Source : Webster's Revised Unabridged Dictionary (1913)

Microcrystalline \Mi`cro*crys"tal*line\, a. [Micro- +
   crystalline.] (Crystallog.)
   Crystalline on a fine, or microscopic, scale; consisting of
   fine crystals; as, the ground mass of certain porphyrics is
   microcrystalline.

Source : WordNet®

microcrystalline
     adj : containing crystals that are visible only under a microscope