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Do bentonites have contradictory characteristics? An attempt to answer unanswered questions

Christidis Georgios

Πλήρης Εγγραφή

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Περίληψη

The formation of bentonites via alteration of volcanic glassy rocks is associated withapparently contradictory mineralogical, geological and geochemical characteristics, which remainpoorly understood. This contribution attempts to answer some of the questions based on the workcarried out in our laboratory over the past 15 y. The first question concerns the formation of opal-CTfreebentonites from acidic precursors. It is suggested that the formation of opal-CT-free bentonites isclosely related to the mechanism of eruption and emplacement of parent volcanic rocks if thetemperature of the parent glass after emplacement is high and/or the establishment of hydraulicgradients coupled with high permeability after deposition of volcanic glass (i.e. diagenetic alteration)if the temperature of the parent glass is low. The second question pertains to the behaviour of Feduring bentonite formation. Based on mineralogical and geological characteristics of differentbentonite deposits, it is suggested that Fe is present mainly as Fe2+ and hence may be mobilizedduring alteration. Oxidation of Fe may take place when bentonites are exposed to more oxidizingconditions close to the surface. Assignment of Fe as Fe2+ increases the layer charge of smectites anddecreases significantly the range of layer charge. Thus the smectites may have considerably differentproperties when formed compared with their counterparts on the surface. The third questionconsiders the possible influence of parent-rock chemistry on the crystal chemistry of smectites. It isproposed that the parent rock may affect the chemistry of the neoformed smectites to some degree;therefore the smectite chemistry may provide indications about the geochemical affinities of theparent rock.

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