In context|geology|lang=en terms the difference between igneous and aphanite

is that igneous is (geology) resulting from, or produced by, the action of great heat; with rocks, it could also mean formed from lava/magma; as, granite and basalt are igneous rocks while aphanite is (geology) certain dark igneous rocks having grain so fine that the individual crystals cannot be seen with the naked eye.

As an adjective igneous

is pertaining to, having the nature of fire; containing fire; resembling fire; as, an igneous appearance.

As a noun aphanite is

(geology) certain dark igneous rocks having grain so fine that the individual crystals cannot be seen with the naked eye.