Abstract
The coarse (0.4–2 µm) clay fraction of an Albian black shale collected in the Atlantic Ocean (Deep Sea Drilling Project leg 11) consists chiefly (90–95%) of smectite and 5–10% illite. Both minerals are locally surrounded by overgrowth structures, such as fine laths about 0.05–0.4 µm long and 0.02–0.1 µm wide. Individual laths or assemblages of laths protrude from the center of smectite flakes at angles of about 60° to each other. Laths occur around illite crystals in a similar manner or coalesce into a rim that consists of 0.05–0.1-µm-size particles. On the basis of scanning transmission electron microscopy: (1) the center of individual illite crystals consists of a dioctahedral mineral, but the overgrowth structures are Al-Fe beidellites; and (2) the smectite flakes have highly variable compositions, but correspond chiefly to Fe-Al-beidellite, whereas the overgrowths are compositionally close to montmorillonite.
The overgrowth structures seem to have formed during early diagenesis. The chemical composition of overgrowths around illite and smectite tend to be similar in response to the new environment, implying an addition of silica to both materials.
Résumé
La fraction argileuse (0,4-2 μm) d’un échantillon de black shale albien de l’Atlantique (Deep Sea Drilling Project leg 11) surtout formée de smectite (90–95%) et d’illite (5–10%) a été étudiée. L’étude morphologique des particules en microscopie électronique montre qu’elles sont souvent entourées de surcroissances généralement formées de lattes très fines longues de 0,05 à 0,4 Mm et larges de 0,02 à 0,1 μm. Autour des flocons de smectite, les lattes peuvent être isolées ou former des assemblages de 4 ou 5 lattes formant fréquemment des angles de 60° entre eux. Des faciès identiques se rencontrent autour des illites mais ces minéraux peuvent aussi être entourés de lattes courtes (0,05 à 0,1 μm) et coalescentes formant une auréole plus ou moins continue autour du cristal.
L’analyse de ces argiles par microscopie électronique analytique montre: (1) que la composition du centre des particules d’illite correspond à des minéraux dioctaédriques mais les lattes qui les entourent sont des beidellites Al-Fe; (2) que la composition du centre des smectites, beaucoup plus variable d’une particule à l’autre, correspond à des beidellites Fe-Al et les lattes qui les entourent à des montmorillonites.
Ces sur-croissances semblent se former au cours de la diagenèse précoce. Le fait que la composition des lattes poussant autour des illites converge vers celle des lattes entourant les smectites constitue une indication de la réponse des minéraux à un changement de milieu. Du point de vue chimique, ces modifications nécessitent toute deux un apport de silice.
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Steinberg, M., Holtzapffel, T. & Rautureau, M. Characterization of Overgrowth Structures Formed Around Individual Clay Particles During Early Diagenesis. Clays Clay Miner. 35, 189–195 (1987). https://doi.org/10.1346/CCMN.1987.0350304
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DOI: https://doi.org/10.1346/CCMN.1987.0350304