The role of argillic alteration in the zeolitization of volcanic glass

The pseudomorphic replacement of glass shards by zeolite minerals is a comm on feature of volcanoclastic sediments. In the majority of cases the initia l stage of this reaction is the alteration of the glass surface to a clay m ineral or celadonite after which the bulk of the glass is altered to zeolit e. This replacement feature is seen particularly well in glass of rhyolitic composition; the zeolite mineral usually being clinoptilolite. Volcanoclas tic rocks of Oligocene age exposed in the Rhodope Massif, Bulgaria offered an opportunity to study this reaction experimentally as rocks containing un altered glass shards are known to be close stratigraphic equivalents of zeo litized tuffs and in this respect are considered to be precursor rocks. Low -temperature hydrothermal reactions conducted on the unaltered glass, which had been clay coated in the laboratory, demonstrates the importance of the clay-glass interface. An hypothesis is put forward to explain this type of zeolitization process and a distinction is drawn between these rocks and o ther sediments in which zeolite minerals form from volcanic glass without t he presence of a clay interface.