Chempet—Calculation for the chemical systematics of igneous rocks based on the CIPW norm

Abstract

CHEMPET is a Pascal program based on the CIPW norm calculation for the classification of igneous rocks. It is suggested that the sequence of desilication steps of the CIPW norm is an approximation for the activity of silica in the rock-forming environment. The main chemical features of rocks can be plotted on the “silica saturation–index of alumina-alkalinity” diagram using the results obtained from the CHEMPET calculations. The program also divides rocks into two series — the Ca- and Fe–Mg ones which differ in their crystallization schemes and metallogeny. The proposed routine gives the same results for major norms as the conventional CIPW norm algorithm which prescribes the successive desilication of normative minerals. The new algorithm is simpler and follows the natural sequence of magmatic crystallization more closely. In addition to the main conventional chemical systematics of rocks (SiO 2 versus (Na 2O+K 2O)) the proposed method enables a rapid assessment of the rock chemistry using the concept of “saturation”. This, in turn makes it possible to mark essential chemical differences between igneous rocks which result in gaps in their CIPW–normative mineral compositions.