phenocryst

[fee-nuh-krist, fen-uh-]
noun Petrology.
any of the conspicuous crystals in a porphyritic rock.

Origin:
1890–95; pheno- + cryst(al)

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World English Dictionary
phenocryst (ˈfiːnəˌkrɪst, ˈfɛn-)
 
n
any of several large crystals that are embedded in a mass of smaller crystals in igneous rocks such as porphyry
 
[C19: from pheno- (shining) + crystal]

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American Heritage
Science Dictionary
phenocryst   (fē'nə-krĭst')  Pronunciation Key 
A large crystal that is surrounded by a finer-grained matrix in an igneous rock. Phenocrysts are usually the first crystals to form from a cooling magma, and therefore have sufficient room to grow to a large size. They are analogous to porphyroblasts in metamorphic rock.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Phenocryst poor, with common olivine and rare plagioclase.
Features such as phenocryst abundance or remanent magnetization provided additional criteria for distinguishing volcanic units.
The pumice component includes coarsely vesiculated, phenocryst-rich and finely vesiculated, phenocryst-poor types.
It is an essential phenocryst in some varieties of lamprophyre.
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