Porphyry (geology)

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"Imperial Porphyry" from the Red Sea Mountains of Egypt "Imperial Porphyry" - porphyritic metadacite to porphyritic meta-andesite (Dokhan Volcanics, Neoproterozoic, ~593-602 Ma; Mons Porphyrites, Red Sea Mountains, Egypt) 2 (30040632451).jpg
"Imperial Porphyry" from the Red Sea Mountains of Egypt
A waterworn cobble of porphyry Sarna-Porphyr P1000115.JPG
A waterworn cobble of porphyry
Rhyolite porphyry from Colorado; scale bar in lower left is 1 cm (0.39 in) Rhyolite porphyry.jpg
Rhyolite porphyry from Colorado; scale bar in lower left is 1 cm (0.39 in)

Porphyry ( /ˈpɔːrfəri/ POR-fə-ree) is a textural term for an igneous rock consisting of coarse-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. [1] In its non-geologic, traditional use, the term porphyry refers to the purple-red form of this stone, valued for its appearance.

Contents

The term porphyry is from the Ancient Greek πορφύρα (porphyra), meaning "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. [2] Thus, "imperial"-grade porphyry was prized for monuments and building projects in Imperial Rome and thereafter.

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 microscopic crystals as in basalt, or crystals easily distinguishable with the eye, as in granite.

Formation

Most igneous rocks have some degree of porphyritic texture. This is because most magma from which igneous rock solidifies is produced by partial melting of a mixture of different minerals. [3] At first the mixed melt slowly cools deep in the crust. The magma begins crystallizing the highest melting point minerals closest to the overall composition first, in a process called fractional crystallization. This forms phenocrysts, which usually have plenty of room for growth, and form large, well-shaped crystals with characteristic crystal faces (euhedral crystals). [4] If they are different in density to the remaining melt, these phenocrysts usually settle out of solution, eventually creating cumulates; however if the partially crystallized magma is then erupted to the surface as a lava, the remainder of the melt is quickly cooled around the phenocrysts and crystallizes much more rapidly to form a very fine-grained or glassy matrix. [5]

Porphyry can also form even from magma that completely solidifies while still underground. The groundmass will be visibly crystalline, though not as large as the phenocrysts. The crystallization of the phenocrysts during fractional crystallization changes the composition of the remaining liquid magma, moving it closer to the eutectic point, with a mixed composition of minerals. As the temperature continues to decrease, this point is reached, and the rock is entirely solidified. The simultaneous crystallization of the remaining minerals produces the finer-grained matrix surrounding the phenocrysts, as they crowd each other out. [5]

The significance of porphyritic texture as an indication that magma forms through different stages of cooling was first recognized by the Canadian geologist, Norman L. Bowen, in 1928. [6]

Porphyritic texture is particularly common in andesite, with the most prominent phenocrysts typically composed of plagioclase feldspar. [7] [8] Plagioclase has almost the same density as basaltic magma, so plagioclase phenocrysts are likely to remain suspended in the magma rather than settling out. [9]

Rhomb porphyry

Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhombus-shaped phenocrysts of feldspar (commonly anorthoclase) embedded in a very fine-grained red-brown matrix. The composition of rhomb porphyry places it in the trachytelatite classification of the QAPF diagram. [10]

Rhomb porphyry is found in continental rift areas, including the East African Rift (including Mount Kilimanjaro), [11] Mount Erebus near the Ross Sea in Antarctica, [12] the Oslo graben in Norway, [10] and south-central British Columbia. [13]

Use in art and architecture

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
Carmagnola, an imperial porphyry head in Venice thought to represent Justinian Carmagnola 1.jpg
Carmagnola , an imperial porphyry head in Venice thought to represent Justinian

Antiquity and Byzantium

Pliny the Elder'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. [14] [ failed verification ] 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. It was also sometimes used in Minoan art, and as early as 1850 BC on Crete in Minoan Knossos there were large column bases made of porphyry. [15]

This particular Imperial grade of porphyry all came from the Gabal Abu Dukhan 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.

