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.[1] 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.[2] In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.

Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as felsites and andesites, although they occur throughout the igneous spectrum including in the ultramafics. The largest crystals found in some pegmatites are often phenocrysts being significantly larger than the other minerals.

Montblanc granite phenocrysts
Granites often have large feldspathic phenocrysts. This granite, from the Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coin (diameter 2.3 cm) for scale.

Classification by phenocryst

Photomicrograph-porphyritic-aphanitic-felsic-rock-USGS
Photomicrograph of a porphyritic-aphanitic felsic rock, from the Middle Eocene in the Blue Ridge Mountains of Virginia. Plagioclase phenocrysts (white) and hornblende phenocryst (dark; intergrown with plagioclase) are set in a fine matrix of plagioclase laths that show flow structure.

Rocks can be classified according to the nature, size and abundance of phenocrysts, and the presence or absence of phenocrysts is often noted when a rock name is determined. Aphyric rocks are those that have no phenocrysts,[3] or more commonly where the rock consists of less than 1% phenocrysts (by volume);[4] while the adjective phyric is sometimes used instead of the term porphyritic to indicate the presence of phenocrysts. Porphyritic rocks are often named using mineral name modifiers, normally in decreasing order of abundance. Thus when olivine forms the primary phenocrysts in a basalt, the name may be refined from basalt to porphyritic olivine basalt or olivine phyric basalt.[5] Similarly, a basalt with olivine as the dominate phenocrysts, but with lesser amounts of plagioclase phenocrysts, might be termed a olivine-plagioclase phyric basalt.

In more complex nomenclature, a basalt with approximately 1% plagioclase phenocrysts, but 4% olivine microphenocrysts, might be termed an aphyric to sparsely plagioclase-olivine phyric basalt, where plagioclase is listed before the olivine, because of its larger crystals.[6] Categorizing a rock as aphyric or as sparsely phyric is often a question of whether a significant number of crystals exceed the minimum size.[7]

Analysis using phenocrysts

Geologists use phenocrysts to help determine rock origins and transformations because crystal formation partly depends on pressure and temperature.

Other characteristics

Plagioclase phenocrysts often exhibit zoning with a more calcic core surrounded by progressively more sodic rinds. This zoning reflects the change in magma composition as crystallization progresses.[8] In rapakivi granites, phenocrysts of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase. In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix. These volcanic phenocrysts often show flow banding, a parallel arrangement of lath-shaped crystals. These characteristics provide clues to the rocks' origins. Similarly, intragranular microfractures and any intergrowth among crystals provide additional clues.[9]

See also

Notes

  1. ^ The minimum size boundary is arbitrary and not precise. It is based upon observation and may vary depending upon whether technical aids, such as a hand lens or a microscope are used or not. One analyst used a 100 µm limit on the size of crystals as that was the minimum that could be point-counted accurately by optical means. Murphy, M. D.; Sparks, R. S. J.; Barclay, J.; Carroll, M. R. & Brewer, T. S. (2000). "Remobilization of andesite magma by intrusion of mafic magma at the Soufriere Hills Volcano, Montserrat, West Indies". Journal of Petrology. 41 (1): 21–42. doi:10.1093/petrology/41.1.21.
  2. ^ Smith, George I. (1964). Geology and Volcanic Petrology of the Lava Mountains, San Bernardino County, California. United States Geological Survey professional paper 457. Washington, D.C.: United States Geological Survey. p. 39. OCLC 3598916.
  3. ^ Gill, Robin (2011). Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. p. 34. ISBN 978-1-4443-3065-6.
  4. ^ Some use a 1% boundary condition, Sen, Bibhas; Sabale, A. B. & Sukumaran, P. V. (2012). "Lava channel of Khedrai Dam, northeast of Nasik in western Deccan Volcanic province: Detailed morphology and evidences of channel reactivation". Journal of the Geological Society of India. 80 (3): 314–328. doi:10.1007/s12594-012-0150-8. and Ocean Drilling Program, Texas A & M University (1991). Proceedings of the Ocean Drilling Program. Part A, Initial report. 140. National Science Foundation (U.S.). p. 52., while others suggest a limit of 5%. Piccirillo, E. M. & Melford, A. J. (1988). The Mesozoic Flood Volcanism of the Paraná Basin: Petrogenetic and Geophysical Aspects. São Paulo, Brazil: Universidade de São Paulo, Instituto Astronômico e Geofísico. p. 49. ISBN 978-85-85047-04-7. and Moulton, B. J. A.; et al. (2008). Volcanology of the Felsic Volcanic Rocks of the Kidd-Munro assemblage in Prosser and Muro Townships and Premininary Correlations with the Kidd Creek Deposit, Abitibi Greenstone Belt, Ontario. Geological Survey of Canada, Current Research, No. 2008-18. Ottawa: Geological Survey of Canada. p. 19. ISBN 978-1-100-10649-6.
  5. ^ Gill, Robin (2011). Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. p. 21. ISBN 978-1-4443-3065-6.
  6. ^ Byerly, Gary R. & Wright, Thomas L. (1978). "Origin of major element chemical trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge". Journal of Volcanology and Geothermal Research. 3 (3): 229–279. doi:10.1016/0377-0273(78)90038-0.
  7. ^ Gangopadhyay, A. M. I. T. A. V. A.; Sen, Gautam & Keshav, Shantanu (2003). "Experimental Crystallization of Deccan Basalts at Low Pressure: Effect of Contamination on Phase Equilibrium" (PDF). Indian Journal of Geology. 75 (1/4): 54.
  8. ^ Williams, Howel; Turner, Francis J. & Gilbert, Charles M. (1954). Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. p. 102–103. ISBN 978-0-7167-0206-1.
  9. ^ Cox, S. F. & Etheridge, M. A. (1983). "Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures". Tectonophysics. 92 (1): 147–170. doi:10.1016/0040-1951(83)90088-4.

