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hk-Ordering in Aluminous Nontronite and Saponite Synthesized Near 90°C: Effects of Synthesis Conditions on Nontronite Composition and Ordering

Published online by Cambridge University Press:  28 February 2024

V. C. Farmer ,
W. J. McHardy ,
F. Elsass  and
M. Robert
V. C. Farmer
Affiliation:
The Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ, United Kingdom
W. J. McHardy
Affiliation:
Station de Science du Sol INRA, Route de Saint-Cyr, 78026 Versailles, France
F. Elsass
Affiliation:
Station de Science du Sol INRA, Route de Saint-Cyr, 78026 Versailles, France
M. Robert
Affiliation:
Station de Science du Sol INRA, Route de Saint-Cyr, 78026 Versailles, France
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Abstract

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In studies on the fate of aluminium in the environment, nontronite and saponite have been obtained by synthesis in reducing alkaline conditions close to those prevailing in poorly drained soils developed from limestones. The two minerals obtained have different structures and organizations corresponding to two different growth and/or maturation mechanisms. High-resolution transmission electron microscopy of ultrathin sections of a synthetic aluminous nontronite embedded in resin showed the presence of crystallites consisting of two to ten co-terminating parallel layers, indicating synchronous growth. Electron diffraction showed that the individual crystallites had hk-ordering, i.e., orientation of layers with respect to the six-fold pseudosymmetry of the unit cell. Deposits of a synthetic saponite included hk-ordered crystallites and crinkled films with turbostratic stacking. The two saponite phases had slightly different b dimensions. Lattice fringe images of sections of saponite embedded in resin showed a high angular disorientation of the layers in the stacking direction, suggesting multiple nucleation and growth of individual layers, subsequently aggregated with imperfect parallelism.

Exploration of the synthetic conditions of the aluminous nontronite indicated that calcium was essential for an hk-ordered product. Syntheses using potassium or sodium hydroxides and carbonates for pH control gave poorly organized nontronites. Hydrazine was not essential for nontronite formation, but better crystallized products—judging by their IR spectra—were obtained in its presence by maintaining reducing conditions in the early stages of synthesis. Attempts to prepare ferruginous beidellites under similar conditions to those in which aluminous nontronites formed were unsuccessful.

Keywords

BeidelliteElectron diffractionHigh-resolution electron microscopyhk-orderLattice imagingLayer stackingNontroniteNontronite synthesisSaponite
Type
Research Article
Information
Clays and Clay Minerals , Volume 42 , Issue 2 , April 1994 , pp. 180 - 186
Copyright
Copyright © 1994, Clay Minerals Society

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