aphanite

from WordNet (r) 3.0 (2006)

aphanite
    n 1: fine-grained homogeneous rock (such as basalt) containing
         minerals undetectable by the naked eye

from The Collaborative International Dictionary of English v.0.48

Aphanite \Aph"a*nite\, n. [Gr. ? invisible; 'a priv. + ? to
   appear.] (Min.)
   A very compact, dark-colored ?ock, consisting of hornblende,
   or pyroxene, and feldspar, but neither of them in perceptible
   grains.
   [1913 Webster]