noun[in singular] Geology
The compact, finer-grained material in which the crystals are embedded in a porphyritic rock.
- Orthoclase occurred as remnant phenocrysts in a groundmass of chloritic material resulting from alteration of iron-rich felsite cobbles.
- The maximum length of most microbes in Group II could not be measured accurately because they are tightly embedded in the silica groundmass.
- The wall rock was highly altered and consisted of a groundmass of pyrophyllite and fine-grained pyrite.
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