Rigid inclusions rotate in geologic materials as shown by torsion experiments

Jeffery showed 86 years ago that a sphere rotates synthetically with applied simple shear, at a rate equal to half the shear strain rate. However, it can be argued that rocks do not behave like viscous fluids hence Jeffery's model does not apply. In order to test this argument, we took a two-phase synthetic aggregate (rock) made of 70% halite and 30% muscovite (as anisotropic ductile matrix) with embedded rigid bodies with contrasting shapes: circular cylinder, sphere and cube. The sphere approximates the shape of a garnet and the cube the shape of pyrite crystals, both so common in nature. When subject to torsion at 250 MPa and 100 °C, the matrix deformed homogeneously and all rigid bodies rotated as theoretically predicted for a viscous fluid matrix. Therefore, we conclude that rocks deforming in a ductile fashion behave macroscopically like a continuum hence fluid mechanics can be used to study rigid inclusion rotation behaviour.