Porphyry (geology)
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Image:Imp porphyry.jpg Porphyry is a very hard igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts. In its non-geologic, traditional use, the term "porphyry" refers to a purple-red stone valued for its appearance.
The term "porphyry" is from Latin and means "purple". Purple was the color of royalty, and the "Imperial Porphyry" was a purple igneous rock with large crystals of plagioclase. This rock was prized for various monuments and building projects in Rome and later. It came from quarries in the Eastern Desert of Egypt, from 600 million year old andesite of the Arabian-Nubian Shield. The quarry seems to have been worked intermittently between 29 and 335, after which it was lost to sight for many centuries. The scientific members of the French Expedition under Napoleon sought for it in vain, and it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by Bruton and Wilkinson in 1823.
Subsequently the name was given to igneous rocks with large crystals. Porphyry now refers to a texture of igneous rocks. Its chief characteristic is a large difference between the size of the tiny matrix crystals and other much larger crystals, called phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals, like basalt, or the individual crystals of the groundmass may be easily distinguished with the eye, as in granite. Many types of igneous rocks may display porphyrytic texture.
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Formation
Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye. The cooling also leads to a separation of dissolved metals into distinct zones. This process is one of the main reasons for the existence of rich, localised metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc and tungsten.
Porphyry in history
Image:Entrance of Colditz Castle chapel.jpg
In the Roman Empire, the palace room reserved for royal births was lined with Imperial Porphyry, and the emperors born in this room were referred to as porphyrogenitus ('born in the purple'). The Romans used the Imperial porphyry for the Pantheon's inlaid panels, for the togas in the sculpted portraiture of their emperors, and for the monolithic pillars of Baalbek's Temple of Heliopolis in Lebanon. Today there are at least 134 porphyry columns in buildings around Rome, all reused from imperial times, since the stone is not naturally present in Italy, and countless altars, basins and other objects.
The Imperial Porphyry was also popular with Byzantium. Constantine the Great celebrated the founding of his new capital, Constantinople (later Istanbul), in 330 by erecting a 30-meter (100') pillar, built of seven porphyry drums, or cylinders, that still stands. Eight monolithic columns of porphyry support Hagia Sophia's exedrae (semicircular niches).
Image:Baptismal Font Magdeburg.jpg
Porphyry was used extensively for decoration in Germany, Poland, and Czechoslovakia. This can be seen in the Mannerist style sculpted portal outside the chapel entrance in Colditz Castle.
Louis XIV King of France obtained the largest collection of porphyry by acquiring the Borghèse collection.
Example Porphyries
- Dacite porphyry
- Trachyte/latite porphyry
- Diorite porphyry
- Granite porphyry
- Rhyolite porphyry
- basalt porphyry
- see also porphyritic texture
- see also list of rock textures
Rhomb porphyry
Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhomb shaped phenocrysts enbedded in a very fine grained red-brown matrix. The composition of rhomb porphyry place it in the trachyte - latite classification of the QAPF diagram.
Rhomb porphyry lavas are known only from three rift areas: The East African Rift (including Mount Kilimanjaro), Mount Erebus near the Ross Sea in Antarctica, and the Oslo graben in Norway.
External links
- Roman occupation of the Eastern Desert of Egypt and the Imperial Porphyry quarries.
- Mons Porphyrites quarries, Egypt
- A visit to the ancient Imperial Porphyry quarries in Egypt.
- Rhomb porphyry lavas
- Flash showing rhomb porphyry formationde:Porphyr
fr:Porphyre (roche) nl:Porfier no:Rombe porfyr pl:Porfir pt:Porfirítica sv:Porfyr