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Porphyry (geology)

From Academic Kids

(For other meanings of Porphyr, see Porphyry)


Porphyry is a very hard igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix (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 igenous rock with large crystals of plagioclase. This 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 AD29 and AD335, 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 Mohamed Ali that the site was rediscovered by Bruton and Wilkinson in 1823.

Missing image
Imp_porphyry.jpg
A slab of imperial porphyry from Egypt, about 15 cm across. The term porphry originally applied to the color of the rock but was later associated with the large white crystals (plagioclase) set in a finer-grained red-purple groundmass.

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.

Contents

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 curst, creating the large crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled rapidly 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.

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Entrance_of_Colditz_Castle_chapel.jpg
The porphyry portal of the "church house" at Colditz Castle, Saxony, designed by Andreas Walther II (1584), is a clear example of the exuberance of "Antwerp Mannerism".

Porphyry in history

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, 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 AD 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, or semicircular niches.

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Baptismal_Font_Magdeburg.jpg
The baptismal font in the Cathedral of Magdeburg is made of rose porphyry from a site near Assuan, Egypt

Porphyry was used extensively for decoration in the south eastern portions of Germany, Poland, and Czechloslovakia. 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

fr:Porphyre (roche) pl:Porfir sv:Porfyr

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