porphyry
Porphyry (from ancient Gr . Πορφύρα porphyra "purple, purple color ") is a common collective term for various volcanic rocks that have large, well-formed individual crystals in a very fine-grained matrix. They usually have an acidic (quartz-rich) to intermediate composition and contain a high proportion of feldspars .
In modern geological terminology, the term porphyry is strictly speaking only for the structure of a rock and not for a specific rock. This structure is correspondingly referred to as a porphyry structure . In addition, “porphyry” is still in use as a colloquial cultural term and as part of the proper names of numerous stones .
Emergence
Rocks with a porphyry structure are formed when magma inside the earth slowly cools down. Few, but large, crystals are already forming in the depths that float in the melt. If there is then a rapid ascent of the magma with a volcanic eruption , the remaining still liquid magma cools down very quickly and crystallizes. This creates numerous, microscopic crystals, which are known as the base mass or matrix . The faster the cooling takes place, the finer-grained the base mass becomes. The large crystals that are easily visible to the naked eye are called sprinkles . They usually have a size between a few millimeters and several centimeters. Most of them are fully developed and therefore typically shaped (“ idiomorphic ”) crystals.
Porphyry species and structurally similar rocks
Porphyries in the real sense
A general distinction is made between high-quartz and low-quartz porphyry. The former has feldspar and quartz crystals as sprinkles. A now outdated name for this rock is therefore quartz porphyry , whereby it was mainly used to denote "old" ( paleozoic ) quartz-rich porphyries. The name to be used today for this rock is rhyolite . However, this name does not make any statements about the structure, but only about the mineral stock and volcanic formation. Therefore, not every rhyolite is necessarily a porphyry. Strictly speaking, one has to speak of porphyry rhyolite if a corresponding structure is present.
Low-quartz porphyries can still contain quartz in the matrix. But it can also be completely absent. Since the exact chemical composition does not play a role in its definition, the term low-quartz porphyry includes several rocks, such as andesite , trachyte or dacite . The same applies here as for rhyolite - not every rock with this name is necessarily porphyry. For paleozoic dazites (sometimes also for "old" low-quartz porphyries in general), the term porphyrite was used, analogous to quartz porphyry , especially when there were plagioclase inserts .
Porphyry granite Ä.
A porphyry granite or granite porphyry is a quartz- and alkali-feldspar-rich plutonic rock or dike rock , the structure of which is between that of a real granite and that of a volcanic porphyry. The rocks with a relatively coarse-grained base mass are referred to as porphyry granite , while the rocks with a very fine-grained matrix, but with a very high proportion of relatively small fragments, are referred to as granite porphyry . Depending on the chemical composition or the mineral content, there are also granodiorite porphyries and diorite porphyries . In the case of corresponding basic rocks , however, one speaks of porphyry dolerites and micro gabbros .
Porphyry tuff
The petrographic rock term porphyry tuff is linguistically based on the "real" porphyries due to the structural similarity of the so-called rocks. Some deposits of porphyries were actually classified as porphyries in the past and the natural stones extracted from these deposits are still sold today as "porphyries". Porphyry tuffs are of volcanic origin and chemically identical to rhyolites, dazites or andesites, but differ significantly from these in the actual formation process : they are not cooled lava flows, but pyroclastic sediments . The porphyry appearance of them is due to the composition of very fine-grained volcanic ash , which forms the basic mass, and the coarser-grained lapilli embedded in it , the "Einsprenglingen". In particular, the ash particles are often cooled so quickly that they are not microcrystalline material, as is the case with the “real” porphyries, but rather volcanic glass with an amorphous ultrastructure. The formation of numerous porphyry tuff deposits is associated with pyroclastic flows . Such porphyry tuffs are also called ignimbrites .
Well-known Porphyrtuffe from Germany are the Rochlitzer Porphyr , the Hilbersdorfer Porphyrtuff ( Zeisigwald -Tuff) and the Rüdigsdorfer Porphyrtuff, all from the Rotliegend of Saxony .
Copper porphyry
Copper- rich subvolcanic porphyries are referred to as copper porphyry or copper porphyry, which are nowadays important as ore deposits for copper , gold or molybdenum . These rocks, bound to subduction zones , are created by the intrusion of fluid-rich (especially water) magma into higher rock layers. As the crystal formation now begins, the fluids collect in the residual magma and penetrate the overburden. The fluids are enriched in heavy metals, which are precipitated during metasomatosis of the overburden. The composition of the heavy metals depends on the place of origin, for example copper porphyries occurring in basaltic island arches are more gold-rich, while porphyries and the like formed in acidic crusts. a. Enrich molybdenum.
