DICKITE AND NACRITE FROM THE LIEGE COAL BASIN (BELGIAN COAL MEASURES GROUP, WESTPHALIAN, UPPER CARBONIFEROUS)
Geological Survey of Belgium, 13, Jenner Street, 1000 Brussels, Belgium. E-mail : eric.goemaere@naturalsciences.be
Abstract
Westphalian sediments of the Belgian Coal Measures contain two primary kaolinite populations, either well crystallised or badly crystallised. The proportion of these two populations was originally linked to the depositional environmentbut changed through diagenesis and atmospheric weathering. Dickite and nacrite, as polymorphs of kaolinite,occur frequently in the tectonised Liège coal measures where they are systematically associated to stress constraints(called tectoglyphs in the present paper). These tectoglyphs and their mineral phases are here described. Dickite fills small sized distensive fractures while sliding surfaces reveal nacrite. Authigenic dickite was precipited from interstitial solutions in open fractures and protected pockets after partial clay dissolution of the host rock. Microtextural relationships suggest that nacrite is a synkynematic mineral resulting from the polymorphic transformation of dickite under stress constraints. No direct relationships can be demonstrated between kaolinite and other polytypes.