7, 10 and 14 Å mixed-layer phyllosilicates studied structurally by TEM in pelitic rocks of the Piemontese zone (Venezuela)

Transmission electron microscopy has been used to study the interstratification of 14 Å chlorite with a 1:1 7 Å phyllosilicate (serpentine type structure) and of 14 Å chlorite with a 2:1 10 Å phyllosilicate (mica type structure). Samples were from pelitic rocks of low grade metamorphism in the Piemontese zone (Venezuela). The 7 Å phase was directly observed as thick packets possibly exhibiting different stacking sequences and, more frequently, as isolated layers intercalated within 14 Å chlorite layers. Such a 7 Å phase is probably a precursor of the 14 Å chlorite. Various interlayered mica- 14 Å chlorite structures were also observed as intermediate states of mica chloritization. In particular, a solid state transformation mechanism, such as a mica layer laterally changing into a chlorite layer (a 1:1 reaction) with increasing volume, as proposed by Olives et al. (1983), was evidenced. Lastly during this study, particular attention was paid to contrast details of TEM images which are very sensitive to experimental parameters (defocus, specimen thickness and orientation,...). Even the use of simple lattice fringe images is to be done with great care, specially for a 7-Å/14-Å mixed multiple layers study. Examples show that, as soon as optimum imaging conditions are left, one-dimensional image analysis of 7 Å and 14 Å phases may be erroneous or ambiguous and such phases confused.La microscopie électronique à transmission (MET) a été utilisée pour étudier l'interstratification de chlorite à 14 Å avec un phyllosilicate 1:1 (de type serpentine) et de chlorite à 14 Å avec un phyllosilicate 2:1 à 10 Å (de type mica). Les échantillons proviennent de roches pélitiques ayant subi un métamorphisme de bas degré dans la zone Piémontèse du Vénézuéla. La phase à 7 Å a été observée soit sous forme de gros paquets montrant différentes séquences d'empilement soit, plus fréquemment, sous forme de couches isolées dans de la chlorite à 14 Å. Des interstratifications variées mica-chlorite à 14 Å ont été également observées et considérées comme des états intermédiaires de chloritisation de mica. En particulier est mis en évidence un mécanisme de transformation mica → chlorite à l'état solide, comme proposé par Olives et al. (1983). Enfin il est montré que même de simples images unidimensionnelles en MET doivent être utilisées avec le plus grand soin, tout spécialement pour étudier des mélanges de phases multiples à 7 Å et 14 Å, comme c'est le cas ici.Amouric Marc, Gianetto Ivanka, Proust Dominique. 7, 10 and 14 Å mixed-layer phyllosilicates studied structurally by TEM in pelitic rocks of the Piemontese zone (Venezuela). In: Bulletin de Minéralogie, volume 111, 1, 1988. pp. 29-37