Mineralogy and plasticity in clay sediments from north-east Tunisia W. Hajjaji a , M. Hachani a , B. Moussi a , K. Jeridi a , M. Medhioub b , A. López-Galindo c, * , F. Rocha d , J.A. Labrincha e , F. Jamoussi a a Géoressources Laboratory, CERTE BP 273, 8020 Soliman, Tunisia b Faculty of Science Sfax, 3018 Sfax, Tunisia c Instituto Andaluz de Ciencias de la Tierra, CSIC – Univ. Granada, Avda. Fuentenueva, 18002 Granada, Spain d Geobiotec, Geosciences Dept., University of Aveiro, 3810-193 Aveiro, Portugal e Ceramics and Glass Engineering Dept. and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal article info Article history: Received 20 March 2009 Received in revised form 14 July 2009 Accepted 23 July 2009 Available online 30 July 2009 Keywords: Tunisian clays Clay mineralogy Plasticity Atterberg limits Equivalent basal spacing abstract Several cross-sections carried out in the Bir M’Cherga area (northern Tunisia) provided a complete Trias- sic–Miocene stratigraphic sequence, rather representative of the whole Tunisian Ridge Field located in the northern Atlas. Mineralogical analysis revealed a predominance of illite in the Early Cretaceous, while smectite is dominant in the Late Cretaceous and Tertiary. In terms of Atterberg limits, the Bir M’Cherga samples can be divided into two groups: one of moderately plastic clay samples until the Early Creta- ceous, and another represented by the Late Cretaceous and Tertiary clays, which are the most plastic. As expected, the Atterberg limits increase with the amount of phyllosilicates present in the sample, which is dependent on the amount of smectite. This analysis was complemented by the use of the equiv- alent basal spacing (EBS) parameter, which gives a good correlation between the mineralogical character- istics of the clays and their plasticity. Using EBS, we can predict the mechanical/plastic behaviour of any clay sample according to its mineralogical composition. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Interest in clays has increased in recent years due to their phys- ical–chemical and plastic properties, which make them some of the most widely used materials in industry. Since Atterberg (1911) much work has been done in soils and sediments in an attempt to evaluate the influence of the various factors involved in the plas- ticity of clay samples, such as their mineralogical composition, shape and size distribution of particles, interaction among clays or with water or dissolved salts, the effect of cementing, clay gen- esis, etc. Casagrande published his well-known soil chart in 1948, and Dumbleton and West (1966) studied the relationships be- tween clay contents and the plastic and liquid limits of natural soils from around the world, in an attempt to define the contribu- tion of clay component to the engineering properties of soil as a whole. More specifically, Bain (1971) focussed on industrial clays (halloysite, kaolinite, illite, mixed layers, several kinds of smectites, sepiolite and paylygorskite), Decleer et al. (1983) correlated min- eral composition, chemistry and granulometry with plastic and li- quid limits in Belgian clays, and Hawkins et al. (1986) did similar analysis in the UK, while Al-Homoud et al. (1996) focussed on clay beds causing landslides and Ohtsubo et al. (2002) on marine clays. Perhaps the most recent and conclusive contribution is by Sch- mitz et al., (2004) who introduced equivalent basal spacing (EBS), a parameter obtained by multiplying the relative amount of a clay with its basal spacing (Å) known from the literature. In this paper we report the Atterberg limits and the mineralogical composition of a significant number (approximately 120) of Tunisian clayey- marly samples in order to investigate their relationships with a good statistical significance. The Diffuse Double Layer (DDL) theory can explain mechanical disparity between different clay minerals (Wersin et al., 2004; Schmitz, 2006). This study examines the relevant mineralogical parameters affecting the plasticity of samples, in order to easily distinguish and select those suited for certain technological applications. With such a large number of samples we can check the validity of the EBS method. Obtaining a direct prediction of plasticity based on this may be useful if other phases such as organic matter (Xiao Gang et al., 2002) or salts (Schmitz and van Paassen, 2003) are absent. 2. Materials and methods Our study area includes the Jebel Oust massif and neighboring areas (near the city of Bir M’Cherga) in north-eastern Tunisia 1464-343X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2009.07.007 * Corresponding author. Tel.: +34 958246207; fax: +34 958243384. E-mail addresses: w.hajjaji@ua.pt (W. Hajjaji), alberto@ugr.es (A. López- Galindo). Journal of African Earth Sciences 57 (2010) 41–46 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci