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 0.5 mm (0.020 in) 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.
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 phenocrystedit
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, or more commonly where the rock consists of less than 1% phenocrysts (by volume); 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. Similarly, a basalt with olivine as the dominant phenocrysts, but with lesser amounts of plagioclase phenocrysts, might be termed an 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. Categorizing a rock as aphyric or as sparsely phyric is often a question of whether a significant number of crystals exceed the minimum size.
Analysis using phenocrystsedit
Geologists use phenocrysts to help determine rock origins and transformations because crystal formation partly depends on pressure and temperature.
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. 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.
In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassymatrix. 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.
^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.
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^Gill, Robin (2011). Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. p. 34. ISBN 978-1-4443-3065-6.
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^Gill, Robin (2011). Igneous Rocks and Processes: A Practical Guide. Hoboken, New Jersey: Wiley. p. 21. ISBN 978-1-4443-3065-6.
^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 (3–4): 229–279. Bibcode:1978JVGR....3..229B. doi:10.1016/0377-0273(78)90038-0.
^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.
^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.
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