phenocryst


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phenocryst

[′fēn·ə‚krist]
(petrology)
A large, conspicuous crystal in a porphyritic rock. Also known as phanerocryst.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Lange, "Phenocryst complexity in andesites and dacites from the Tequila volcanic field, Mexico: resolving the effects of degassing vs.
is a dense porphyritic basalt with about twenty percent of tabular plagioclase and minor augite and olivine phenocrysts. Phenocryst [cores-An.sub.47-60], [microprobe-An.sub.56-59] [groundmass-An.sub.44-63], [microprobe-An.sub.47].
One of these, a medium-grained gabbroid, contains up to 3 cm long phenocrysts of plagioclase.
2A) comprising phenocrysts of plagioclase (30%), amphibole (10%), quartz (15%), K-feldspar (10%), and biotite (3%) in a fine-grained matrix of the same minerals with accessory zircon, apatite, and sphene (Fig.
The irregularity displayed by the phenocryst shapes in quartz syenite is higher than that in both the megaporphyritic and microporphyritic types of the alkali granite but comparable to that in nepheline syenite, which is fine to medium grained.
Francois Mountains Complex regardless of inferred mode or depth of crystallization (volcanic phenocryst versus volcanic matrix versus hypabyssal ring pluton versus subvolcanic massif) or crystallization age (1.47 Ga versus 1.36-1.38 Ga) to be evidence for a single, pervasive alteration event subsequent to (or coincident with) the last major igneous event in the area.
Phenocryst-poor latite and trachyte are interpreted as the result of mineral separation, and phenocryst-rich tephrite porphyry and olivine diabase porphyry are the result of phenocryst accumulations in the magma chamber.
Plagioclase is the major phenocryst, which occupies ~20 vol% of the rock, and it is typically zoned and rimed by K-feldspar.
The phenocryst population can be identified as crystals inherited from one of the parental magmas, but fine grains were inherited from one of the parent magmas or must have formed after the mixing event.
The small compositional differences can largely be explained (and numerically modelled) by slight variations in phenocryst populations; relatively large variations in concentrations in some of the LIL elements (e.g., Rb, Ba, and Sr) seen in some samples are apparently due to alteration and weathering (Table 3, analysis lh).
This interpretation is consistent with petrographic observations that magnetite is a common phenocryst in basaltic calc-alkalic basalts bur is rare in tholeiitic basalts.