Porphyry (geology)

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A piece of porphyry Sarna-Porphyr P1000115.JPG
A piece of porphyry
Rhyolite porphyry. Scale bar in lower left is 1 cm. Rhyolite porphyry.jpg
Rhyolite porphyry. Scale bar in lower left is 1 cm.

Porphyry is a textural term for an igneous rock consisting of large-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate rich, generally aphanitic 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.

Rock (geology) A naturally occurring solid aggregate of one or more minerals or mineraloids

A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition and the way in which it is formed. Rocks are usually grouped into three main groups: igneous rocks, metamorphic rocks and sedimentary rocks. Rocks form the Earth's outer solid layer, the crust.

Crystal solid material whose constituent atoms, molecules, or ions are arranged in an ordered pattern extending in all three spatial dimensions

A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.

Feldspar A group of rock-forming tectosilicate minerals

Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.

Contents

The term porphyry is from Ancient Greek (πορφύρα porphúra) and means "purple". Purple was the color of royalty, and the "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. [1] "Imperial" grade porphyry was thus prized for monuments and building projects in Imperial Rome and later. Porphyry typically has hardness 7 on the Mohs scale of mineral hardness,[ citation needed ] corresponding to steel and quartz.

Ancient Greek Version of the Greek language used from roughly the 9th century BCE to the 6th century CE

The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BCE by Mycenaean Greek and succeeded by medieval Greek.

Purple Range of colors with the hues between blue and red

Purple is a color intermediate between blue and red. It is similar to violet, but unlike violet, which is a spectral color with its own wavelength on the visible spectrum of light, purple is a secondary color made by combining red and blue. The complementary color of purple in the RYB color model is yellow.

Plagioclase feldspars, albite - anorthite solid solution series

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Ancient Greek for "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi3O8 to CaAl2Si2O8), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.

Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition. Its chief characteristic is a large difference in size between the tiny matrix crystals and the much larger phenocrysts. Porphyries may be aphanites or phanerites, that is, the groundmass may have invisibly small crystals as in basalt, or crystals easily distinguishable with the eye, as in granite. Most types of igneous rocks display some degree of porphyritic texture.

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. Porphyritic rocks may be aphanites or extrusive rock, with large crystals or phenocrysts floating in a fine-grained groundmass of non-visible crystals, as in a porphyritic basalt, or phanerites or intrusive rock, with individual crystals of the groundmass easily distinguished with the eye, but one group of crystals clearly much bigger than the rest, as in a porphyritic granite. Most types of igneous rocks may display some degree of porphyritic texture. One main type of rock that has a porphyritic texture are porphyry, though not all porphyritic rocks are porphyries.

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. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

Aphanite Igneous rocks which are so fine-grained that their component mineral crystals are not detectable by the unaided eye

Aphanite, or aphanitic as an adjective, is a name given to certain igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye. This geological texture results from rapid cooling in volcanic or hypabyssal environments. As a rule, the texture of these rocks is not the same as that of volcanic glass, with volcanic glass being non-crystalline (amorphous), and having a glass-like appearance.

Formation

Porphyry deposits are formed when a column of rising magma is cooled in two stages. In the first, the magma is cooled slowly deep in the crust, creating the large crystal grains with a diameter of 2 mm or more. In the second and 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.

Magma Mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth

Magma is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma is produced by melting of the mantle and/or the crust at various tectonic settings, including subduction zones, continental rift zones, mid-ocean ridges and hotspots. Mantle and crustal melts migrate upwards through the crust where they are thought to be stored in magma chambers or trans-crustal crystal-rich mush zones. During their storage in the crust, magma compositions may be modified by fractional crystallization, contamination with crustal melts, magma mixing, and degassing. Following their ascent through the crust, magmas may feed a volcano or solidify underground to form an intrusion. While the study of magma has historically relied on observing magma in the form of lava flows, magma has been encountered in situ three times during geothermal drilling projects—twice in Iceland, and once in Hawaii.

Volcano A rupture in the crust of a planetary-mass object that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface

A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Porphyry copper

Diagram of zonation in a porphyry copper deposit USGS PorphyryCu.jpg
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 that create porphyritic textures in intrusive and hypabyssal porphyritic rocks also lead to a separation of dissolved metals into distinct zones.

