differentiation index The sum of the normative constituents (see CIPW NORM CALCULATION) Q + Ab + Or + Ne + Kp + Lc in an igneous rock, where Q = quartz, Ab = albite, Or = orthoclase, Ne = nepheline, Kp = kaliophilite, and Lc = leucite. The index, defined in 1960 by two American petrologists, Thornton and Tuttle, seeks to quantify the degree of differentiation a rock has undergone. The greater the degree of differentiation, the more enriched the rock is in felsic minerals and hence the higher the differentiation index.
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