Introduction

The quantitative study of grain orientations (fabric) is very important in structural geology and materials science (Wenk, 1985, Kocks et al., 2000, Randle & Engler, 2000, Wenk, 2002, Wenk & Van Houtte, 2004). It is less established in igneous petrology (Nicolas, 1992, Smith, 2002), where fabric has been used mainly to establish flow directions and mechanisms in lava flows and dykes. Fabric studies find their way in geophysics as there is considerable interest in the anisotropy of seismic velocities (e.g. Mainprice & Nicolas, 1989, Xie et al., 2003). Fabric is also important in engineering geology, as it affects the physical properties of rocks (e.g. Akesson et al., 2003). It is also of interest in palaeontology and medicine for quantifying the structure of bone (Ketcham & Ryan, 2004). Knowledge of grain orientations is also necessary to calculate size distributions from the dimensions of grain intersections in all the domains already mentioned (see Section 3.3.4).

The grains that define the fabric of a material can be a variety of different objects: commonly crystals, clasts and enclaves. One measure of fabric is based on the orientations of the grain shapes (shape preferred orientations: SPO). Here the nature of the material is unimportant for the measurement of the fabric. However, the contrast in viscosity between the grain and matrix during production of the fabric will define what parameter is recorded by the fabric.