Publication: Occurrence of Fe–Mg-rich smectites and corrensite in the Morrón de Mateo
bentonite deposit (Cabo de Gata region, Spain): A natural analogue
of the bentonite barrier in a radwaste repository
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Publication Date
2011
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Elsevier
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Abstract
The Morrón de Mateo bentonite deposit is being studied as a natural analogue of the thermal and geochemical
effects on a bentonite barrier in a deep geological repository of high level radioactive wastes.
This bentonite deposit and its host rocks were intruded by a rhyodacitic volcanic dome that induced a
hydrothermal metasomatic process affecting the biocalcarenite beds close to the dome. In this work,
the mineralogical and chemical features of the clay minerals of the hydrothermally altered pyroclastic
(white tuffs) and epiclastic rocks (mass flow), located in the NE sector of the Morrón de Mateo deposit
are described. White tuffs have a high content of phyllosilicates, mainly composed of dioctahedral smectites,
while mass flow have a higher proportion of inherited minerals, the neoformed phyllosilicates are
dioctahedral smectites and an interlayer chlorite/smectite mineral of corrensite type. The chemical composition
of smectites reflects the different nature of the parent rocks, in such a way that smectites from
white tuffs have a quite homogeneous chemical composition and their structural formulae correspond to
montmorillonite type, while smectites from mass flow show more chemical variability, higher Fe and Mg
contents and a mean structural formulae corresponding to Fe–Mg-rich beidellite and/or to an intermediate
smectite member between beidellite and saponite. In addition, chemical composition and textural
features of corrensite-like clay minerals in relation to Fe–Mg-rich smectites in the samples have also been
studied, suggesting that the former seems to be formed from Fe–Mg-rich smectites. The presence of corrensite
in the epiclastic rocks suggests that in the Morrón de Mateo area a hydrothermal alteration process
occurred after bentonite formation, which transformed Fe–Mg-rich smectites into corrensite. This
transformation was probably favoured by the intrusion of the Morrón de Mateo volcanic dome, which
produced a temperature increase in the geological media and a supply of Fe–Mg-rich solutions. These
physicochemical conditions were also responsible for the metasomatic transformations observed in
the biocalcarenite beds located on the top of the bentonite deposit. All these data suggest that the Morrón
de Mateo natural system could be a good natural analogue of both thermal and chemical effects on a bentonite
barrier related to the radioactive decay of fission products and the interaction between the corrosion
products of steel over-packs and the bentonite. These circumstances would favour the
transformation of the candidate Al-rich smectites into Fe–Mg-rich smectites and corrensite, as steps prior
to formation of chlorite. In this case, all the physicochemical and mechanical properties of Al-rich smectites
would disappear and the clayey barrier would fail.