GeoClassroom Physical Geology Historical Geology Structure Lab Mineralogy Petrology

Alumina Saturation

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.

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