References

  • Best, Myron (2002). Igneous and Metamorphic Petrology (second ed.). Oxford, England: Blackwell Publishing. ISBN 978-1-4051-0588-0.
  • Williams, Howel; Turner, Francis J. & Gilbert, Charlse M. (1954). Petrography: An introduction to the study of rocks in thin sections. San Francisco: W. H. Freeman. ISBN 978-0-7167-0206-1.
  • The Integrated Ocean Drilling Program (IODP). (2001) Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports.[1]
Amygdule

Amygdules or amygdales form when the gas bubbles or vesicles in volcanic lava (or other extrusive igneous rocks) are infilled with a secondary mineral such as calcite, quartz, chlorite or one of the zeolites. Amygdules usually form after the rock has been emplaced, and are often associated with low-temperature alteration. Amygdules may often be concentrically zoned. Rocks containing amygdules can be described as amygdaloidal.

The word is derived from the Latin word "amygdala" for almond tree and the Greek word "αμυγδαλή" for almond, reflecting the typical shape of an infilled vesicle.

"Amygdule" is more common in US usage, while "amygdale" is more common in British usage.

Andesite

For the extinct cephalopod genus, see Andesites.

Andesite ( or ) 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 rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts they are a major component of the Martian crust. The name andesite is derived from the Andes mountain range.

Basalt

Basalt (US: , UK: ) 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.

Biotite

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2. It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalysed dark micas in the field. Biotite was named by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica.Members of the biotite group are sheet silicates. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" (muscovite) – both form in the same rocks, and in some instances side-by-side.

Clinopyroxene thermobarometry

In petrology, the mineral clinopyroxene is used for temperature and pressure calculations of the magma that produced igneous rock containing this mineral. Clinopyroxene thermobarometry is one of several geothermobarometers. Two things makes this method especially useful: first, clinopyroxene is a common phenocryst in igneous rocks and easy to identify; second, the crystallization of the jadeite component of clinopyroxene implies a growth in molar volume being thus a good indicator of pressure.

The minerals and liquids involved in clinopyroxene thermobarometry are:

