In the QAP diagrams of, for example, Streckeisen, the different types of magmatic rocks are dsicriminated according to their modeal proportions of quartz, K- feldspar and alkali feldspar. This is certainly a simplification of the matter since all types of rocks also contain various proportions (more than some percentages) of other non-accessory components, such as micas, amphiboles, pyroxenes, garnets, or even olivine. Our question is whether there are literature compilations, in which the natural variability of modal compositions is addressed for those types of magmatic rocks discriminated by Streckeisen. Most importantly, which components other than QAP occur in the individual types of rock and in which proportions.
Dear Dr. Förster, may I recommend the schematic representation of the relationships between the mineral content, the SiO2 content and the well-known classification into acidic, intermediate, basic and ultra-basic magmatites (on page 37)
by
W. Maresch, H.-P. Schertl, O. Medenbach (2016): "Rocks - Systematic, Determination, Origin" 3rd corrected and expanded edition, Schweizerbarth - Stuttgart 2016, Best regards, Guenter Grundmann
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