Crystal Structure of Clays 

Most clay minerals have two types of basic structural units  - tetrahedral and octahedral.  The tetrahedral unit is ^4- ₄  SiO  in which silicon is equidistant from the four oxygen atoms.  The octahedral unit consists of an aluminium, iron or magnesium ion, surrounded by oxygen or hydroxyl ions in the form of an octahedron.  The tetrahedral and octahedral units giv e rise to their respective sheets by joining of the units.  These structures are depicted schematically. An aluminium-dominated sheet is known as a dioctahedral sheet and the one dominated by magnesium is called trioctahedral sheet.