Porphyry (geology)
Overview
 
Porphyry is a variety of igneous rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 consisting of large-grained crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s, such as feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 or quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, dispersed in a fine-grained feldspathic
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...

 or groundmass. The larger crystals are called phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

s. 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
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 and means "purple
Purple
Purple is a range of hues of color occurring between red and blue, and is classified as a secondary color as the colors are required to create the shade....

". Purple was the color of royalty, and the "Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

.
Encyclopedia
Porphyry is a variety of igneous rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 consisting of large-grained crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s, such as feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 or quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, dispersed in a fine-grained feldspathic
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...

 or groundmass. The larger crystals are called phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

s. 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
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 and means "purple
Purple
Purple is a range of hues of color occurring between red and blue, and is classified as a secondary color as the colors are required to create the shade....

". Purple was the color of royalty, and the "Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

. This rock was prized for various monuments and building projects in Imperial Rome
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 and later.

Subsequently the name was given to igneous rocks with large crystals. Porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

now refers to a texture
Rock microstructure
Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...

 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 aphanite
Aphanite
Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks which are so fine-grained that their component mineral crystals are not detectable by the unaided eye...

s or phanerite
Phanerite
A phanerite is an igneous rock having the grains of its essential minerals large enough to be seen macroscopically, i.e. are distinguishable with the unaided eye. Synonymous with coarse grained or megascopically crystalline.Cf: aphanitic....

s, that is, the groundmass may have invisibly small crystals, like basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

, or the individual crystals of the groundmass may be easily distinguished with the eye, as in granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

. Most types of igneous rocks may display some degree of porphyritic texture.

Formation

Porphyry deposits are formed when a column of rising magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 is cooled in two stages. In the first stage, the magma is cooled quickly on the surface, creating the small crystal grains, with a diameter of 2 mm or more. In the final stage, the magma is cooled slowly at relatively deep depth or as it erupts from a volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

, creating small grains that are usually invisible to the unaided eye.

Porphyry Copper

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
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 in the world. This enrichment occurs in the porphyry itself, other related igneous rocks or surrounding country rocks, especially carbonate rock
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

 (in a process similar to skarn
Skarn
Skarn is an old Swedish mining term originally used to describe a type of silicate gangue, or waste rock, associated with iron-ore bearing sulfide deposits apparently replacing Archean age limestones in Sweden's Persberg mining district. In modern usage the term "skarn" has been expanded to refer...

s). Collectively, these type of deposits are known as Porphyry copper deposits.

Rhomb porphyry

Rhomb porphyry is a volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...

 with gray-white large porphyritic
Porphyritic
Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

 rhomb
Rhombus
In Euclidean geometry, a rhombus or rhomb is a convex quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.Every...

 shaped phenocrysts embedded in a very fine grained red-brown matrix
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...

. The composition of rhomb porphyry place it in the trachyte
Trachyte
Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present....

 - latite
Latite
Latite is an igneous, volcanic rock, with aphanitic-aphyric to aphyric-porphyritic texture. Its mineral assemblage is usually alkali feldspar and plagioclase in approximately equal amounts. Quartz is less than five percent and is absent in a feldspathoid-bearing latite, and olivine is absent in a...

 classification of the QAPF diagram
QAPF diagram
A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...

.

Rhomb porphyry lava
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

s are known only from three rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....

 areas: The East African Rift
East African Rift
The East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...

 (including Mount Kilimanjaro
Mount Kilimanjaro
Kilimanjaro, with its three volcanic cones, Kibo, Mawenzi, and Shira, is a dormant volcano in Kilimanjaro National Park, Tanzania and the highest mountain in Africa at above sea level .-Geology:...

), Mount Erebus
Mount Erebus
Mount Erebus in Antarctica is the southernmost historically active volcano on Earth, the second highest volcano in Antarctica , and the 6th highest ultra mountain on an island. With a summit elevation of , it is located on Ross Island, which is also home to three inactive volcanoes, notably Mount...

 near the Ross Sea
Ross Sea
The Ross Sea is a deep bay of the Southern Ocean in Antarctica between Victoria Land and Marie Byrd Land.-Description:The Ross Sea was discovered by James Ross in 1841. In the west of the Ross Sea is Ross Island with the Mt. Erebus volcano, in the east Roosevelt Island. The southern part is covered...

 in Antarctica, and the Oslo graben
Oslo Graben
The Oslo Graben or Oslo Rift is a graben formed during a geologic rifting event in Permian time, the last phase of the Variscan orogeny. The main graben forming period began in the late Carboniferous, which culminated with rift formation and volcanism, with associated rhomb porphyry lava flows...

 in Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

.
It is intrusive.

Historical and cultural uses

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. It came from a single quarry
Stone quarries of ancient Egypt
The stone quarries of ancient Egypt once produced quality stone for the construction of decorative monuments such as sculptures and obelisks. These quarries are now recognised archaeological sites...

 in the Eastern Desert
Eastern Desert
The Eastern Desert is the section of Sahara Desert east of the Nile River, between the river and the Red Sea. It extends from Egypt in the north to Eritrea in the south, and also comprises parts of Sudan and Ethiopia.-Features:...

 of Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...

