QAPF

QAPF is a mineralogical classification framework used in petrology to categorize felsic igneous rocks based on their modal abundances of four mineral groups: quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F). To apply the scheme, geologists determine the rock’s percentage by volume of these minerals—typically by petrographic analysis of a thin section—and plot the results on a QAPF diagram. The resulting position places the rock into established lithologic fields such as granite, granodiorite, tonalite, diorite, quartz diorite, and related varieties. The diagram also accounts for feldspathoid-bearing rocks (where F is significant), which require a Q-A-P-F representation rather than the simpler Q-A-P approach.

The QAPF diagram was developed in the 1970s within the broader framework of the IUGS (International Union of Geological Sciences) igneous rock classification, with Streckeisen’s work in rock classification providing the foundation. It remains a standard tool for differentiating felsic to intermediate igneous rocks in both plutonic and volcanic contexts. The exact boundaries between rock types are defined in reference volumes and classifications maintained by IUGS and its successors.

Practical use of QAPF depends on reliable modal analyses; alterations, metamorphism, or cumulate textures can complicate plotting and interpretation. The method is most effective for silica-rich rocks and may be less applicable to mafic or highly altered samples. Despite limitations, QAPF provides a concise, widely adopted framework for reporting and comparing felsic igneous rock compositions across studies.