Igneous rocks may be simply classified according to their chemical/mineral composition as felsic, intermediate, mafic, and ultramafic, and by texture or grain size:
Is igneous fine grained?
Extrusive igneous rocks have a fine-grained or aphanitic texture, in which the grains are too small to see with the unaided eye. The fine-grained texture indicates the quickly cooling lava did not have time to grow large crystals. These tiny crystals can be viewed under a petrographic microscope [1].
Why are igneous rocks fine grained?
How Extrusive Igneous Rocks are Formed: While molten, they flow out onto the surface or flow into cracks near the Earth's surface. They cool quickly with no time for large crystals to grow. They are, thus, very fine grained.
