Planar intergrowths and polysomes

Examples abound, especially in minerals. A classic polysomatic series is given by the minerals that are formed by an ordered intergrowth 

2Mica also forms polytypes. These occur when the component layers of a mica stack in alternative ways. In these polytypes the composition does not change from one member of the series to another and remains that of the parent phase. 

Many other examples of Ruddleston-Popper phases have been synthesised. The generalised homologous series formula for these compounds Is fWl n03 + 1 [ = (A0)(AB03) or (H)(P) ] where A is a large cation, typically an alkali 

---