- Porphyry (geology)
Porphyry is a variety of
igneousrock consisting of large-grained crystals, such as feldsparor 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 the purple-red form of this stone, valued for its appearance.
The term "porphyry" is from Greek 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. This rock was prized for various monuments and building projects in Imperial Rome and later. Pliny's Natural Historyaffirmed 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 Desertof Egypt, from 600 million year old andesiteof the Arabian-Nubian Shield. The road from the quarry westward to Qena (Roman Maximianopolis) on the Nile, which Ptolemyput 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 Sophiaand 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
Napoleonsought 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.
Subsequently the name was given to
igneous rockswith 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.
Porphyry deposits are formed when a column of rising
magmais 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, zincand tungsten.
As early as 1850
BCon Cretein Minoan Knossosthere were large columns made of porphyry. [ [http://letmespeaktothedriver.com/site/10854/knossos.html#fieldnotes C. Michael Hogan, "Knossos fieldnotes", Modern Antiquarian (2007)] ] All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon, [ [http://www.saudiaramcoworld.com/issue/199806/via.porphyrites.htm A visit to the ancient Imperial Porphyry quarries in Egypt. "Saudi Aramco World" Louis Werner, "Via Porphyrites" November/December 1998] ] as well as the altars and vases and fountain basins reused in the Renaissanceand dispersed as far as Kiev, all came from the one quarry at "Mons Porpyritis" [ [http://www.arch.soton.ac.uk/Projects/projects.asp?ProjectID=34 University of Southhampton, "Mons Porphyrites quarries, Egypt"] ] ("Porphyry Mountain", the Arabic "Jabal Abu Dukhan"), which seems to have been worked intermittently between 29and 335 AD, [ [http://weekly.ahram.org.eg/1999/417/special.htm Rushdi Said, Al-Ahram Weekly, 18 - 24 February 1999 Issue No. 417, "Roman occupation of the Eastern Desert of Egypt and the Imperial Porphyry quarries"] ] when Constantine Icelebrated the founding of his capital Constantinoplewith 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 eleventh 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 Sicilyand the Empress Constance are preserved in the cathedrals of Palermoand Monreale.
The Romans used the Imperial porphyry for the monolithic pillars of
Baalbek's Temple of Heliopolisin 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
Franceobtained the largest collection of porphyry by acquiring the Borghesecollection.
Bonapartists recovered the body of Napoleon Ifrom 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 quartzitefrom Finland was chosen, in spite of its purchase from the Russian Empire, an enemy of France.
* see also
* see also
list of rock textures
Rhomb porphyry is a
volcanic rockwith gray-white large porphyriticrhomb shaped phenocrystsenbedded in a very fine grained red-brown matrix. The composition of rhomb porphyry place it in the trachyte- latiteclassification of the QAPF diagram.
lavas are known only from three riftareas: The East African Rift(including Mount Kilimanjaro), Mount Erebusnear the Ross Seain Antarctica, and the Oslo grabenin Norway.
* [http://www.euratlas.com/mons_porphyrites/index.html Pictures of the Mons Porphyrites, Red Sea, Egypt.]
* [http://www.toyen.uio.no/geomus/nettutstillinger/Osloriften/rombeporfyr-eng.html Rhomb porphyry lavas]
* [http://www.ig.uit.no/geostudiesamling/ Flash showing rhomb porphyry formation]
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