phenocryst

phenocryst — noun

1. A crystal that is noticeably larger than the surrounding rock grains, formed whe

1.nounC2
Definition

A crystal that is noticeably larger than the surrounding rock grains, formed when magma cools at different stages — the crystal develops early while the surrounding material is still liquid, then becomes enclosed when the rest hardens.

Example

When Nkechi broke open the dark volcanic rock, several white phenocrysts were visible inside.

visible + phenocryst — common collocation for describing what you can see

Large feldspar phenocrysts stood out clearly in the porphyry rock that Caio and Asher collected.

phenocryst + of + [mineral] — pattern for specifying the crystal type

Common collocations
  • large phenocryst
  • visible phenocryst
  • feldspar phenocryst
  • phenocryst-bearing rock
  • phenocryst of quartz
  • euhedral phenocryst
  • isolated phenocryst
Synonyms
  • embedded crystal

    A general descriptive phrase rather than a technical term; less precise than phenocryst.

  • porphyritic crystal

    Used interchangeably in some geology writing, but porphyritic describes the rock texture, not the crystal itself.

Grammar Patterns

phenocryst + of + [mineral]

[mineral] + phenocryst

Usage Note

Almost always used in technical geology writing or teaching contexts. Because a rock typically contains many such crystals, the plural form phenocrysts is more common than the singular.

Common Mistakes

A porphyry is a rock with many porphyries.
A porphyry is a rock with many phenocrysts.
porphyry is the rock itself, not the crystals inside it.
The phenocryst melted when the rock was heated.
The phenocryst remained solid while the surrounding rock began to melt.
a phenocryst typically has a higher melting point than the groundmass.

Word family

  • phenocrystic
    adjectiveC2

Synonym discussion

Sense: noun/1

Phenocryst, porphyry, and groundmass describe different parts of the same rock. A phenocryst is a single large crystal inside the rock. The groundmass (or matrix) is the fine-grained material that surrounds all the phenocrysts. Porphyry is the name for the whole rock type that has this texture — it is not a synonym for the crystals themselves. Crystal is a much broader term for any solid with a regular atomic structure; a phenocryst is always a crystal, but most crystals are not phenocrysts. Inclusion refers to any solid, liquid, or gas trapped inside a crystal — an inclusion is inside a phenocryst, not the phenocryst itself. For example: 'The porphyry sample contained large feldspar phenocrysts, and under the microscope we could see tiny liquid inclusions inside them.'

porphyrygroundmass ↗crystal ↗inclusion ↗

Common collocations

Nouns

  • phenocryst size
  • phenocryst content
  • phenocryst assemblage
  • phenocryst texture

Phrases

  • large phenocrysts of [mineral]
  • visible phenocrysts in the rock

Prepositions

  • phenocryst of [mineral]
  • phenocryst in [rock]
  • phenocryst within [groundmass]

Etymology

GreekEnglish

Coined in the late 19th century from Greek φαίνω (phainein, 'to show' or 'to appear') and κρύσταλλος (krystallos, 'crystal' or 'ice'). The name describes how these crystals visibly 'show themselves' inside the finer-grained rock that surrounds them.

First known use

1889

1889, in the meaning defined above