This study aims at building a theoretical framework allowing for estimation of the macroscopic friction coefficient of a mixed quartz/clay fault gouge from the description of the microstructure and from relevant properties at the lower scale. The effect of clay content is particularly highlighted through two opposite morphology cases. The first one corresponds to a grain framework-supported microstructure with a predominant content of quartz grains forming a connected structure and some clay filling the intergranular space. The second considered morphology corresponds to a clay matrix-supported microstructure in which quartz grains are embedded. The friction coefficient of the gouge is derived with respect to the clay content for both morphologies because of theoretical homogenization techniques. The models prove to be in good agreement with experimental results from the literature. Copyright © 2012 John Wiley & Sons, Ltd.