Phenocryst

Classification by phenocryst

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 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. 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 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.

Notes

References

• 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

References

1. (2000). "Remobilization of andesite magma by intrusion of mafic magma at the Soufriere Hills Volcano, Montserrat, West Indies". Journal of Petrology.
2. Smith, George I.. (1964). "Geology and Volcanic Petrology of the Lava Mountains, San Bernardino County, California". United States Geological Survey.
3. Gill, Robin. (2011). "Igneous Rocks and Processes: A Practical Guide". Wiley.
4. Ocean Drilling Program, Texas A & M University. (2012). "Lava channel of Khedrai Dam, northeast of Nasik in western Deccan Volcanic province: Detailed morphology and evidences of channel reactivation". National Science Foundation (U.S.).
5. Gill, Robin. (2011). "Igneous Rocks and Processes: A Practical Guide". Wiley.
6. (1978). "Origin of major element chemical trends in DSDP Leg 37 basalts, Mid-Atlantic Ridge". Journal of Volcanology and Geothermal Research.
7. (2003). "Experimental Crystallization of Deccan Basalts at Low Pressure: Effect of Contamination on Phase Equilibrium". Indian Journal of Geology.
8. (1954). "Petrography: An introduction to the study of rocks in thin sections". W. H. Freeman.
9. "Crystal zoning." Oxford Reference. Accessed 8 Aug. 2020. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095651756.