groundmass

American  

[ground-mas] / ˈgraʊndˌmæs /

noun

1. the crystalline, granular, or glassy base or matrix of a porphyritic or other igneous rock, in which the more prominent crystals are embedded.

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groundmass British  

/ ˈɡraʊndˌmæs /

noun

1. the matrix of igneous rocks, such as porphyry, in which larger crystals (phenocrysts) are embedded

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Etymology

Origin of groundmass

First recorded in 1875–80; ground ¹ + mass

Example Sentences

This sample is mostly fine groundmass with a few small green phenocrysts that are the mineral olivine.

From Textbooks • Jan. 1, 2017

Typically, smaller sediment grains, collectively called groundmass or matrix, fill in much of the volume between the larger clasts, and hold the clasts together.

From Textbooks • Jan. 1, 2017

The large crystals are called phenocrysts and the fine-grained matrix is called the groundmass or matrix.

From Textbooks • Jan. 1, 2017

Pyroxene, quartz and augite form the groundmass, as seen in section. 

From The Long Labrador Trail by Wallace, Dillon

Olivine rarely occurs as an ingredient of the groundmass.

From Encyclopaedia Britannica, 11th Edition, Volume 3, Part 1, Slice 3 "Banks" to "Bassoon" by Various