Phenocryst

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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.......
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