Chemical processes causing cementation in heat-affected smectite - the Kinnekulle bentonite

摘要

Numerical calculation of silica migration and precipitation that can cause cementation of smectite buffer clay has been made using the Grindrod/Takase chemical model. It is used here to investigate whether the silicification of the bentonite and surrounding sediments at Kinnekulle, southwestern Sweden, can be explained by the heat pulse caused by the diabase intrusion that took place in Permian time. Compilation of data concerning silica cementation and associated microstructural and rheological changes showed that significant silica precipitation should have occurred in the Kinnekulle case and this is also documented. Thus, precipitation of quartz has taken place to an extent that can be explained by the chemical model, which also showed conversion of smectite to illite by neoformation of the latter mineral but only for the 3000 years long heating period. Introduction of a criterion for non-reversible illitization is hence a necessary improvement of the model for explaining the actual presence of neoformed illite, which may in fact be wholly or partly responsible for the cementation. (The report is made up of two articles: 'Cementation processes in smectite clay associated with conversion of smectite to illite as exemplified by the Kinnekulle bentonites' and 'Nonisothermal modelling of geochemical evolution in the Kinnekulle bentonite layer. Mathematical modelling and simulation') 33 refs, 40 figs.