- © The Mineralogical Society
The formation of bentonites via alteration of volcanic glassy rocks is associated with apparently contradictory mineralogical, geological and geochemical characteristics, which remain poorly understood. This contribution attempts to answer some of the questions based on the work carried out in our laboratory over the past 15 y. The first question concerns the formation of opal-CT-free bentonites from acidic precursors. It is suggested that the formation of opal-CT-free bentonites is closely related to the mechanism of eruption and emplacement of parent volcanic rocks if the temperature of the parent glass after emplacement is high and/or the establishment of hydraulic gradients 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 Fe during bentonite formation. Based on mineralogical and geological characteristics of different bentonite deposits, it is suggested that Fe is present mainly as Fe2+ and hence may be mobilized during alteration. Oxidation of Fe may take place when bentonites are exposed to more oxidizing conditions close to the surface. Assignment of Fe as Fe2+ increases the layer charge of smectites and decreases significantly the range of layer charge. Thus the smectites may have considerably different properties when formed compared with their counterparts on the surface. The third question considers the possible influence of parent-rock chemistry on the crystal chemistry of smectites. It is proposed 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 the parent rock.