Assuming dislocation glide as the prevailing mechanism of flow in the minerals, we tried to get some insight into the structure and preferred orientations resulting from a pure shear or a simple shear deformation through a numerical model. Our approach is a purely geometrical one. The crystalline aggregate is modelled as a two-dimensional collection of polygonal cells. Each cell has a unique slip system of random orientation. A given final shape is imposed as a boundary condition and is achieved through that deformation for each cell (rotation, translation, and internal homogeneous slip) which minimizes the gaps and overlaps with its nearest neighbours. When the final fit of the cell boundaries is not acceptable, fracture of the cells is allowed in order to improve it. This simulation, though the mechanisms are quite simplified, contributes nevertheless towards understanding the deformation of an aggregate. The results are in good agreement with experimental deformation of rocks whose grains deform preferentially on one glide system. It leads to a better understanding of the behaviour of individual grains and preferred orientation of slip directions, in some natural deformations.