Occurrence in Central Europe
Porphyries are relatively widespread in Germany. They originated mainly at the time of the Rotliegend in the Lower Permian . There are large deposits in the Thuringian Forest , in north-west Saxony (the Rochlitzer Porphyry or near Dornreichenbach ), in the northern Saalekreis and in Halle (Saale) (see also Hallescher Porphyry Complex ) and on Haarstrang . Other significant deposits in Germany can be found near the Bruchhauser stones , on the Battert , in the Odenwald , in the Tharandt forest and in the Meißener Land ( Leutewitz / Andesite ).
Furthermore, Scandinavian porphyries are quite common and very widespread as glacial debris in northern Germany . With the exception of the Permian porphyries from the Oslo Trench , they are mostly Precambrian in age. Occasionally, certain porphyries as guiding debris are characteristic of a defined area of origin. This includes B. the rhombic porphyry from the Oslo trench.
The occurrence within the Etschtaler Vulkanit-Gruppe in South Tyrol and Trentino is also very pronounced .
use
On the one hand, porphyries serve as a mass raw material for the building materials industry, in particular for the production of gravel and grit. On the other hand, they are also a sought-after natural stone . Especially when polished, they can have a very decorative effect due to their porphyry structure. They are used both outdoors and indoors, for example as facade cladding or as a worktop in the kitchen. Due to the rapid cooling close to the surface, most of the porphyry deposits are criss-crossed by a close-knit network of cooling fissures, so that no larger blocks of stone can be extracted from them, which somewhat limits the possible uses.
history
Porphyry was already mined in the Old Egyptian Empire at Mons Porphyrites in Egypt , the only known mining area at the time. Larger signs of degradation come from there also from Roman times. Porphyry was very popular during the Roman Tetrarchy and then also in Constantinian times . Due to its purple color, it was reserved exclusively for the emperors and their portraits. A well-known example are the statues of the four tetrarchs on St. Mark's Basilica in Venice .
For Emperor Constantine, there were porphyry circles in the floors of his reception halls that only he was allowed to enter, and his sons were born in porphyry- paneled rooms ( porphyra ) and buried in porphyry sarcophagi.
Another well-known example of the use of porphyry is the porphyry disk, which marks the place in St. Peter's Basilica where Charlemagne is said to have been crowned. In the cathedral of Palermo are u. a. the tombs of Emperor Heinrich VI. , Emperor Frederick II as well as King Roger II of Sicily and Constance of Sicily , these were also made of porphyry. The sarcophagus of King William I of Sicily in the Cathedral of Monreale is also made of porphyry.
A porphyry stone that has been used south of the Alps since ancient times is the Kroke stone .
Antique bust of Minerva , 2nd century (with additions from the 17th century), Louvre
Baptismal font in Magdeburg Cathedral , from Porfido rosso
The late antique Venetian tetrarch group made from propyr at St. Mark's Basilica in Venice
The tombs of Frederick II and Rogers II in the Cathedral of Palermo
Sarcophagus of Wilhelm I in Monreale
literature
- Aschaffenburg Natural Science Association (ed.): Porphyre. Announcements from the Natural Science Museum of the City of Aschaffenburg. Vol. 26 (Proceedings of the Porphyry Conference on October 21 and 22, 2011 in Weilbach and Amorbach, Miltenberg district). Helga Lorenz Verlag, Karlstein am Main 2012, ISSN 0939-1924 .
- Gabriele Borghini (ed.): Marmi antichi . Edizioni de Luca, Rome 2001, ISBN 88-8016-181-4 .
- Raymond Perrier: Les roches ornementales . Edition Pro Roc, Ternay 2004, ISBN 2-9508992-6-9 .
- Arnd Peschel: Natural stones . 2nd revised edition. Deutscher Verlag der Grundstoffindindustrie, Leipzig 1983 ( series of monographs usable rocks and industrial minerals 1, ZDB -ID 554920-6 ).
- Roland Vinx: Rock determination in the field . Elsevier, Munich 2005, ISBN 3-8274-1513-6 .
- Yvonne Schmuhl: Porphyry . In: RDK Labor (2016).
Web links
- Mineral Atlas: Porphyry
- Porfido verde antico the striking green rock (andesitic porphyry) from Greece; Porfido rosso antico the imperial (red) porphyry from Egypt and other porphyries.
Individual evidence
- ↑ a b Wolfhardt Wimmenauer: Petrography of igneous and metamorphic rocks. Enke-Verlag, Stuttgart 1985, ISBN 3-432-94671-6 , p. 175.
- ↑ Heiner Siedel: Saxon "Porphyrtuffe" from the Rotliegend as building stones: occurrence and mining, application, properties and weathering. Institute for Stone Conservation e. V. Report No. 22, 2006 ( PDF ( Memento of the original from December 5, 2014 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. ) .
- ↑ Jamie J. Wilkinson: Triggers for the formation of porphyry ore deposits in magmatic arcs , nature geoscience, October 13, 2013 doi: 10.1038 / ngeo1940
