Independant of the silica saturation, alumina saturation is based on the 1:1 alkali:alumina ratio of feldspars and feldspathoids. Any excess or deficiency in alumina in a rock is reflected in the mineralogy. Four classes of alumina saturation/undersaturation are:

1. Peraluminous - Al2O3 > (CaO + N2O + K2O)

   corundum appears in the norm

   Minerals present in the rock are: muscovite, topaz, tourmaline, spessartine-almandine, corundum, andalusite and sillimanite.

2. Metaluminous - Al2O3 < (CaO + Na2O + K2O) but Al2O3 > (Na2O + K2O).

   anorthite appears in the norm

   Al-bearing minerals are typical, e.g. biotite, hornblende.

3. Subaluminous - Al2O3 = (Na2O + K2O)

   normative anorthite is small

   Feldspars and feldspathoids are the only minerals with essential Al2O3.

4. Peralkaline - Al2O3 < (Na2O + K2O) and rarely Al2O3 < K2O

   Acmite, sodium silicate and rarely potassium silicate appear in the norm.

   Alkali ferromagnesium minerals common, e.g. aegerine, riebeckite, richerite.