After the fifth century the quarry was lost to sight for many centuries. Byzantium scholar Alexander Vasiliev suggested this was the consequence of the Council of Chalcedon in 451 CE and the subsequent troubles in Egypt. [16] 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 the English Egyptologists James Burton and John Gardner Wilkinson in 1823.

All the porphyry columns in Rome, the red porphyry togas on busts of emperors, the porphyry panels in the revetment of the Pantheon, [14] as well as the altars and vases and fountain basins reused in the Renaissance and dispersed as far as Kyiv, all came from the one quarry at Mons Porphyrites [17] ("Porphyry Mountain", the Arabic Jabal Abu Dukhan), which seems to have been worked intermittently between 29 and 335 AD. [18] Porphyry was also used for the blocks of the Column of Constantine in Istanbul. [19]

Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia [20] and in the "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople, giving rise to the phrase "born in the purple". [21]

Choosing porphyry as a material was a bold and specific statement for late Imperial Rome. As if it were not enough that porphyry was explicitly for imperial use, the stone's rarity set the emperors apart from their subjects as their superiors. The comparative vividness of porphyry to other stones underscored that these figures were not regular citizens, but many levels above, even gods, and worthy of the respect they expected. Porphyry made the emperors unapproachable in terms of power and nature, belonging to another world, the world of the mighty gods, present for a short time on earth. [22]

Porphyry also stood in for the physical purple robes Roman emperors wore to show status, because of its purple colouring. Similar to porphyry, purple fabric was extremely difficult to make, as what we now call Tyrian purple required the use of rare sea snails to make the dye. [23] The colour itself reminded the public to how to behave in the presence of the emperors, with respect bordering on worship for the self-proclaimed god-kings. [24]

Roman and late Roman imperial sarcophagi

Porphyry sarcophagus, Istanbul Archaeological Museum Istanbul - Museo archeologico - Sarcofago imperiale bizantino - Foto G. Dall'Orto 28-5-2006.jpg
Porphyry sarcophagus, Istanbul Archaeological Museum

A uniquely prestigious use of porphyry was its choice as material for imperial sarcophagi in the 4th and early 5th centuries. That tradition appears to have been started with Diocletian's porphyry sarcophagus in his mausoleum, which was destroyed when the building was repurposed as a church but of which probable fragments are at the Archaeological Museum in Split, Croatia. [25] The oldest and best-preserved ones are now conserved at the Vatican Museums and known as the Sarcophagi of Helena and Constantina.

Nine other imperial porphyry sarcophagi were long held in the Church of the Holy Apostles in Constantinople. They were described by Constantine VII Porphyrogenitus in the De Ceremoniis (mid-10th century), who specified them to be respectively of Constantine the Great, Constantius II, Julian, Jovian, Theodosius I, Arcadius, Aelia Eudoxia, Theodosius II, and Marcian. Of these, most still exist in complete or fragmentary form, despite depredations by later Byzantine Emperors, Crusaders, and Ottoman conquerors. [16] Four presently adorn the facade of the main building of the İstanbul Archaeology Museums, [26] including one whose rounded shape led Alexander Vasiliev to suggest attribution to Emperor Julian on the basis of Constantine Porphyrogenitus's description. Vasiliev conjectures that the nine imperial sarcophagi, including one which carries a crux ansata or Egyptian cross, were carved in Egypt before shipment to Constantinople. [16]

Porphyry sarcophagi in post-Roman Western Europe

The imperial porphyry sarcophagi tradition was emulated by Ostrogothic King Theodoric the Great (454-526), whose mausoleum in Ravenna still contains a porphyry tub that was used as his sarcophagus. Similarly Charles the Bald, King of West Francia and Roman Emperor, was buried at Saint-Denis in a porphyry tub [27] which may be the same one known as "Dagobert's tub" (cuve de Dagobert), now in the Louvre. [28]

The tomb of Peter III of Aragon, in the Monastery of Santes Creus near Tarragona, reuses a porphyry tub or alveus, which has been conjectured to be originally the sarcophagus of Late Roman Emperor Constans in his mausoleum at Centcelles, a nearby site with a well-preserved 4th-century rotunda. [29]

