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Structure and Growth Mechanism of Glauconite as Seen by High-Resolution Transmission Electron Microscopy

Published online by Cambridge University Press:  02 April 2024

Marc Amouric  and
Claude Parron
Marc Amouric
Affiliation:
Centre de Recherche sur les Mécanismes de la Croissance Cristalline CNRS-Campus de Luminy, Case 913, 13288 Marseille cedex 09, France
Claude Parron
Affiliation:
Laboratoire de Géologie Dynamique et Laboratoire associé au CNRS no. 132, Faculté des Sciences Saint-Jérôme, 13397 Marseille cedex 13, France
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Abstract

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The internal fabric of glauconite pellets has been studied by high-resolution transmission electron microscopy (HRTEM) for a better understanding of the glauconitization process. Typical “lamellae” which make up the glauconite pellets showed a spindle-like arrangement of layered crystallite packets. Three main mineral phases were detected: (1) well-ordered glauconite sensu stricto (d(001) = 10 Å) generally in the middle of the spindles; (2) a poorly ordered and undetermined layered-phase “X” with d(001) ~ 12.5 Å, usually sandwiching glauconite such that the interface between the two materials is very sharp; and (3) a noncrystalline or gel-like phase located between the lamellae. A 14-Å smectite-like phase was rarely observed at the periphery of some grains. The glauconite crystallites clearly showed characteristic growth features (e.g., growth steps), whereas the unknown phase X exhibited destabilization characteristics. A structural analysis of the pure glauconite indicates that this dioctahedral mica was present in the IMd (disordered), 1M, and, to a much lesser extent, 2M1 polytypic forms. HRTEM revealed no interlayering of glauconite with the other layered phases. Rather, it appeared to have formed by a layer-growth mechanism at the expense of the unknown phase X which apparently converted into non-crystalline matter before converting to glauconite. The precursor function of the interlamellae “gel” phase during the evolutive process of glauconitization is not understood.

Résumé

Résumé

La structure interne de grains de glauconie provenant de roches sédimentaires paléocènes de Côte d'Ivoire, a été étudiée par la microscopie électronique en transmission à haute résolution (METHR) afin de mieux comprendre le processus de glauconitogenèse. Les lamelles typiques, observées au microscope électronique à balayage, qui composent la glauconite, révèlent en METHR une organisation en fuseaux ou navettes constitués de paquets de cristallites à structure en feuillets. Trois phases principales ont pu être détectées: (1) La glauconite s.S. bien ordonnée (d(001) = 10 Å) au coeur des fuseaux; (2) une phase mal ordonnée et indéterminée (phase X), telle que (d(001) ~ 12,5 Å), entoure communément les paquets de cristallites de glauconite; et (3) une phase non cristallisée, ressemblant à un gel, localisée entre les lamelles. Une phase à 14 Å de type smectitique a pu être rarement observée à la périphérie de certains grains.

L'analyse structurale des cristallites de glauconite indique que ce mica dioctaédrique présente le plus souvent les formes polytypiques 1M et 1Md et moins fréquemment la forme 2M1, mise en évidence ici pour la première fois. Concernant les relations entre les différentes phases observées, la METHR ne révèle aucune interstratification des feuillets de glauconite avec les autres phases. Les cristallites de glauconite montrent clairement des figures caractéristiques de croissance (gradins par exemple) aux endroits ou la phase X présente des caractères de déstabilisation, tels que sa transformation en un matériau pauvrement cristallisé ou amorphe. Ainsi l’étude en METHR des grains de glauconie révèle que la glauconitisation constitue un processus évolutif au cours duquel le premier stade cristallisé semble représenter par la formation, peut être à partir d'un gel, d'une phase X en feuillets à 12,5 Å (smectite Fe ou nontronite?). La cristallisation de la phase glauconitique, par un mécanisme de croissance par couches, suit la déstabilisation (amorphisation) de la phase à 12,5 Å. Ces nouvelles observations militent fortement pour la théorie de la néoformation de la glauconite plutôt que pour la “layer-lattice theory”.

Keywords

GlauconiteGrowth mechanismHigh-resolution transmission electron microscopyMicaPolytype
Type
Research Article
Information
Clays and Clay Minerals , Volume 33 , Issue 6 , December 1985 , pp. 473 - 482
Copyright
Copyright © 1985, The Clay Minerals Society

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