Authigenic siliceous minerals in the Tremadoc graptolitic argillite of Estonia

Graptolitic argillite contains in addition to rockforming minerals (hydromicas, chlorite, elastic quartz of pelitic fraction) and accessory minerals also authigenic ones. Numerous clay minerals, carbonates, sulphides, and ,siliceous minerals are authigenic. Siliceous minerals form lenticular or pocket-shaped laminae on several levels in the eastern part of the distribution area of grapto!itic argillite that contains spicules of siliceous sponges and siliceous dioxide. Siliceous dioxide occurs in various forms: as amorphous silicon, opal, chalcedony, and authigenic quartz. Opal may occur in the composition, of spicules and partly also of globules. Chalcedony is found as aggregates with a fibrous composition and collomorphic structure. It forms spherulites, bundles, and incrustites around elastic grains and pyrite crystals and on the walls of cavities. There are metacolloidal pellets, plats, and .rods of irregular shape among the chalcedonic globules. Edgings and coatings of authigenic quartz are abundant. The siliceous laminae were formed during the early phase of diagenesis. Amorphous silica dioxide was replaced by chalcedony and then by quartz during the diagenesis. Their formation became possible thanks to a short-term input of a large amount of siliceous material into the basin. The leaching of silica as colloidal solutions in the denudation area was not sufficient for this. It is supposed that hydrothermae were a source of silicon. The deep origin of silicon is supported by the high content of several typical hydrothermal elements in the siliceous interlayers.