Rotation of uniaxial ellipsoidal particles during simple shear revisited: the influence of elongation ratio, initial distribution of a multiparticle system and amount of shear in the acquisition of a stable orientation

The rotation of uniaxial ellipsoidal particles has provided the base for many studies of shape preferred orientations of mineral grains in a large variety of rock types. In this paper we present reports of a study of the evolution of a multiparticle system as a function of the elongation ratio of the particles and of the initial particle distribution. Our model reveals that both factors are important in controlling the acquisition of a stable orientation upon deformation. The more elongated particles (r=short axis/long axis≤0.5) will define a stable fabric in almost any situation of geologic interest (γ=shear deformation<10), but less elongated particles (r>0.5) can also be considered to have achieved a stable orientation depending on the total amount of shear experienced by the rock. However, in systems with the less elongated particles (r≥0.8) it is possible to find mean orientations that are perpendicular to the direction of shear, although the occurrence of such behavior constitutes only a minor fraction of the studied examples. In any case, it is suggested that a systematic study of mineral fabrics in samples from the same rock can yield enough information to evaluate whether the mean shape preferred orientation of the minerals is parallel to flow direction.