International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C, ch. 6.4, p. 612

The parameters describing the microstructure of the crystal are the mosaicblock size and the angle between the mosaic blocks. These are not constrained in any way to be isotropic with respect to the crystal axes. In particular, they are not constrained by symmetry. For example, in a facecentredcubic crystal under uniaxial stress, slip will occur on one set of {111} planes, leading to a dislocation array of noncubic symmetry. In principle, anisotropy can be incorporated into the formal theory by allowing and g to depend on the Miller indices of the reflections. This has not been done in this work, but reference should be made to the work of Coppens & Hamilton (1970).
References
Coppens, P. & Hamilton, W. C. (1970). Anisotropic extinction corrections in the Zachariasen approximation. Acta Cryst. A26, 71–83.