aphanitic

Aphanite (from the Greek αφανης, invisible) is a name given to certain typically dark-coloured igneous rocks which are so fine-grained that their component mineral crystals are not detected by the unaided eye. This texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments.
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Adjective
1. of or relating to aphanite
(pertainym) aphanite

(a.)
Resembling aphanite; having a very fine-grained structure.
  

Of or being an igneous rock containing grains that are so small as to be barely visible to the naked eye.

AFANITICO