Augite – (Ca,Mg,Fe)SiO3

Diopside – MgCaSi2O6

Hedenbergite – CaFeSi2O6

Jadeite – Na(Al,Fe3+)Si2O6

Corral de Coquena

Corral de Coquena is a volcanic spatter rampart in the Andes, over the Tropic of Capricorn. The rampart at its highest point is 4,572 metres (15,000 ft) high.The structure has a width of 2.5 by 2.7 kilometres (1.6 mi × 1.7 mi) and is a discontinuous rampart 30–210 metres (98–689 ft) high. This rampart surrounds a sediment-filled crater that is 30–80 metres (98–262 ft) deep beneath the surrounding terrain. The deepest point is 4,363 metres (14,314 ft) high above sea level. The rampart is formed from two main bodies each up to 800 metres (2,600 ft) wide with gaps separating them. It is located in the southeastern portion of the moat which surrounds the 4 mya 60 by 35 kilometres (37 mi × 22 mi) La Pacana caldera.Coquena formed along an outer ring fault. The caldera wall reaches its highest height in the area of Corral de Coquena. The basement beneath Corral de Coquena is slightly higher than the general Pacana caldera floor. The ring belongs to the post-caldera activity phase of La Pacana. The dating is uncertain; the ring is constructed on top of the Pampa Chamaca ignimbrite (2.4 mya) but one date obtained from the ring is 4.4 ± 0.3 mya, obtained on biotite. The date of the ring is more likely to be incorrect. Other estimates indicate that Pampa Chamaca overlies the Corral de Coquena deposits, an as yet unsettled question. The Atana ignimbrite that clearly pre-dates the Coquena ring has been reassessed as being 3.9-4.2 ± 0.1-0.2 mya old, reducing the estimated age of Corral de Coquena as well.The rampart is formed by glassy rhyolite, or dacite, typical of the potassium-rich calc-alkaline series of the Central Volcanic Zone. It has a phenocryst content of 20%.Dacite clasts cover the inward-sloping walls of the rampart. Outwards, lobes and terraces are found possibly formed by agglutinating dacite forming lava flows. The rhyolites too show evidence of flow structures and bedding.In the Pliocene, destruction of a lava dome resulted in the formation of a pyroclastic deposit around Corral de Coquena. These deposits consists of volcanic ash, pumice and rhyolite, forming layers with angular pumice and ash and an abovelying layer of vitric rhyolite, similar to the rampart wall. The deposits are up to 10 metres (33 ft) thick and cover a surface of c. 50 square kilometres (19 sq mi). Total volume is less than 1 cubic kilometre (0.24 cu mi). A later layer of reworked Atana and Corral de Coquena pyroclastics extends 2–3 kilometres (1.2–1.9 mi) away.Aside from the lower phenocryst content, this lava is very similar to Morro Negro, another Pacana lava dome. Ilmenite, magnetite and quartz are found in the rhyolite. A water content of 3-4% and temperatures of 800 ± 50 °C (1,472 ± 90 °F) have been estimated on the basis of composition. The magma that formed Corral de Coquena is probably related to magmas that formed the Atana ignimbrite and were not erupted during that activity phase. Subsequently, part of these leaked out and formed Corral de Coquena.The appearance of Corral de Coquena is similar to a maar. Despite the arid climate in the area which has persisted since the Miocene, the local water table (150 metres (490 ft) beneath the ground on the basis of water levels in nearby lakes) may have been high enough to trigger phreatomagmatic activity. This activity formed the crater. Later, lava itself erupted in the form of lava fountaining. Spatters formed by the fountaining then formed the Corral de Coquena rampart. This is an unusual mode of activity for silicic magmas but also documented at Huaynaputina and the Cerro Chascon-Runtu Jarita complex.

Felsic

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Felsic magma or lava is higher in viscosity than mafic magma/lava.

Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).

Fish Canyon Tuff

The Fish Canyon Tuff is the large volcanic ash flow deposit resulting from one of the largest known explosive eruptions on Earth, estimated at 1,200 cu mi (5,000 km3). (see List of largest volcanic eruptions) The eruption was centered at La Garita Caldera in southwest Colorado. The tuff can be assumed to belong to one eruption due to its high chemical consistency (SiO2=bulk 67.5–68.5% (dacite), matrix 75–76% (rhyolite) and consistent phenocryst content (35–50%) and composition (plagioclase, sanidine, quartz, biotite, hornblende, sphene, apatite, zircon, Fe-Ti oxides are the primary phenocrysts). This tuff and eruption is part of the larger San Juan volcanic field and Mid-Tertiary ignimbrite flare-up.

Geology of São Tomé and Príncipe

São Tomé and Príncipe both formed within the past 30 million years due to volcanic activity in deep water along the Cameroon line. Long-running interactions with seawater and different eruption periods have generated a wide variety of different igneous and volcanic rocks on the islands with complex mineral assemblages.

Iddingsite

Iddingsite is a microcrystalline rock that is derived from alteration of olivine. It is usually studied as a mineral, and consists of a mixture of remnant olivine, clay minerals, iron oxides and ferrihydrites. Debates over iddingsite's non-definite crystal structure caused it to be de-listed as an official mineral by the IMA; thus it is properly referred to as a rock.

Iddingsite forms from the weathering of basalt in the presence of liquid water and can be described as a phenocryst, i.e. it has megascopically visible crystals in a fine-grained groundmass of a porphyritic rock. It is a pseudomorph that has a composition that is constantly transforming from the original olivine, passing through many stages of structural and chemical change to create a fully altered iddingsite.

Because iddingsite is constantly transforming it does not have a definite structure or a definite chemical composition. The chemical formula for iddingsite has been approximated as MgO * Fe2O3 * 3Si2O2 * 4 H2O where MgO can be substituted by CaO. The geologic occurrence of iddingsite is limited to extrusive or subvolcanic rocks that are formed by injection of magma near the surface. It is absent from deep-seated rocks and is found on meteorites. As it has been found on Martian meteorites, its ages have been calculated to obtain absolute ages when liquid water was at or near the surface of Mars.

