SEDIMENTARY ROCKS/Clays and Their Diagenesis 63 Figure Change in percentage of calcite, quartz, K feldspar, illite, kaolinite, and chlorite with depth Adapted from Hower J, Eslinger EV, Hower ME, and Perry EA (1976) Mechanism of burial metamorphism of argillaceous sediments: 1: Mineralogical and chemical evidence Geological Society of America Bulletin 87: 725 737 kaolinite or chlorite to be the predominant clay, special conditions are required, whilst the preservation of smectite at depth requires the inhibition of diagenesis, usually through overpressuring Illitization of Smectite The most studied and first recognized aspect of clay diagenesis is the illitization of smectite with increasing depth This has been most intensively studied in the Gulf Coast region of the USA, where Tertiary smectite-rich argillaceous sediments have undergone progressive burial With increasing depth (i.e., increasing temperature), K feldspar, kaolinite, and smectite decrease, whilst illite and chlorite increase In its simplest form, this reaction can be written as the dissolution of K feldspar to yield Al3ỵ and Kỵ, which react with smectite to form illite smectite and ultimately illite: smectite ỵ Al3ỵ ỵ Kỵ ! illite þ Si4þ þ Fe2þ þ Naþ þ Mg2þ or smectite þ Kþ ! illite þ Si4þ þ Fe2þ þ Naþ þ Mg2þ The second reaction conserves aluminium and requires the dissolution of some smectite The rock evidence suggests that both reactions are possible K feldspar may also react with kaolinite to form illite It is widely claimed that the Kỵ for these reactions is a