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Textural Transition and Genetic Relationship between Precursor Stevensite and Sepiolite in Lacustrine Sediments (Jbel Rhassoul, Morocco)

Published online by Cambridge University Press:  28 February 2024

Azeddine Chahi ,
Bertrand Fritz ,
Joelle Duplay ,
Francis Weber  and
Jacques Lucas
Azeddine Chahi
Affiliation:
Centre de Géochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France Faculté des Sciences Semlalia, Boulevard Amir Moulay Abdellah, BP S.15 Marrakech, Maroc
Bertrand Fritz
Affiliation:
Centre de Géochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France
Joelle Duplay
Affiliation:
Centre de Géochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France
Francis Weber
Affiliation:
Centre de Géochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France
Jacques Lucas
Affiliation:
Centre de Géochimie de la Surface, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, France Université Louis Pasteur, Institut des Sciences de la terre, 1, rue Blessig, 67084 Strasbourg Cedex, France
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Abstract

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In the Tertiary lacustrine sediments of the Jbel Rhassoul (Morocco), stevensite and sepiolite, confined to the dolomitic facies, are commonly mixed at lower parts of the so-called “Formation Intermédiaire”. Transmission electron microscopy (TEM) observations reveal a relation between these 2 magnesian clay minerals. One can observe the different transition steps, from the initial folded layers of stevensite to the fibers emerging from the layers, and finally to the complete replacement of stevensite by sepiolite. That transition can also be observed by scanning electron microscopy (SEM), where the fibers seem to grow at the expense of stevensite. Thermodynamic calculations have been applied to provide geochemical conditions for the formation of sepiolite after stevensite. Early deposition of the “Formation Intermédiaire” occurred under climatic conditions varying between dry and wet. During dry periods, the relative silica enrichment and the pH decrease in the lake water should destabilize stevensite, leading to the formation of sepiolite. Subsequently, a perennial wet climate would lead to the formation of pure stevensite without any trace of sepiolite.

Keywords

Jbel RhassoulSepioliteStevensiteTextural TransitionThermodynamic Calculation
Type
Research Article
Information
Clays and Clay Minerals , Volume 45 , Issue 3 , June 1997 , pp. 378 - 389
Copyright
Copyright © 1997, The Clay Minerals Society

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