Siliceous Rocks

a group of sedimentary rocks entirely or more than 50 percent composed of free or hydrous silica. The rock-forming minerals are opal, chalcedony, and quartz. Consequently, a distinction is made among opal, chalcedony, quartz, and mixed siliceous rocks. The structure is fine-grained and cryptocrystalline.

Depending on occurrence, siliceous rocks may be stratified or nodular. By origin, a distinction is made between chemogenic (jaspilities and siliceous sinters) and organogenic (diatomite, radiolarite, and spongolite) siliceous rocks. In addition, cryptogenic siliceous rocks are singled out (opoka, tripoli). The vol-canogenic-sedimentary process plays a considerable role in the formation of many siliceous rocks (jaspers, geyserites, some jas-pilites). Chalcedony and quartz siliceous rocks occur as a result of the crystallization of opaline rocks. The origin of many siliceous rocks is a matter of dispute. Siliceous rocks of young deposits (beginning with the Cretaceous system) are composed primarily of opal, while in the Jurassic and Triassic they are composed of chalcedony and quartz. In the Paleozoic and more ancient times they are composed predominantly of quartz. In ancient rocks opal is found only in the form of secondary segregations.

The distribution of siliceous rocks in a stratigraphic column and in space reflects the evolution of siliceous sedimentation. In the Precambrian period (under geosyncline and platform conditions) ferruginous-siliceous beds of jaspilites were deposited from materials from the continents and from materials of volcanic origin. Jaspilites are not found in deposits younger than Cambrian. Organisms (radiolaria and sponges) acquire an important role in the formation of siliceous rock in the Paleozoic. Geosyn-clines, with the volcanic-sedimentary process that is characteristic of them, became the principal zones of siliceous rock accumulation. Volcanogenic SiO₂ precipitated out into sediment through chemical and biochemical means. In the Cretaceous period the organogenic formation of siliceous rocks became dominant after diatoms appeared at the end of the Jurassic. Siliceous rocks became widespread in ocean sediments and on the continental platforms, primarily in the high latitudes of the northern and southern hemispheres; they also continued their significant development in the geosynclines. In modern times, seawaters are not saturated with silica and chemogenic deposition of siliceous rocks is not taking place; only organogenic siliceous rocks are accumulating.