Abstract

The CIPW norm gives results which closely resemble the mode for basalt and many other quartz-saturated igneous rocks, but which depart widely from the mode for alkaline and peralkaline assemblages. A modified norm, deleting halite, thenardite, sodium metasilicate, potassium metasilicate, sodium carbonate, and larnite, and substituting sodalite, enigmatite, eudialyte, riebeckite, andradite, and Ca-tschermaks molecule (CaAl2SiO6), gives a good representation of most alkaline and peralkaline igneous rocks while giving results identical to the CIPW system for common rocks. Estimates of silica saturation, differentiation indices, and disposition of alkalies are substantially improved.

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