Two-dimensional XRD study

The most common two-dimensional technique employs a precession camera, but any technique giving two-dimensional undistorted images of the reciprocal lattice is suitable as well. From these undistorted images, the geometry of the diffraction pattern can be analyzed by simple visual inspection. In the case of a precession camera study, the crystal must be mounted so as to have the (001) plane perpendicular to the goniometer rotation axis. In fact, the stacking of layers in micas is along c and the periodicity in reciprocal 

5xFe + (1 - 0.5x)Mg M2 layers M2 = (1 - x)Fe + xMg Ml, Mi O.SxFe + (1 - 0.5x)Mg. If the value of x is far from 2/3, the presence of the M2 layers should, in principle, be revealed even by a structure refinement employing only the homomorphic sequence as starting model. However, with the approach of x to 2/3 (where the difference between Ml and M2 disappears), the distinction between N layers of type Ml and N-n) layers of type Ml plus n layers of type M2 becomes difficult. The presence of an M2 layer may be erroneously interpreted for disorder in the cation distribution. The site occupancies in the O sheet should be carefully checked otherwise important information about the nature of the polytype under investigation can be easily overlooked (see also Nespolo 2001). 

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