QAPF diagram

QAPF diagrams are not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).

Because F and Q groups cannot simultaneously form in plutonic rocks—due to the difference in their respective silica contents—the QAPF diagram is drawn as two mutually exclusive triangle plots, i.e., QAP and FAP.

To use this classification method, the concentrations (the modes) of the four mineral groups must be determined or estimated, and then normalized to 100%.

This diagram makes no distinction between rock types at the same QAPF plot position and classification, but of different bulk chemical compositions with respect to other minerals such as olivine, pyroxenes, amphiboles or micas.

For example, because non-Q, -A, -P and -F minerals are disregarded the system does not distinguish between gabbro, diorite, and anorthosite.