Form vs habit

In the description of crystals and crystal structures the two terms/om and habit have very specific and very different meanings. Form refers to the internal crystal structure and etymologically is the descendant of the Greek morph. Hence, polymorph refers to a number of different crystal modifications or different crystal structures, and the naming of different structures as Form F or a Form follows directly from this definition and usage. As we have seen above, the difference in crystal structure is very much, although not exclusively, a function of thermodynamics. Certainly, only the structures which are thermodynamically accessible can ever exist, but there often is a question of thermodynamic vs kinetic control over which particular structure may be obtained under any particular set of crystal growth conditions. 

The important point is that differences in external crystal shape, habit, or crystal morphology may not necessarily indicate a change in the polymorphic form or polymorphic crystal structure. An example of this distinction is given in Eig. 2.13. 

As Herbstein (2001) has noted, consideration of polymorphism in terms of classical thermodynamics ignores the structural aspects of the phenomenon, an approach he terms as purely physical. At the other extreme of his proposed physics-chemistry scale for polymorphism, molecules of a substance would exhibit appreciable chemical differences between the chemical entities in the two polymorphs. At the chemical end of the scale one is dealing with structural characterization and comparison, and a number of typical examples along the physics-chemistry scale are discussed in detail by Herbstein. 

In dealing with a polymorphic system perhaps the fundamental question is how similar or how different are the various crystal structures. Although Gavezzotti and Fillippini (1995) have attempted to provide a quantitative measure to this comparison, we believe that investigation of this question should include a close visual examination of the crystal structures as outlined in the next section. 

---