Igneous rock classification is based mostly on the two factors discussed above: chemical composition and crystal size. In simple practice, we divide composition into three categories: rich in iron, magnesium, and calcium (also called mafic or basic); rich in potassium, aluminum and silicon (also called felsic, acidic, or silica); and intermediate. 
 

Crystal size also falls into three simple categories: large enough to be seen with the naked eye (phaneritic); too small to be seen with the naked eye, but still present (aphanitic); and non-crystalline (glassy). Note that the glassy category is not broken down on the basis of mineral composition, because there are no crystals, and therefore there are no minerals. When there are two distinct crystal sizes in the same rock the texture is porphyritic. 
The last category, fragmental, is one of convenience, allowing us to classify igneous rocks formed from volcanic ash and other materials ejected from volcanos (without reference to their composition or texture). 
 
 

All igneous rocks do not occur in the same proportions or distributions. In fact, two of them, granite and basalt, make up some 90% of all igneous rocks. If the material is going to leak to the surface, it will generally be early on before the magma chamber has time to seal itself off by cooling. If the material leaks out early, it will not have time to undergo differentiation and will be mafic in composition. Because it cools rapidly at the surface, it does not have time to form large crystals; therefore most of the extrusive rocks formed by volcanic activity are basalts. Conversely, if the magma cools below the surface, it will do so over long periods of time, and will undergo differentiation and have large crystals. Therefore, most of the intrusive rocks we see exposed after erosion are granites.