ANALYSES OF POLYTYPES

Since the pioneering study of Mauguin (1928), polytypism of micas has been extensively investigated, both experimentally and theoretically. Polytypism in micas 

Another example is the determination of the stacking sequence of a very long-period (inhomogeneous), mixed-rotation polytype, discovered in an oxybiotite (dehydrogenated biotite with ferric iron) sample from the Ruiz Peak rhyodacite. New Mexico, USA. In this sample, Ross et al. (1966) identified several inhomogeneous polytypes by X-ray diffraction. SAD patterns revealed the existence of many kinds of polytypes, namely IM, 

It is easy to find other mica specimens to investigate with similar stacking sequences 

The insets at the bottom-right in (b) and (c) are magnified images to show the correspondence between HRTEM contrast and atom positions, where T, O and K indicate tetrahedral sheet, octahedral sheet and potassinm ions respectively. The black arrow in (b) indicates a cleaved surface and white, fine arrows in (b) and (c) correspond to the projection of the stacking vector in each unit layer (Kogure and Nespolo 1999b). 

Beside polytypism, defect structures in crystals are a most suitable subject for TEM. 

---