Igneous Rocks are classified based upon their

a. Mineralogy and composition

b. Crystal texture and density

c. Crystal size and mineral composition

d. Hardness and Texture

igneous rocks can be "primary" rocks because they crystallize from a liquid. Sedimentary and metamorphic rocks

Igneous rocks may be simply classified according to their chemical/mineral composition as felsic, intermediate, mafic, and ultramafic

and by texture or grain size: intrusive rocks are course grained (all crystals are visible to the naked eye) while extrusive rocks may be fine-grained Volcanic rocks, especially felsic and intermediate, often have a porphyritic texture characterized by visible crystals floating in a fine-grained groundmass.