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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 fewer than 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 phyric is often a question of whether a sufficient number of crystals exceeds ~1 mm in size.
- 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.