Elastic moduli of clay minerals and their aggregates: A review

The main objective of this review paper is to take a crossed look at all of the data of elastic moduli of clay minerals acquired from various experimental and theoretical methods. The comparison of these data of various origins, rarely made in practice, has been conducted qualitatively and quantitatively by calculating the Voigt-Reuss-Hill (VRH) averages of tensorial estimates. This comparison highlights four sets of results. First, the elastic constants of 2:1 nonswelling minerals can be reasonably estimated by molecular modeling of the density functional theory (DFT) type. Second, all the estimates on swelling minerals mainly obtained by molecular modeling confirm that their elastic properties are sensitive to the presence of interlayer water: they globally decrease with the number of water molecules existing in interlayer space. However, these theoretical estimates face a major difficulty: their experimental validation. The measurement on macroscopic analogs is of interest in this context. Third, most of the values provided by micromechanical inversion of geophysical or geomechanical data acquired on clay rocks or clayey composites and the unjacketed bulk modulus Ks values classically measured in geomechanics are rather representative of clay mineral assemblages and not of a given single clay mineral. Considering these mineral assemblages, the mechanical interactions between minerals and the fundamental role of interlayer water, as shown in this review, should be considered. Fourth, the results of a micromechanical model, a Mori-Tanaka scheme, applied in particular to the isotropic VRH moduli suggest a solid nature of the elastic properties of interlayer water. The elastic stiffness of the latter would be rather representative of those of the mineral solid itself than those of ice, as the literature often suggests.

Automated summary

The objective of this review paper is to compare and analyze the data of elastic moduli of clay minerals acquired through different experimental and theoretical methods. The comparison reveals four sets of results, including the estimation of elastic constants of nonswelling minerals through molecular modeling, the sensitivity of elastic properties of swelling minerals to interlayer water, the representation of clay mineral assemblages by values obtained from geophysical or geomechanical data, and the solid nature of the elastic properties of interlayer water.