| Abstract: | Two mechanisms have been suggested whereby preferred orientation of long and short axes might be produced in a small number of marker grains dispersed in a granular matrix under shear. One relies on the varying angular velocity of non equidimensional grains as they rotate in a sheared fluid, the other on grains tending to take up attitudes such that minimum angular momentum is transferred during collisions as they are sheared past each other. The two mechanisms are not necessarily mutually exclusive: experiments have been performed to test their relative effectiveness in producing preferred orientation.The style of orientation varies. When the marker and matrix grains are the same size and geometrically similar the orientation appears dominated by the effects of intergranular collisions. In particular, there is an angle, which maybe almost 30°, between the preferred direction of long axis orientation and the direction of flow. When the marker grains are much larger than the matrix their orientation is like that of grains in a viscous fluid, the preferred direction of long axes orientation being in or near to the flow direction. The results are relevant to the understanding of grain orientation in sedimentary and igneous rocks. |
