KAOLINITE AND DICKITE FORMATION DURING SHALE DIAGENESIS - ISOTOPIC DATA

X-ray diffraction, scanning electron microscopy and O-isotope geochemi stry have been used to investigate the origin and possible controls on polymorphic transformation of kaolin minerals filling veins in Cretac eous shales from the Gibraltar Strait area (southern Spain). The miner alogy of the enclosing shales indicates that kaolin minerals formed fr om smectite dissolution, a process that silmultaneously originated I/S mixed-layers and quartz. Kaolinite and dickite delta(18)O values sugg est that an increase in the water isotopic composition, from Cretaceou s sea water values (-1 parts per thousand) to values of about 3 parts per thousand, occurred parallel to smectite dissolution, the intensity of this process being the main factor controlling the isotopic compos ition of kaolin minerals. The minimum formation temperature ranges fro m 62 degrees C for kaolinite to 86-96 degrees C for dickite, indicatin g that the depth of burial was the main control on polymorph formation . This temperature range agrees with that deduced for illite/smectite ordering. The passage from kaolinite-to dickite-rich veins was accompa nied, as deduced from SEM examination, by a morphologic evolution char acterized by the division of large vermiculae, dominant in kaolinite s amples, and the formation of short stacks and platy crystals, which ar e predominant in dickite. The mechanism of dickite formation, however, remains uncertain. (C) 1998 Elsevier Science Ltd. All rights reserved .