Porphyry (geology)
Porphyry ( AltGr. Πορφύρα porphyra " purple, purple color " ) is a widely used collective term for various volcanic rocks, which have large, well-educated individual crystals in a fine-grained groundmass. They usually have an acidic ( quartz- rich ) to intermediate composition and contain a high proportion of feldspars.
In the modern geological jargon, the term porphyry applies strictly only for the micrograph of a rock and not to a specific rock. In addition, he is still in use as a colloquial term culture and as a proper name for building stones. After the porphyries as type- formative rocks the porphyritic structure is named.
The similar names Porphyrit features more intermediate to basic ( silica- poor ) rocks that belong to the andesite / basalt family. The current term for the mostly greenish, Palaeozoic rocks is incurred Paläoandesit.
Granite porphyry, however, represents a transitional forms between granite and porphyry represents the individual forms are in turn into each other.
Formation
Rocks with porphyritic structure formed when magma cools slowly at first within the earth. In the depth of already formed few but large crystals that float in the melt. Then it comes to a rapid ascent of the magma with a volcanic eruption, the remaining still molten magma cools very rapidly and crystallized. This creates numerous microscopic crystals, which are called matrix or matrix. The faster the cooling takes place, the more fine-grained is the basic mass. The large, easily visible to the naked eye crystals are called phenocrysts. They usually have a size from a few millimeters to several centimeters. Mostly it is fully developed and therefore typically shaped ( " euhedral " ) crystals.
Species
It is generally distinguished between quartz- rich and quarzarmem porphyry. In the former can be found next to feldspar, quartz crystals as inclusions. It is therefore also referred to as " quartz porphyry ". The currently valid name for this rock is rhyolite. Quartz porphyries, quartz arms nor in the matrix contain. But it can also be completely absent. Due to the variable chemical composition fall within the definition of low-quartz porphyry several rocks, such as andesite, trachyte and dacite. Also ignimbrites were formerly known as quartz- poor porphyries.
Deposits in Central Europe
Porphyries are relatively widespread in Germany. They were primarily at the time of the Lower Permian Rotliegend. Large deposits are found among others in the Thuringian Forest, in North West Saxons (with thorn Reichenbach ), in the northern Saalekreis (among Löbejüner porphyry ) in Halle ( Saale) and the hair strand. Other significant deposits in Germany can be found in the Bruchhauser stones on Battert, in the Odenwald, in Tharandter forest and in Meissen Country ( Leutewitz / andesite ).
Furthermore Scandinavian porphyries as glacial boulders in northern Germany are quite often very widespread and. They are, with the exception of the Permian porphyries from the Oslo Graben, mostly Precambrian age. Sometimes, certain porphyries as Leitgeschiebe characteristic defined for a region of origin. These include, for example, the Rhombenporphyr from the Oslo Graben.
Very pronounced is also the deposits of the Brenner road north of Bolzano within the Etschtaler volcanic group.
Use
Porphyries serve both as a raw material for the construction industry, especially for the gravel and gravel production. On the other hand, they are also a popular natural stone. Especially polished they can be very decorative. Even indoors find porphyries use, such as a countertop in the kitchen or other decorative applications. The application as natural stone, however, strict limits are usually set as a result of the rapid, near-surface cooling, the most porphyry deposits runs through a dense network of cooling fractures and larger building stone therefore can not be won.
Porphyry
A linguistically related form to porphyries are rocks which are referred to in petrography as Porphyrtuffe and ignimbrites. Although they are chemically identical to rhyolites, dacites and andesites, distinguished by their origin but by this much. It is not about cold lava but pyroclastic solidified. The porphyritic appearance goes with them to the composition of very fine-grained volcanic ash, which forms the matrix and embedded therein, grobkörnigerer lapilli, the " phenocrysts " back. Especially the ash particles are often cooled so quickly that it is not, as in the "real" porphyries is microcrystalline material, but volcanic glass with amorphous ultrastructure.
Known Porphyrtuffe from Germany are the Zeisigwaldtuff in Chemnitz- Hilber village and Rochlitzer porphyry from Saxony. The latter is a light to dark ruby-colored rock with some yellow color, which has been used for about 1000 years as a building material for walls, stairs, door and window surrounds and decorative elements.
History
Porphyry was at Mons mined in the Old Egyptian Empire Porphyrites in Egypt, the then only known mining area. Larger mining tracks come from there also from Roman times. Porphyry was at the time of the Roman Tetrarchy and then in the time of Constantine very popular. Because of its purple color, it was reserved exclusively for the emperors and their portraits. A well-known example are the statues of the four Tetrarch at St. Mark's Basilica in Venice. For Emperor Constantine there were porphyry circles in the floor of his reception halls that only he was allowed to enter, and his sons were born in porphyrgetäfelten rooms ( Porphyra ) and buried in porphyry sarcophagi.
Other well-known examples of the use of porphyry porphyry is the disc that marks the spot in St. Peter, said to have been crowned at the Charlemagne. In the cathedral at Palermo are, inter alia, to the tombs of Emperor Henry VI. , Emperor Frederick II and of King Roger II of Sicily and Constance of Sicily, these were also made of porphyry. Also, the sarcophagus of King William I of Sicily in the Cathedral of Monreale is porphyry.
A since ancient south of the Alps used Porphyry stone is the Krokeische stone.
Pictures
Late antique porphyry statues of the four Tetrarch at St. Mark's Basilica in Venice
The tombs of Henry VI. and Roger II
The tomb of Henry VI.
Sarcophagus of William I in Monreale