physical Properties of Bentonites

“…In the case of smectites characterization of layer charge is of economic and geologic importance, because layer charge strongly affects key smectite properties such as swelling (McEwan and Wilson 1980), cation exchange capacity, and ion exchange selectivity (Maes and Cremers 1977). Smectites often are compositionally and structurally heterogeneous (Stul and Mortier 1974, Lagaly and Weiss 1975, Talibudeen and Goulding 1983, Nadeau et al 1985, Decarreau et al 1987, Iwasaki and Watanabe 1988, Lagaly 1994, Christidis and Eberl 2003. This heterogeneity contributes significantly to layer charge heterogeneity, both in terms of charge location (tetrahedral or beidellitic versus octahedral or montmorillonitic charge) and charge magnitude (i.e., individual smectite 2:1 layers may differ in charge, with the total layer charge for a sample being an average of these different charges).…”