Clay minerals, definition mineral names

The layers may be regularly or randomly interstratified. The latter are by far the more common and are probably the second most abundant clay mineral species, following illite (which in most cases is a mixed-layer clay). More regularly interstratified clay minerals have a definite periodicity and some are given specific mineral names. The randomly interstratified clay minerals are described in terms of the type and proportion of the two or more types of layers. Many of them exhibit some degree of regular interlayering. 

We will see that kaolinite Al2(Si205)(0H)4 - which is the main mineral of clays, hence the name kaolin given to some clays rich in kaolinite - undergoes irreversible reactions that transform it into metakaolinite when it is fired above approximately 500°C, which could justify the temperamre of 540°C selected in the definition used to make the distinction between dried earth and terra cotta. 

---