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Porphyry

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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 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. This rock was prized for various monuments and building projects in Imperial Rome and later. Pliny's Natural History afirmed that the "Imperial Pophyry" 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. 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 the Great erected in Constantinople a 30-meter pillar, built of seven stacked porphyry drums, which still stands. A triumphant last use were the eight monolithic columns of porphyry that support exedrae 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). In Constantinople, the imperial family were "born to the purple" in a room in the palace veneered with purple porphyry that was 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, the first to be immured in a porphyry sarcophagus. Roman sarcophagi were re-used for imperial burials in Sicily: the poprphyry 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. 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 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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