Feldspars are the abundant minerals that occurs in igneous rocks, and alumina is one of the most important element in the feldspars. Thus alumina saturation is based on weather there is an excess or lack of Al to make up the feldspars –
- Orthoclase = 1/2 K2O, 1/2 Al2O3
- Albite = 1/2 Na2O, 1/2 Al2O3
- Anorthite = 1CaO, 1Al2O3
Based on the relative amount of K2O, Na2O, CaO and Al2O3 the rocks can be divided into three groups –
- Peraluminous rocks :- These rocks have more Al2O3 than combined K2O, Na2O, CaO.
- The alumina rich minerals will be present like Muscovite, Corundom, Topaz, Kyanite, Sillimanite, Andalusite.
- They will have corundom in NORM and no diopside.
2. Meta-luminous rock :- These rocks have Al2O3 < (K2O+Na2O+CaO) but Al2O3 > (K2O+Na2O).
- These are most common type of igneous rocks.
- There will be lack of an Al2O3 rich mineral and lack of Na-pyroxene.
3. Peralkaline rocks :- These rocks have Al2O3 < (Na2O +K2O).
- There will be Na-rich minerals like aegerine, riebeckite, aenigmatite etc.
REFERENCES :-
- google images
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