An element or compound in two or more layer-like crystal structures that differs in layer-stacking sequences. Polytypism differs from polymorphism in permitting small differences in chemical composition between structures, not to exceed 0.25 atoms per formula unit of any constituent element. Layer structures that differ from one another by more than this amount are to be called polytypoids rather than polytypes. (Quot Bailey, 1977)
Cf., polymorphism