, from 600 million year old andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...

 of the Arabian-Nubian Shield
Arabian-Nubian Shield
The Arabian-Nubian Shield is an exposure of Precambrian crystalline rocks on the flanks of the Red Sea. The crystalline rocks are mostly Neoproterozoic in age. Geographically - and from north to south - the ANS includes the nations of Israel, Jordan. Egypt, Saudi Arabia, Sudan, Eritrea,...

. The road from the quarry westward to Qena
Qena
Qena is a city in Upper Egypt, and the capital of the Qena Governorate. Situated on the east bank of the Nile, it was known as Kaine during the Greco-Roman period and as Cainepolis in antiquity.- Overview :...

 (Roman Maximianopolis) on the Nile, which Ptolemy
Ptolemy
Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

 put on his second-century map, was described first by Strabo
Strabo
Strabo, also written Strabon was a Greek historian, geographer and philosopher.-Life:Strabo was born to an affluent family from Amaseia in Pontus , a city which he said was situated the approximate equivalent of 75 km from the Black Sea...

, and it is to this day known as the Via Porphyrites, the Porphyry Road, its track marked by the hydreuma
Hydreuma
In Hellenistic and Roman Arabia and Egypt, a hydreuma was an enclosed "watering station" at wadis in dry regions. A hydreuma was a manned and fortified watering hole or way station along a caravan route, providing a man-made oasis...

ta, 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
Hagia Sophia
Hagia Sophia is a former Orthodox patriarchal basilica, later a mosque, and now a museum in Istanbul, Turkey...

 and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople
Great Palace of Constantinople
The Great Palace of Constantinople — also known as the Sacred Palace — was the large Imperial Byzantine palace complex located in the south-eastern end of the peninsula now known as "Old Istanbul", modern Turkey...

.

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
Muhammad Ali of Egypt
Muhammad Ali Pasha al-Mas'ud ibn Agha was a commander in the Ottoman army, who became Wāli, and self-declared Khedive of Egypt and Sudan...

 that the site was rediscovered by Burton and Wilkinson in 1823.

As early as 1850 BC on Crete
Crete
Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...

 in Minoan
Minoan civilization
The Minoan civilization was a Bronze Age civilization that arose on the island of Crete and flourished from approximately the 27th century BC to the 15th century BC. It was rediscovered at the beginning of the 20th century through the work of the British archaeologist Arthur Evans...

 Knossos
Knossos
Knossos , also known as Labyrinth, or Knossos Palace, is the largest Bronze Age archaeological site on Crete and probably the ceremonial and political centre of the Minoan civilization and culture. The palace appears as a maze of workrooms, living spaces, and store rooms close to a central square...

 there were large columns made of porphyry. All the porphyry columns in Rome, the red porphyry toga
Toga
The toga, a distinctive garment of Ancient Rome, was a cloth of perhaps 20 ft in length which was wrapped around the body and was generally worn over a tunic. The toga was made of wool, and the tunic under it often was made of linen. After the 2nd century BC, the toga was a garment worn...

s on busts of emperor
Emperor
An emperor is a monarch, usually the sovereign ruler of an empire or another type of imperial realm. Empress, the female equivalent, may indicate an emperor's wife or a woman who rules in her own right...

s, the porphyry panels in the revetment of the Pantheon
Pantheon, Rome
The Pantheon ,Rarely Pantheum. This appears in Pliny's Natural History in describing this edifice: Agrippae Pantheum decoravit Diogenes Atheniensis; in columnis templi eius Caryatides probantur inter pauca operum, sicut in fastigio posita signa, sed propter altitudinem loci minus celebrata.from ,...

, as well as the altars and vases and fountain basins reused in the Renaissance
Renaissance
The Renaissance was a cultural movement that spanned roughly the 14th to the 17th century, beginning in Italy in the Late Middle Ages and later spreading to the rest of Europe. The term is also used more loosely to refer to the historical era, but since the changes of the Renaissance were not...

 and dispersed as far as Kiev
Kiev
Kiev or Kyiv is the capital and the largest city of Ukraine, located in the north central part of the country on the Dnieper River. The population as of the 2001 census was 2,611,300. However, higher numbers have been cited in the press....

, 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 335 AD.

The remains of Napoleon are entombed in a porphyry sarcophagus in the crypt under the dome at Les Invalides
Les Invalides
Les Invalides , officially known as L'Hôtel national des Invalides , is a complex of buildings in the 7th arrondissement of Paris, France, containing museums and monuments, all relating to the military history of France, as well as a hospital and a retirement home for war veterans, the building's...

.

Porphyry was used for the blocks of the Column of Constantine
Column of Constantine
The Column of Constantine is a Roman monumental column constructed on the orders of the Roman emperor Constantine the Great in 330 AD. It commemorates the declaration of Byzantium as the new capital city of the Roman Empire...

in Istanbul.

External links

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