This process, which occurs primarily when fluids are driven off the cooling magma, is one of the main reasons for the existence in the world of rich, localized metal ore deposits such as those of gold, copper, molybdenum, lead, tin, zinc, rhenium and tungsten. This enrichment occurs in the porphyry itself, or in 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]

Gold Chemical element with atomic number 79

Gold is a chemical element with symbol Au and atomic number 79, making it one of the higher atomic number elements that occur naturally. In its purest form, it is a bright, slightly reddish yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. Gold often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits. It occurs in a solid solution series with the native element silver and also naturally alloyed with copper and palladium. Less commonly, it occurs in minerals as gold compounds, often with tellurium.

Copper Chemical element with atomic number 29

Copper is a chemical element with symbol Cu and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.

Molybdenum Chemical element with atomic number 42

Molybdenum is a chemical element with symbol Mo and atomic number 42. The name is from Neo-Latin molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered in 1778 by Carl Wilhelm Scheele. The metal was first isolated in 1781 by Peter Jacob Hjelm.

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 places it in the trachytelatite classification of the QAPF diagram.

Rhomb porphyry lavas are only known 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.

Uses

Historical

The Tetrarchs, a porphyry sculpture sacked from the Byzantine Philadelphion palace in 1204, Treasury of St. Marks, Venice Venice - The Tetrarchs 03.jpg
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] Ancient Egyptians used other decorative porphyritic stones of a very close composition and appearance, but apparently remained unaware of the presence of the Roman grade although it was located in their own country.

This particular Imperial grade of porphyry 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 first described 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 it in vain, and it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by James Burton and John Gardiner 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 also used for the blocks of the Column of Constantine in Istanbul. [9]

Modern

In countries where many cars have studded winter tires such as Sweden, Finland and Norway, it is common that highways are paved with asphalt made of porphyry aggregate to make the wearing course withstand the extreme wear from the spiked tires.

See also

Related Research Articles

Granite A common type of intrusive, felsic, igneous rock with granular structure

Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar.

Porphyry may refer to:

Basalt A magnesium- and iron-rich extrusive igneous rock

Basalt is a mafic extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or a moon. More than 90% of all volcanic rock on Earth is basalt. Basalt lava has a low viscosity, due to its low silica content, resulting in rapid lava flows that can spread over great areas before cooling and solidification. Flood basalt describes the formation in a series of lava basalt flows.

Dacite Volcanic rock intermediate in composition between andesite and rhyolite

Dacite is an igneous, volcanic rock. It has an aphanitic to porphyritic texture and is intermediate in composition between andesite and rhyolite. The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains where the rock was first described.

Trachyte igneous rock

Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. It is the volcanic equivalent of syenite. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present.. Biotite, clinopyroxene and olivine are common accessory minerals.

Latite igneous rock

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 quartz-bearing latite. When quartz content is greater than five percent the rock is classified as quartz latite. Biotite, hornblende, pyroxene and scarce olivine or quartz are common accessory minerals.

Volcanic rock rocks composing or associated with volcanoes, volcanic activity or volcanism

Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Matrix (geology) Finer-grained material in a rock within which coarser material is embedded

The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals or clasts are embedded.

Phenocryst

A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.

Quartz monzonite

Quartz monzonite or adamellite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende constitute the dark minerals. Because of its coloring, it is often confused with granite, but whereas granite contains more than 20% quartz, quartz monzonite is only 5–20% quartz. Rock with less than five percent quartz is classified as monzonite. A rock with more alkali feldspar is a syenite whereas one with more plagioclase is a quartz diorite. The fine grained volcanic rock equivalent of quartz monzonite is quartz latite.

Lamprophyre igneous rock

Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.

Quartz-porphyry A type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

Porphyroblast

A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic.

Cathedral Peak Granodiorite

The Cathedral Peak Granodiorite (CPG) was named after its type locality, Cathedral Peak in Yosemite National Park, California. The granodiorite forms part of the Tuolumne Intrusive Suite, one of the four major intrusive suites within the Sierra Nevada. It has been assigned radiometric ages between 88 and 87 million years and therefore reached its cooling stage in the Coniacian.

Igneous rock Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. The magma can be derived from partial melts of existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure, or a change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust.

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. 1 2 "Via Porphyrites". Saudi Aramco World. Retrieved 2012-10-14.
  4. Emerson Howland Swift. Hagia Sophia . Retrieved 2012-10-14.
  5. A. G. Paspatēs (2004-04-30). The Great Palace Of Constantinople . 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. Archived from the original on 2008-03-21. Retrieved 2012-10-14.
  8. "Al-Ahram Weekly | Special: East of Edfu". Weekly.ahram.org.eg. 1999-02-24. Archived from the original on August 13, 2012. Retrieved 2012-10-14.
  9. The Cambridge companion to the Age of Constantine, Volume 13 By Noel Emmanuel Lenski , p. 9, at Google Books