Picrit is (according to the current definition of the International Union of Geological Sciences ) the name for a volcanic rock, which has a chemical composition within the following limit values: 52%> SiO 2 > 30%; MgO> 12%; (K 2 O + Na 2 O) <2%. In the TAS diagram , rocks that move within these limits can fall into the field of foidites as well as the fields of picrobasalts or basalts ; in this case the term “Pikrit” takes precedence.
The term was previously used in a broader sense; Thus, in the older literature one also finds the definitions as “greened fine to coarse-grained volcanic rocks belonging to the basalt family ” or “ meta basalts with a proportion of over 50% olivine” or “a variety of dolerite or basalt that is extreme is rich in olivine and pyroxene ”.
Picrites often occur together with diabases and they mostly have a porphyry structure . In the mineral inventory they have light green olivine and dark augite . They can also contain hornblende , bronzite and biotite . Accessory mixtures below one percent are apatite and magnetite . Pikrites are very dense and dark ( ultramafic ) rocks.
Occurrence and use
One of the few deposits mined in Europe is in Hirzenhain in Hesse. The natural stones quarried there are used for sculptures and grave monuments. This stone, called "Hessisch Neugrün", can be polished and is frost-resistant. In the vicinity there are other occurrences of similar natural stones ("Dillenburger", "Rachelshäuser", "Bottenhorner" and "Aßlarer Pikrit"), all of which are more or less largely altered, and it is therefore not clear whether they are the correspond to the modern definition of a picrit. In the 20th century, a Pikrit near Seibis (today a district of Rosenthal am Rennsteig in the Bad Lobenstein region ) was dismantled and used for regional and supra-regional building projects. There are several picrite deposits in Russia. In Iceland , picrites can be found in the Reykjanes Peninsula and the Mývatn area .
Natural stone types
- Hessisch Neugrün , mining near Hirzenhain in Hesse (eastern Rhenish Slate Mountains )
- Seibis , near Seibis in Thuringia ( Thuringian Slate Mountains ), mining has now ceased
- Dietmar Reinsch: Natural stone studies. An introduction for civil engineers, architects, preservationists and stonemasons. Enke, Stuttgart 1991, ISBN 3-432-99461-3 .
- R. W. LeMaitre (Ed.): Igneous Rocks. A Classification and Glossary of Terms . 2nd Edition. Cambridge University Press, Cambridge 2004, ISBN 0-521-61948-3 , pp. 34, 128 .
- Dietmar Reinsch: Natursteinkunde , p. 125, see Ref.
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks. Enke, Stuttgart 1985 ISBN 3-432-94671-6 .
- Enno Steindlberger: Volcanic rocks from Hessen and their properties as natural stones . In: Hessisches Landesamt für Umwelt und Geologie (Hrsg.): Geologische Abhandlungen Hessen . tape 110 . Wiesbaden 2003, ISBN 3-89531-812-4 , p. 138-147 .
- Ari Trausti Gudmundsson: Living Earth . Mál og menning, Reykjavik 2007, ISBN 978-9979-3-2778-3 , p. 43 .
- K. Saemundsson, Á. Hjartarson, I. Kaldal, M. Á. Sigurgeirson, SG Kristinsson, S. Vikingsson: Geological Map of the Northern Volcanic Zone, Iceland. Northern Part. 1: 100,000. Ed .: Iceland Geosurvey and Landsvirkjun. Reykjavik 2012.
- Angela Ehling: Diabase - Rock of the Year 2017: Diabase as a stone. Info sheet, Federal Institute for Geosciences and Natural Resources, Geozentrum Hannover, 2017 ( online )