In twelfth- and thirteenth-century Sicily, another group of porphyry sarcophagi were produced from the reign of Roger II onward and used for Royal and then Imperial burials, namely those of King Roger II, King William I, Emperor Henry VI, Empress Constance, and Emperor Frederick II. They are all now in the Palermo Cathedral, except William's in Monreale Cathedral. Scholar Rosa Bacile argues that they were carved by a local workshop from porphyry imported from Rome, the latter four plausibly (based on observation of their fluting) all from a single column shaft that may have been taken from the Baths of Caracalla or the Baths of Diocletian. She notes that these Sicilian porphyry sarcophagi "are the very first examples of medieval free-standing secular tombs in the West, and therefore play a unique role within the history of Italian sepulchral art (earlier and later tombs are adjacent to, and dependent on walls)." [30]

Six grand porphyry sarcophagi are featured along the walls of the octagonal Cappella dei Principi (Chapel of the Princes) that was built as one of two chapels in the architectural complex of the Basilica of San Lorenzo, in Florence, Italy, for the de' Medici family. Purple porphyry was used lavishly throughout the opulent chapel as well, with a revetment of marbles, inlaid with other colored marbles and semi-precious stone, that covers the walls completely. Envisioned by Cosimo I, Grand Duke of Tuscany (1537–1574), it was initiated by Ferdinand I de' Medici, following a design by Matteo Nigetti that won an informal competition held in 1602 by Don Giovanni de' Medici (a son of Cosimo I), which was altered somewhat during execution by Buontalenti. [31]

The tomb of Napoleon at Les Invalides in Paris, designed by architect Louis Visconti, is centered on the deceased emperor's sarcophagus that often has been described as made of red porphyry although this is incorrect. Napoleon's sarcophagus is made of quartzite, however, its pedestal is made of green andesite porphyry from Vosges. [32]

Modern uses

In countries where many automobiles 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 winter tires. [33]

See also

Related Research Articles

Granite Common type of intrusive, felsic, igneous rock with granular structure

Granite is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.

Porphyry may refer to:

Rhyolite Igneous, volcanic rock, of felsic (silica-rich) composition

Rhyolite is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent to granite.

Dacite Volcanic rock intermediate in composition between andesite and rhyolite

Dacite is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.

Trachyte Extrusive igneous rock

Trachyte is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and alkali metals. It is the volcanic equivalent of syenite.

Latite Type of volcanic 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. Feldspathoid-bearing latite is sometimes referred to as tristanite.

Andesite Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

Aphanite Igneous rock composed of very small crystals invisible to the naked eye

Aphanites are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked 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.

Extrusive rock Mode of igneous volcanic rock formation

Extrusive rock refers to the mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff. In contrast, intrusive rock refers to rocks formed by magma which cools below the surface.

Porphyritic Igneous rock with large and small crystals

Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning all types of igneous rocks can display some degree of porphyritic texture. Most Porphyritic rocks have bimodal size ranges, meaning the rock is composed of two distinct sizes of crystal.

Volcanic rock Rock formed from lava erupted from a volcano

Volcanic rock is a rock formed from lava 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.

Phenocryst Crystal larger than the rock grains that surround it in an igneous rock

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.

Intrusive rock Magmatic rock formed below the surface

Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form intrusions, such as batholiths, dikes, sills, laccoliths, and volcanic necks.

Lamprophyre Ultrapotassic igneous rocks

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.

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.

The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt, ferro-basalt, tholeiitic basaltic andesite, tholeiitic andesite, dacite and rhyolite. The variety of basalt in the series was originally called tholeiite but the International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to that term.

Fractional crystallization (geology) Process of rock formation

Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within crust and mantle of a rocky planetary body, such as the Earth. It is important in the formation of igneous rocks because it is one of the main processes of magmatic differentiation. Fractional crystallization is also important in the formation of sedimentary evaporite rocks.

Igneous textures

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

Cathedral Peak Granodiorite Suite of intrusive rock in the Sierra Nevada

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.

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