groundmass
/ˈɡraʊndˌmæs/
noun
- The fine-grained material in which larger crystals or particles are embedded in a rock.
- The large feldspar crystals stood out against the fine-grained groundmass.
- Geologists study the groundmass to learn about the cooling history of magma.
- Under the microscope, the groundmass of the volcanic rock appeared dark and glassy.