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Earth and Space Sciences · Glossary

What is Aluminium saturation index?

Also known as: peraluminous · metaluminous · peralkaline

Definition 12.7 University Earth and Space Sciences — Year 2 · Chapter 12 — Crustal Melting: Migmatites and Granites

The aluminium saturation index of a rock is the molar ratio

A/CNK=n(AlX2OX3)n(CaO)+n(NaX2O)+n(KX2O),\mathrm{A/CNK} = \frac{n(\ce{Al2O3})}{n(\ce{CaO}) + n(\ce{Na2O}) + n(\ce{K2O})},

and its companion is A/NK=n(AlX2OX3)/[n(NaX2O)+n(KX2O)]\mathrm{A/NK} = n(\ce{Al2O3})/[n(\ce{Na2O}) + n(\ce{K2O})]. A rock is peraluminous if A/CNK>1\mathrm{A/CNK} > 1, metaluminous if A/CNK<1<A/NK\mathrm{A/CNK} < 1 < \mathrm{A/NK}, and peralkaline if A/NK<1\mathrm{A/NK} < 1.

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