An aphanitic texture indicates that an igneous rock cooled too quickly for macroscopic crystals to form, typical of volcanic extrusions or shallow intrusions. When geologists describe a rock as aphanitic, it means a hand lens or microscope is required to identify its mineral components.
Heard on the job
The dike has an aphanitic margin where it chilled against the cold country rock.
Do not mix it up with
- Phaneritic: Aphanitic rocks have invisible grains, whereas phaneritic rocks have large, easily visible crystals.