Radiolarite belong to the group of sedimentary siliceous rocks. As the name indicates, it is formed of Radiolaria, i.e., the skeletal elements of siliceous unicellular beings. Apart from siliceous sponges and diatoms, Radiolaria are the most important biogene sources for the formation of sedimentary siliceous rocks. They are present in the seas and oceans since at least the Palaeozoic period. Typically, they form only a small fraction of the fauna, even the microfauna and they disappear unnoticed among the multitude of organic and mineral components forming the marine sediments turning to well-known sedimentary rocks by way of diagenesis. They are very small, typically in the range of 10-100 mms. Radiolaria are accumulated in rock-forming quantities only among very special circumstances. For this, deep and cold water is ideal where other marine fauna is scarce and the temperature of the water prevents the accumulation of carbonic rocks. Being small in size radiolarite accumulates from radiolarian silt very slowly, by a rate of some cms per million years. The mass occurrence of Radiolaria in the present day Carpathian basin and formation of radiolarite took place in the Mesozoic period when the world ocean (Tethys) was of equatorial position. Radiolarite was formed in a long, west-eastern arch along the Alp-Carpathian system, across the Balkans till the Himalayas. The resulting siliceous rocks can be found at many places in south-central Europe. On the basis of physical qualities, mainly colour, lustre and character of bedrock, regionall meaningful macroscopic groups could be differentiated among radiolarites. These groups are currently investigated by detailed chemical studies to check the validity of these categories and provide objective means for distribution studies. Transdanubian radiolarites Transdanubian radiolarites are the most important raw material types for the territory west of the Danube. Their use is amply documented since the Lower Palaeolithic (Vértesszőlős, KRETZOI-DOBOSI 1990) throughout prehistory and into historical times (BIRÓ 1986, BIRÓ-REGENYE 1991, HÁLA 1986). Macroscopically separated types vary much less than we could observe in limnoquartzites. The two important source areas in Hungary (the Bakony and Gerecse-Pilis mountains, respectively) differ from each other mainly in the quality and the character of the mother rock (porous, light porcelanite and hard limestone, respectively (KONDA 1986)). Colour varieties also help to differentiate specimens, more or less, from the two source regions. Clear, more vivid variants are typically found in the Bakony sources. Macroscopic analysis indicate that the better quality and more spectacular Bakony material was preferentially used. Most important colour varieties include the vivid red Szentgál type radiolarite, reddish brown and brown (Hárskút type) with more or less porcelanite and a special mustard-yellow variety, named after the source regions, the Urkút-Eplény type radiolarite. Radiolarites of the Gerecse Mts. are typically liver-coloured and/or grey.
Mecsek radiolarites
It is fairly easy to distinguish Mecsek radiolarite from Transdanubian radiolarites on the macroscopical level, due to its lilac-brownish brown, silky greenish-bluish grey basic variants. On the microscopic level, the difference is less marked due to the basic fossil component (radiolarians) dominating in both varieties. Certain differences, however, are observable in matrix and texture as well. The geohistorical position of the two formations is equally different - Transdanubian radiolarites belong to the African (Alpidic) plate of the Tethys sediments while the Mecsek radiolarite belongs to the European plate, that is, the Carpathian radiolarites. There is a small difference in the formation period as well. Transdanubian radiolarites formed in the Middle Jurassic while Mecsek (and other Carpathian radiolarites) formed during the Upper Jurassic-Lower Cretaceous.