Phenocryst

Phenocryst

A phenocryst is a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of a porphyritic igneous rock. Phenocrysts often have euhedral forms either due to early growth within a magma or by post-emplacement recrystallization.

Plagioclase phenocrysts often exhibit zoning with a more calcic core surrounded by progressively more sodic rinds. This zoning is reflective of the changing magma composition as crystallization progresses. 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.

Phenocrysts are often used when the rock name is determined. For example, olivine may form the primary phenocrysts of some materials, and as such is used to define the subtype of that material (e.g., a 'porphyritic olivine basalt'). Phenocrysts are commonly found in materials such as felsite and andesite.

Volcanic rocks classified according to the nature and abundance of phenocryst assemblages are often described as aphyric when 1% phenocrysts are visible with a hand lens. Porphyritic volcanic rocks are further classified by phenocryst type using mineral name modifiers given in the order of decreasing abundance. The term phenocryst is used for a crystal that is significantly larger than the average size of the groundmass crystals; in practice, these are generally >~1 mm. Thus, olivine-plagioclase phyric basalt contains >10% phenocrysts, the dominant phenocryst being olivine, with lesser amounts of plagioclase. The suffix -phyric includes all of the phenocryst phases that occur in the rock, as long as the total content >1%.

Aphyric to sparsely plagioclase-olivine phyric basalts contain 1% prismatic to tabular plagioclase phenocrysts and 1%-5% olivine microphenocrysts. Categorizing the rock as aphyric or as sparsely plagioclase-olivine phyric basalt is often a question of whether a sufficient number of crystals exceeds ~1 mm in size. In general, aphyric rocks contain phenocrysts of plagioclase and microphenocrysts of olivine larger than 1 mm.

A similar metamorphic texture is the porphyroblast.

References

* Best, Myron (2002) "Igneous and Metamorphic Petrology", Blackwell Publishing, 2nd ed., ISBN 1-4051-0588-7
* Williams, Howel; Francis J. Turner and Charlse M. Gilbert (1954) "Petrography", W. H. Freeman
* The Integrated Ocean Drilling Program (IODP). (2001) Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports. [http://www-odp.tamu.edu/publications/187_IR/chap_02/chap_02.htm]


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  • Phenocryst — Phe no*cryst, n. [Gr. ? to appear + ? crystal.] (Geol.) One of the prominent embedded crystals of a porphyry. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • phenocryst — [fē′nə krist] n. [Fr phénocryste < Gr phainein, to show (see FANTASY) + krystallos,CRYSTAL] a relatively large and usually conspicuous crystal found in a fine grained matrix in porphyritic igneous rock …   English World dictionary

  • phenocryst — noun Etymology: French phénocryste, from Greek phainein to show + krystallos crystal more at fancy Date: 1893 one of the prominent embedded crystals of a porphyry • phenocrystic adjective …   New Collegiate Dictionary

  • phenocryst — /fee neuh krist, fen euh /, n. Petrol. any of the conspicuous crystals in a porphyritic rock. [1890 95; PHENO + CRYST(AL)] * * * …   Universalium

  • phenocryst — noun any relatively large crystal embedded in a more fine grained or glassy igneous rock …   Wiktionary

  • phenocryst — ALASKA VOLCANO OBSERVATORY GLOSSARY A conspicuous, usually large, crystal embedded in porphyritic igneous rock …   Glossary of volcanic terms

  • phenocryst — [ fi:nə(ʊ)krɪst, fɛn ] noun Geology a large or conspicuous crystal in a porphyritic rock. Origin C19: from Fr. phénocryste, from Gk phainein to show + krustallos crystal …   English new terms dictionary

  • phenocryst — pheno·cryst …   English syllables

  • phenocryst — phe•no•cryst [[t]ˈfi nə krɪst, ˈfɛn ə [/t]] n. pet any of the conspicuous crystals in a porphyritic rock • Etymology: 1890–95; pheno +cryst ( al) …   From formal English to slang

  • phenocryst — /ˈfinəkrɪst/ (say feenuhkrist), /ˈfɛnə / (say fenuh ) noun any of the relatively large and ordinarily conspicuous crystals in a porphyritic rock. {pheno + cryst(al)} …  

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