The main aspects of OD theory have been presented and discussed in the contribution by Ďurovič (1997). It clearly transpires from that contribution how the theory favours a deep insight into the various related phenomena of one-dimensional disorder, twinning (generally polysynthetic twinning), polytypism, and how it presents a comprehensive interpretation of the diverse anomalous features frequently displayed by diffraction patterns: diffraction enhancement of symmetry, diffuse spots, continuous streaking.
However, notwithstanding the theory is firmly built and various practical tools for its utilization are available, it is still far from constituting a ‘normal’ professional equipment not only for mineralogists, inorganic chemists, material scientists, but also for crystal chemists and crystallographers.
I think that the main reasons for that are the widespread assumption that the OD structures are relatively few in number and that their arrangements may be solved and effectively described without introducing a largely new theoretical approach and its abstruse terminology.
I do not agree with this point of view. First of all, the number of substances which display OD features is very large, especially among inorganic compounds, both synthetic and natural (Merlino, 1990); moreover I think that number would be even larger if the OD procedures were widely known and carefully applied.
Second, sometimes it is only through the application of OD procedures that the ‘real’ structure of the compounds under study may be understood and, in all cases, the OD approach gives a deeper insight into the problem, suggesting interesting relations and pointing to new possible polytypes.
Figures & Tables
Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.