porphyry

A piece of porphyry

Rhyolite porphyry. Scale bar in lower left is 1 cm.

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 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 purple igneous rock with large crystals of plagioclase. This rock was prized for various monuments and building projects in Imperial Rome and later.

Subsequently the name was given to igneous rocks with large crystals. Porphyritic 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 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. Most types of igneous rocks may display some degree of porphyritic texture.

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.

Porphyry Copper

Diagram of zonation in a porphyry copper deposit

The term porphyry is also used for a mineral deposit called a copper porphyry. The different stages of cooling which creates porphyritic textures in intrusive and hypabyssal porphyritic rocks also leads to a separation of dissolved metals into distinct zones. This process, which is produced primarily via fluids driven off the cooling magma, is one of the main reasons for the existence of rich, localized metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc and tungsten in the world. This enrichment occurs in the porphyry itself, other related igneous rocks or surrounding country rocks, especially carbonate rock (in a process similar to skarns). Collectively, these type of deposits are known as Porphyry copper deposits.^[1]

Rhomb porphyry

Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhomb shaped phenocrysts embedded 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. It is intrusive.

Historical and cultural uses

The baptismal font in the Cathedral of Magdeburg is made of red porphyry from Gebel Abu Dokhan near Hurghada, Egypt

Pliny's Natural History affirmed that the "Imperial Porphyry" had been discovered at an isolated site in Egypt in AD 18, by a Roman legionary named Caius Cominius Leugas.^[2] 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 ^[3]and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople. ^[4]

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.

As early as 1850 BC on Crete in Minoan Knossos there were large columns made of porphyry.^[5] All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon,^[2] 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^[6] ("Porphyry Mountain", the Arabic Jabal Abu Dukhan), which seems to have been worked intermittently between 29 and 335 AD.^[7]

The remains of Napoleon are entombed in a porphyry sarcophagus in the crypt under the dome at Les Invalides.^[8]

Porphyry was used for the blocks of the Column of Constantine in Istanbul.^[9]

See also

• List of rock textures

References

Wikimedia Commons has media related to: Porphyry

External links

• Pictures of the Mons Porphyrites, Red Sea, Egypt.
• Rhomb porphyry lavas
• Flash showing rhomb porphyry formation