Porphyry (geology)
Porphyry is a variety of 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 traditional, nongeologic use, the term "porphyry" refers to the purple-red form of this 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 deep brownish purple igneous rock with large crystals of plagioclase.
Subsequently the name was given to igneous rocks with large crystals.
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
777777777777 This rock was prized for various monuments and building projects in Imperial Rome and later. Pliny's Natural History affirmed that the "Imperial Porphyry" had been discovered at an isolated site in Egypt in AD 18, by a Roman legionnaire named Caius Cominius Leugas (Werner 1998). It came from a single quarry in the Eastern Desert of Egypt, from 600 million year old andesite of the Arabian-Nubian Shield. The road from the quarry westward to Qena (Roman Maximianopolis) on the Nile, which Ptolemy put on his second-century map, was described first by Strabo, and it is to this day known as the Via Porphyrites, the Porphyry Road, its track marked by the hydreumata, or watering wells that made it viable in this utterly dry landscape. Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia and in the "Porphyra," the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople.
After the fourth century the quarry 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 Burton and Wilkinson in 1823.
7777777777
All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon, as well as the altars and vases and fountain basins reused in the Renaissance and dispersed as far as Kiev, all came from the one quarry at Mons Porpyritis ("Porphyry Mountain," the Arabic Jabal Abu Dukhan), which seems to have been worked intermittently between 29 and 330, when Constantine I celebrated the founding of his capital Constantinople with a 30-meter (100') pillar, built of seven stacked porphyry drums, which still stands. A triumphant last use were the eight monolithic columns of porphyry that support exedrae (semicircular niches) in Hagia Sophia. Justinian's chronicler, Procopius, called the columns "a meadow with its flowers in full bloom, surely to make a man marvel at the purple of some and at those on which the crimson glows." (noted by Werner).
Byzantine historians distinguish two sorts of emperors: those who won power through a coup and those "born to the purple." These porphyrogenites were born to the imperial family in a room in the Great Palace veneered with purple porphyry, as described by Anna Comnena, daughter of the eleventhth-century emperor Alexius I.
The imperial family were entombed in the purple as well, beginning with Nero, who was the first to be immured in a porphyry sarcophagus. Roman sarcophagi were re-used for imperial burials in Sicily: the porphyry sarcophagi of Holy Roman Emperors Frederick II and Henry IV and king William I of Sicily and the Empress Constance are preserved in the cathedrals of Palermo and Monreale.
The Romans used the Imperial porphyry 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.
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 Borghese collection.
In 1840, Bonapartists recovered the body of Napoleon I from Saint Helena and intended to bury it in a porphyry sarcophagus in Les Invalides, Paris. However, the Egyptian quarry was not available and a similar red quartzite from Finland was chosen, in spite of its purchase from the Russian Empire, an enemy of France.
Example Porphyries
- Dacite porphyry
- Trachyte/latite porphyry
- Diorite porphyry
- Granite porphyry
- Rhyolite porphyry
- basalt porphyry
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.
See also
ReferencesISBN links support NWE through referral fees
- Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W.H. Freeman. ISBN 0716724383.
- McBirney, Alexander R. 2006. Igneous Petrology. 3rd ed. Jones & Bartlett. ISBN 0763734489.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Skinner, Brian J., Stephen C. Porter, and Jeffrey Park. 2004. Dynamic Earth: An Introduction to Physical Geology. 5th ed. Hoboken, NJ: John Wiley. ISBN 0471152285.
External links
- Pictures of the Mons Porphyrites, Red Sea, Egypt.
- 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 formation
- Saudi Aramco World Louis Werner, "Via Porphyrites" November/December 1998
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
