Applying the criteria above for identifying silica undersaturated rocks would tell us that this is a silica-undersaturated rock, which we know to be correct. Next, let's look at a silica oversaturated composition, such as one just to the left of the point labeled 'P' in the diagram. If this liquid is cooled to the liquidus and olivine is allowed to crystallize, and is then quenched on the surface, it will contain phenocrysts of Fo-rich olivine in a glassy groundmass. Applying the criteria above would suggest that this rock is also silica undersaturated, but we know it is not. After silica, alumina is the second most abundant oxide constituent in igneous rocks.
Feldspars are, in general, the most abundant minerals that occur in igneous rocks.
Thus, the concept of alumina saturation is based on whether or not there is an excess or lack of Al to make up the feldspars. If there is an excess of Alumina over that required to form feldspars, we say that the rock is peraluminous . These are the more common types of igneous rocks. They are characterized by lack of an Al 2 O 3 -rich mineral and lack of sodic pyroxenes and amphiboles in the mode. Na 2 SiO 3 will occur as normative minerals.