Mesonorms of granitic rock analyses

SUMMARY. A modification to the CIPW norm, to account for the presence of biotite or muscovite or both in granite rocks, is described. The modification is most significant in norms that have been reduced to three components and plotted on experimental phase diagrams, such as the systems Q-Or-Ab and Or-Ab-An. SINCE Cross, Iddings, Pirsson, and Washington 09o3) first published their system of deriving normative minerals from chemical analyses, the CIPW norm has been success-fully used to identify, compare, and classify trends in igneous rocks with similar or dif-fering mineral compositions. The norm has been used for metamorphic rocks and rocks of somewhat uncertain mode of origin (Bowes, ~967). Various minor modifications to the CIPW norm have been advocated (Eskola, I954; Barth, I959; Kelsey, I965), and the further modification presented here is considered esirable when chemical data for granitic rocks are to be compared graphically with phase diagrams. This paper repre-sents an expansion, into the igneous field, of Barth's (I96Z) mesonorm calculation. The basis of his mesonorm calculation, unfortunately a rather neglected technique, is that certain metamorphic rocks require mica and amphibole representation in normativ