Aphanite is a field classification for any dense, fine-grained igneous rock where the mineral constituents are microscopic. It forms when magma cools rapidly at or near the surface, preventing large crystals from growing. Basalt, andesite, and rhyolite are typical aphanites.
Heard on the job
The core sample is mostly a dark aphanite, so we'll need thin sections to figure out its exact mineralogy.