Phenocryst
A phenocryst is an early forming, relatively large and usually conspicuous
Phenocrysts are more often found in the lighter (higher silica) igneous rocks such as
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,[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 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.[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
In shallow
See also
Notes
- .
- OCLC 3598916.
- ISBN 978-1-4443-3065-6.
- ISBN 978-1-100-10649-6.
- ISBN 978-1-4443-3065-6.
- .
- ^ 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.
- ISBN 978-0-7167-0206-1.
- ^ "Crystal zoning." Oxford Reference. Accessed 8 Aug. 2020. https://www.oxfordreference.com/view/10.1093/oi/authority.20110803095651756.
- ^ 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. .
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]