[in singular] The compact, finer-grained material in which the crystals are embedded in a porphyritic rock.
- ‘The maximum length of most microbes in Group II could not be measured accurately because they are tightly embedded in the silica groundmass.’
- ‘Only in rare examples are there discontinuities in the silica groundmass around the microbes that may denote the outer limit of the silica that originally encrusted the microbe.’
- ‘The wall rock was highly altered and consisted of a groundmass of pyrophyllite and fine-grained pyrite.’
- ‘Orthoclase occurred as remnant phenocrysts in a groundmass of chloritic material resulting from alteration of iron-rich felsite cobbles.’
- ‘In all specimens the groundmass is peppered with opaque minerals, rare apatite laths and rare euhedral blade-like calcite.’