Clay mineral-brine interactions during evaporite diagenesis: lanthanide distribution in WIPP samples

In order to study the behavior of clay minerals considered for overpack in salt media, the detrital clay minerals were examined by sannning electron microscopy (SEM), neutron activation analysis (NAA), X-ray diffraction (XRD) and supporting techniques. While special emphasis is placed on lanthanide elements, data for other trace and some major elements are reported as well. Fourteen samples were selected which, from SEM and XRD examination, showed clear evidence for some interaction between clay minerals and brine. These analyses were undertaken to help establish a data base for clay minerals which have been immersed in an evaporite brine solution. It was also felt that the trace element data would be of use in explaining possible reactions between clay minerals used as overpack and brines formed at or near a canister interface. Further, to understand the geochemical systematics of the evaporite rocks of the Delaware Basin, it is desirable to determine the effectiveness of clay minerals as ion exchange scavengers. The studies revealed the following: trace element concentrations, exclusive of the lanthanides, for the clay minerals are significantly higher than for both anhydrites and polyhalites; where local concentrations of trace elements are noted in the evaporites, these are due to inclusions of very small amounts of clay minerals and/or oxide coatings on evaporite grains; lanthanide distribution patterns for the clay minerals are very close to the pattern for pelagic sediments but significantly different from the sea water pattern; and despite the alterations of the original smectites during the formation of chlorite-saponite, the essential trace element chemistry of the clay minerals is preserved