Measurement and cartography of clay matrix orientations by image analysis and grey-level diagram decomposition

The organization in three dimensions of clay particles in natural or man-made clayey materials is a dominant parameter influencing their mechanical, porosity, and permeability properties (Foster & De, 1971; Arch & Maltman, 1990; Luo et al., 1992; Schneider et al., 1994). In order to explain the relationships between clay textures and permeability or retention capacities, it is necessary to quantify the crystal]ite organization in the clay matrix. Although qualitative descriptions of the clay matrix are commonly reported (Morgenstern & Tchalenko, 1967; Foster & De, 1971), attempts at quantitative descriptions are more complicated (Bai et al., 1994). The analysis of soil mechanics requires three observation scales: (1) naked eye observation of the whole test-piece which allows the main fissures and shears induced by the geotechnic tests to be located; (2) optical microscopic observation which allows the whole arrangement of clay particles related to the shear planes to be specified; and (3) scanning electron microscopy (SEM) observation which allows the measurement of the orientation directions of selected clay particles along the shear discontinuity and associated fissures or kink band boundary shear planes (Sloane & Kell, 1966; Gillot, 1969; McKyes & Yong, 1971; Foster & De, 1971; Gmmberger et al., 1994). The limits of the SEM observation are the qualitative aspect of the particle arrangement analysis. The analysis of the SEM image of an homogeneous mineralogical matrix is limited by the lack of contrast. The back-scattered electron images do not give significant grey-level differences. Consequently, the quantitative measurement of the particle orientations is partial and limited to selected disseminated particles in microsites in the observation plane. The optic microscopic analysis is an intermediate scale observation which is actually insufficiently developed. There are difficulties with clay matrix descriptions using the polarizing microscope because of their cryptocrystalline texture (particle size < 2 ~m). It is unrealistic to try to characterize each microcrystal; statistical observations have to be used to quantify some parameters. q:) 1998 The Mineralogical Society 630 P. Dudoignon and A. Pantet This paper proposes a method for the measurement and the quantification of the clay mineral orientations in a clay matrix observed on thinsections under an optical microscope. It is an image analysis method based on the study of the greylevel patterns (256 grey levels) of digitized images. Firstly, the evolution of the grey-level diagrams is explained by the observation of a monocrystalline example and of a bimodal example, which are a quartz crystal and a twinned plagioclase crystal, respectively. Secondly, the method uses a mathematical decomposition of the grey-level diagram obtained on two bimodal clay matrices. This method is not restricted to the calculation of an anisotropy index but allows: (1) the measurements of the different clay mineral orientations in the three directions; (2) the quantification (%) of the different clay orientation modes; and (3) their location and area thresholding in the analysed thin-sections. The measurements were carried out using crossed-polarized light and the revolving stage of a conventional polarizing microscope, without complementary optical material except for the CCD camera and the associated image analysis software.