PITCHSTONE, RHYOLITE AND KAOLIN NEAR MEISSEN, SAXONY

The postconference field trip led to the classic occurrence of rhyolit ic to rhyodacitic glasses in the surroundings of Meissen, the thickest and best-developed uppermost Carboniferous vitreous lava now in Europ e. The pitchstones are the oldest vitreous volcanic rocks known at pre sent in Europe. The red, black or green coloured glasses are rhyolitic -rhyodacitic composition (normative mineral content: quartz: 49.4 vol% ; K-feldspar: 26.8 vol%; plagioclase: 20.8 vol%; accessories: 2.9 vol% ). Characteristic for the pitchstone is a water content >5%. Fine crys talline rhyolite-rhyodacite volcanic flow structures (so-called ''quar tz porphyry''), which were extruded in the same volcanic period, are o bserved in contact with the vitreous rocks. The origin of these two ro ck types is controversially discussed in the literature. New results c oncerning gas content and degassing behaviour (gas release profiles) a re presented below. A large-scale kaolinization of the vitreous and cr ystalline rocks is observed in the Volcanic region of Meissen. The raw material for the famous porcelain manufactory Meissen is currently ta ken from these deposits.