Phenocryst

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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. Montblanc granite phenocrysts.JPG
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.

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.

Contents

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.

Classification by phenocryst

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. Photomicrograph-porphyritic-aphanitic-felsic-rock-USGS.jpg
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] This is described as normal zoning if the rim of the crystal shows a lower-temperature composition than the core of the crystal. Reverse zoning describes the more unusual case where the rim shows a higher-temperature composition than the core. Oscillatory zoning shows period fluctuations between low- and high-temperature compositions. [9]

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. [10]

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. 102103. ISBN   978-0-7167-0206-1.
  9. "Crystal zoning." Oxford Reference. Accessed 8 Aug. 2020. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095651756.
  10. 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.

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Basalt A magnesium- and iron-rich extrusive igneous rock

Basalt is a mafic extrusive igneous rock formed from the rapid cooling of lava rich in magnesium and iron 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, and the eruption of basalt lava is observed by geologists at about 20 volcanoes per year.

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. It is composed predominantly of plagioclase feldspar and quartz.

Trachyte

Trachyte is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and 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.

Andesite An intermediate volcanic rock

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

Extrusive rock

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.

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase.

Porphyritic Characteristic texture of igneous rocks containing crystals of contrasting size (large and small crystals)

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.

Volcanic rock

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.

Peridotite A coarse-grained ultramafic igneous rock

Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.

Lamprophyre

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.

Picrite basalt Variety of high-magnesium basalt that is very rich in the mineral olivine

Picrite basalt or picrobasalt is a variety of high-magnesium olivine basalt that is very rich in the mineral olivine. It is dark with yellow-green olivine phenocrysts (20-50%) and black to dark brown pyroxene, mostly augite.

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.

Cumulate rock

Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded 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. The International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to the term "tholeiite".

Shoshonite is a type of igneous rock. More specifically, it is a potassium-rich variety of basaltic trachyandesite, composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored volcanic glass. Shoshonite gives its name to the shoshonite series and grades into absarokite with the loss of plagioclase phenocrysts and into banakite with an increase in sanidine. Shoshonite was named by Iddings in 1895 for the Shoshone River in Wyoming.

Alkali basalt Type of volcanic rock

Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt is characterized by relatively high alkali (Na2O and K2O) content relative to other basalts and by the presence of olivine and titanium-rich augite in its groundmass and phenocrysts, and nepheline in its CIPW norm.

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.

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.

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