Occurrence of Fe–Mg-rich smectites and corrensite in the Morrón de Mateoudbentonite deposit (Cabo de Gata region, Spain): A natural analogueudof the bentonite barrier in a radwaste repository

摘要

The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemicaludeffects on a bentonite barrier in a deep geological repository of high level radioactive wastes.udThis bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced audhydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work,udthe mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclasticud(white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo depositudare described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites,udwhile mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates areuddioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical compositionudof smectites reflects the different nature of the parent rocks, in such a way that smectites fromudwhite tuffs have a quite homogeneous chemical composition and their structural formulae correspond toudmontmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mgudcontents and a mean structural formulae corresponding to Fe–Mg-rich beidellite and/or to an intermediateudsmectite member between beidellite and saponite. In addition, chemical composition and texturaludfeatures of corrensite-like clay minerals in relation to Fe–Mg-rich smectites in the samples have also beenudstudied, suggesting that the former seems to be formed from Fe–Mg-rich smectites. The presence of corrensiteudin the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration processudoccurred after bentonite formation, which transformed Fe–Mg-rich smectites into corrensite. Thisudtransformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, whichudproduced a temperature increase in the geological media and a supply of Fe–Mg-rich solutions. Theseudphysicochemical conditions were also responsible for the metasomatic transformations observed inudthe biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrónudde Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentoniteudbarrier related to the radioactive decay of fission products and the interaction between the corrosionudproducts of steel over-packs and the bentonite. These circumstances would favour theudtransformation of the candidate Al-rich smectites into Fe–Mg-rich smectites and corrensite, as steps priorudto formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectitesudwould disappear and the clayey barrier would fail.