Domain Segregation in Ni-Fe-Mg-Smectites

A. Decarreau1, 6, F. Colin2, A. Herbillon3, A. Manceau4, 6, D. Nahon2, H. Paquet5, D. Trauth-Badaud1 and J. J. Trescases2
1 Laboratoire de Géochimie des Roches Sédimentaires, UA-CNRS 723, Université Paris XI, bât. 504, 91405 Orsay Cédex, France
2 Laboratoire de Pétrologie de la Surface, UA-CNRS 721, Université de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cédex, France
3 Groupe de Physico-Chimie Minérale et de Catalyse, Université Catholique de Louvain, Place Croix du Sud 1, 1348 Louvain la Neuve, Belgique
4 Laboratoire de Minéralogie-Cristallographie, UA-CNRS 9, Universités Paris VI et VII, 4 place Jussieu, 75252 Paris Cédex 05, France
5 Centre de Sédimentologie et Géochimie de la Surface du CNRS, 1 rue Blessig, 67084 Strausbourg Cédex, France
6 Laboratoire pour l'Utilisation du Rayonnement Electromagnétique (LURE), CNRS, 91405 Orsay, France

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, Mössbauer, 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 Å in diameter and about 75 Å 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. Mössbauer results show that all Fe cations are Fe3+ in octahedral sites. The structural formula of the smectite is: (Ca0.01 K0.05)(Al0.17Fe0.5Mg0.48Ni1.47Cr0.02)(Si3.92Al0.08)O10(OH)2 The 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 Al 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.

Key Words: Chemical composition • Laterite • Mössbauer spectroscopy • Nickel • Nontronite • Pimelite • Smectite • Transmission electron microscopy

Clays and Clay Minerals; February 1987 v. 35; no. 1; p. 1-10; DOI: 10.1346/CCMN.1987.0350101
© 1987, The Clay Minerals Society
Clay Minerals Society (www.clays.org)