groundmass

groundmass — noun

1. in a porphyritic rock, the small-grained or glass-like material surrounding the

1.nounC2
Definition

in a porphyritic rock, the small-grained or glass-like material surrounding the larger crystals you can see with the naked eye.

Example

Under the microscope, Samir saw bright crystals of quartz set in a dark groundmass.

collocation: set in a (dark/fine) groundmass

The groundmass of this lava sample is so fine that it looks almost like glass.

pattern: the groundmass of [rock sample] is [texture]

Common collocations
  • fine-grained groundmass
  • glassy groundmass
  • dark groundmass
  • set in a groundmass
  • the groundmass of the rock
Synonyms
  • matrix

    broader term used across petrology and sedimentology; groundmass is reserved for the fine material around phenocrysts.

  • mesostasis

    the very last material to crystallise between earlier crystals; a narrower technical subset of the groundmass.

Antonyms
  • phenocryst

    the large visible crystals embedded WITHIN the groundmass — the contrasting component, not the same field of view.

Grammar Patterns

the groundmass of [rock type]

Usage Note

Almost always uncountable and used in petrology or igneous-rock contexts. Subject of the surrounding sentence is usually a rock sample, thin section, or specimen rather than a person.

Common Mistakes

The big crystals are inside a groundmasses of glass.
The big crystals are inside a groundmass of glass.
groundmass is uncountable; do not add -es.

Synonym discussion

Sense: noun/1

In petrology, groundmass, matrix, and mesostasis describe related but distinct parts of a rock. Groundmass is the fine-grained or glassy material that surrounds the larger crystals (phenocrysts) in a porphyritic igneous rock — it always implies a contrast with bigger crystals you can see. Matrix is the wider term: petrologists use it for the fine material around larger grains in any rock, including sedimentary ones, and many writers use matrix and groundmass interchangeably for igneous rocks. Mesostasis is narrower still: it names the very last bit of melt that crystallises between earlier formed crystals, often glassy or rich in unusual minerals. Phenocryst is the opposite component — the large visible crystals sitting INSIDE the groundmass. So you might write: large feldspar phenocrysts sit in a glassy groundmass; the matrix of this sandstone is mostly clay; the mesostasis between the olivine crystals is dark and glassy.

matrix ↗mesostasisphenocryst ↗

Common collocations

Nouns

  • groundmass texture
  • groundmass crystals
  • groundmass composition

Phrases

  • set in a fine-grained groundmass
  • the groundmass of the rock

Prepositions

  • in a groundmass
  • of the groundmass

Etymology

GermanEnglish

A 19th-century compound formed in English from 'ground' (in the older sense of 'base' or 'background') and 'mass' (a body of material), modeled on the German petrology term 'Grundmasse'. The word entered English petrology vocabulary as geologists studied porphyritic rocks under the microscope and needed a name for the fine background material around the visible crystals.