| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 4437006 | Applied Geochemistry | 2014 | 15 Pages |
Samples of Opalinus Clays from Mont Terri Underground Research Laboratory in the Swiss Jura were analyzed repetitively relative to the duration of their exposure to atmosphere. The objective was the evaluation of such a progressive exposure on the chemical composition of whole-rock samples, and on the chemical and Sr isotopic compositions of leachates obtained by leaching the rock powders with dilute acetic acid. This chemical study was complemented by scanning electron microscope observations to identify the related mineral alterations. The chemical data for the rock powders remained quite constant whatever the duration of the storage, whereas significant changes were observed for the leachates. Similar changes were observed in the leachates of samples collected progressively closer to the walls of a previously excavated niche. When storage time or distance to gallery wall increases, the main variations are: (1) a progressive decrease of the element contents and of the 87Sr/86Sr ratio in the leachates, (2) an alteration of pyrite followed by precipitation of Ca-sulfate in the rocks, both observed by SEM, and (3) a probable precipitation of new mineral phases in the rocks, such as Fe-oxyhydroxides and/or jarosite that could not be visualized by electron microscopic observation because of their very limited amount and very small grain size. The modifications call for an oxidation of pyrite that probably induced also an oxidation of the organic matter. It could also be shown that the reactions were enhanced by temperature increase, and that they were less pronounced in the samples behind the gallery wall than in the cores stored in laboratory conditions.Of importance for the waste-disposal assessment is the fact that the chemical characteristics, and therefore the mineral assemblages, of the studied argillaceous rocks are reactive to changing oxidation/reduction conditions monitored by dehydration. The resulting mineral reorganization seems to contribute to a sealing of the rocks, as the amounts of elements released into the leachates decrease progressively with storage time. This tendency is detectable after several months for the core samples stored under vacuum in Al-foil at room temperature but with periodic exposure to atmosphere, and after several years for the in situ samples collected behind an excavation wall.
