World Library  
Flag as Inappropriate
Email this Article

Porphyry (geology)

Article Id: WHEBN0000305399
Reproduction Date:

Title: Porphyry (geology)  
Author: World Heritage Encyclopedia
Language: English
Subject: Mount Banda Banda, Kiruna Mine, Economy of Balochistan, Pakistan, Breccia, Hagia Sophia
Collection: Igneous Rocks, Porphyritic Rocks, Sculpture Materials, Stone, Volcanic Rocks
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Porphyry (geology)

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. The rock was the hardest known in antiquity[1] and was prized for 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.

Contents

  • Formation 1
    • Porphyry copper 1.1
    • Rhomb porphyry 1.2
  • Historical and cultural uses 2
  • See also 3
  • References 4
  • External links 5

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, rhenium 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".[2]

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 Tetrarchs, a porphyry sculpture sacked from the Byzantine Philadelphion palace in 1204, Treasury of St. Marks, Venice.

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.[3] 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 [4] and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople.[5]

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 column bases made of porphyry.[6] All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon,[3] 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[7] ("Porphyry Mountain", the Arabic Jabal Abu Dukhan), which seems to have been worked intermittently between 29 and 335 AD.[8]

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

See also

References

  1. ^ "PORPHYRY" in The Oxford Dictionary of Byzantium, Oxford University Press, New York & Oxford, 1991, p. 1701. ISBN 0195046528
  2. ^ Dietrich, R. and Skinner, B., 1979, Rocks and Rock Minerals, pg. 125.
  3. ^ a b "Via Porphyrites". Saudi Aramco World. Retrieved 2012-10-14. 
  4. ^ Hagia Sophia - Emerson Howland Swift - Google Books. Books.google.com.tr. Retrieved 2012-10-14. 
  5. ^ The Great Palace Of Constantinople - A. G. Paspatēs - Google Books. Books.google.com.tr. 2004-04-30. Retrieved 2012-10-14. 
  6. ^ C. Michael Hogan (2007). "Knossos fieldnotes". The Modern Antiquarian. Retrieved 2012-10-14. 
  7. ^ "Archaeology". Arch.soton.ac.uk. 2012-09-25. Retrieved 2012-10-14. 
  8. ^ "Al-Ahram Weekly | Special: East of Edfu". Weekly.ahram.org.eg. 1999-02-24. Retrieved 2012-10-14. 
  9. ^ The Cambridge companion to the Age of Constantine, Volume 13 By Noel Emmanuel Lenski

External links

  • Pictures of the Mons Porphyrites, Red Sea, Egypt.
  • Rhomb porphyry lavas
  • Flash showing rhomb porphyry formation
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from Hawaii eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.