It was named after Joseph P. Iddings, an American petrologist.

Igneous petrology

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks utilizes a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Igneous rock

Igneous rock (derived from the Latin word ignis meaning fire), 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.

Megacryst

In geology, a megacryst is a crystal or grain that is considerably larger than the encircling matrix. They are found in igneous and metamorphic rocks. Megacrysts can be further classified based on the nature of their origin, either as:

Phenocrysts, which crystallize in molten rock material (lava or magma) and are hence an earlier crystallization than the matrix in which they are embedded

Porphyroblasts, which develop in solid rock as the result of metamorphism or metasomatism

Phlogopite

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.

Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O10(F,OH)2. Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content. For physical and optical identification, it shares most of the characteristic properties of biotite.

Porphyroblast

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

The most common porphyroblasts in metapelites (metamorphosed mudstones and siltstones) are garnets and staurolites, which stand out in well-foliated metapelites (such as schists) against the platy mica matrix.

A similar type of crystal is a phenocryst, a large crystal in an igneous rock. Porphyroblasts are often confused with porphyroclasts, which can also be large outstanding crystals, but which are older than the matrix of the rock.

If a porphyroblastic mineral has small inclusions of minerals within it, the mineral is described as poikiloblastic. This observation can help interpret deformation history.

A rock which has many porphyroblasts is described as having a porphyroblastic texture.

As porphyroblasts grow, the foliation may be preserved as oriented inclusions trapped by the porphyroblast as it overgrows them, and this is helpful for tracking changing deformation planes.

In metamorphic rocks that experience deformation during metamorphism, porphyroblasts may grow before, during, or after the phase of deformation recorded by the matrix minerals. The relationship of porphyroblast growth to deformation is typically evaluated by comparing the shape orientation of trails of mineral inclusions in the porphyroblast to the matrix fabric.

Some garnet porphyroblasts contain curving trails of quartz and other mineral inclusions that record rotation of the crystals relative to their surroundings. However, the question of how much porphyroblasts actually rotate in an external reference frame fixed to the Earth's surface during metamorphism and deformation has long been the subject of debate. The question focused on so-called "spiral garnets", also known as "snowball garnets", whose inclusion trails define spiral patterns. These microstructures are interpreted classically as having formed by shearing induced rotation of a growing garnet crystal. Later research, however, led to an alternative formation model in which a porphyroblast grows over a developing microfold while maintaining a stable position in the external reference frame. Repetition of this process can then produce complex spiral-shaped patterns. Although many researchers continue to adopt the classic rotational model, most reseachers who have published research testing both models by measuring the orientations of porphyroblasts have come to support the modern interpretation.

Porphyry

Porphyry (; Greek: Πορφύριος, Porphyrios "purple-clad") may refer to:

Porphyry (geology), an igneous rock with large crystals in a fine-grained matrix, or associated mineral deposit

Porphyritic, the general igneous texture of a rock with two distinct crystal (phenocryst) sizes

Porphyry copper deposit, a primary (low grade) ore deposit of copper, consisting of porphyry rocks

Porphyry Island in Lake Superior

Porphyry (philosopher) (234–305), Neoplatonic philosopher

Porphyry of Gaza (or "St. Porphyry of Gaza", 347–420), Bishop of Gaza

Porphyry, a system of astrological house division

Porphyry, a vineyard near Seaham, New South Wales

Quartz latite

A quartz latite is a volcanic rock or fine grained intrusive rock equivalent to a latite with a phenocryst modal composition containing 5-20% quartz. Above 20% quartz, the rock would be classified as a rhyolite. It is the fine grained equivalent of a quartz monzonite containing approximately equal amounts of plagioclase and alkali feldspar.

Quimsachata (Canchis)

Quimsachata (possibly from Aymara and Quechua kimsa three, Pukina chata mountain) is an extinct volcano in the Andes of Peru. It is located in the Cusco Region, Canchis Province at about 24 kilometres (15 mi) northwest of the town of Sicuani. This volcano is constructed from two separate centres, one active 11,500 years ago which formed a scoria cone and a lava field and another active 4450 BCE which formed two lava flows and a lava dome.

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

Textures are penetrative fabrics of rocks; they occur throughout the entirety of the rock mass on a microscopic, hand specimen and often on an outcrop scale. This is similar in many ways to foliations, except a texture does not necessarily carry structural information in terms of deformation events and orientation information. Structures occur on hand-specimen scale and above.

Microstructure analysis describes the textural features of the rock, and can provide information on the conditions of formation, petrogenesis, and subsequent deformation, folding or alteration events.

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