A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized (recalculated so that their sum is 100%).
QAPF diagrams were created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks fostered by Albert Streckeisen (whence their alternative name: Streckeisen diagrams). Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks.
QAPF diagrams are mostly used to classify plutonic rocks (phaneritic rocks), but are also used to classify volcanic rocks if modal mineralogical compositions have been determined. QAPF diagrams are not used to classify pyroclastic rocks or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification (Total-Alkali-Silica) is used. TAS is also used if volcanic rock contains volcanic glass (such as obsidian). QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).
An exact name can be given only if the mineralogical composition is known, which cannot be determined in the field.
Reading QAPF diagram
The QAPF diagram utilizes four minerals, or mineral groups, to classify igneous rocks. These minerals are quartz (Q), Alkali feldspars (A), plagioclase feldspars (P), and feldspathoids (F). F and Q cannot form in plutonic rocks simultaneously due to the difference in their respective silica contents. Other minerals may occur in samples, but they are not utilized by this classification method.
The QAPF diagram is composed of two ternary plots (QAP and FAP) joined along one side. To use this classification method, the concentration (the mode) of these minerals must be determined and normalized to 100%. For example: a plutonic rock that contains no alkali feldspar and no feldspathoids, but contains lots of pyroxenes (unlabeled), plagioclase-feldspar, and few quartz grains is probably gabbro (located at the right edge of the diagram, near P). This diagram makes no distinction between rock types of the same chemical composition, but different physical characteristics (such as gabbro, diorite, and anorthosite).
Note that this diagram is not used for all plutonic rocks. Ultramafic rocks are the most important plutonic rocks that have separate classification diagrams.
- Streckeisen, A. L., 1974. Classification and Nomenclature of Plutonic Rocks. Recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks. Geologische Rundschau. Internationale Zeitschrift für Geologie. Stuttgart. Vol.63, p. 773-785.
- Streckeisen, A. L., 1978. IUGS Subcommission on the Systematics of Igneous Rocks. Classification and Nomenclature of Volcanic Rocks, Lamprophyres, Carbonatites and Melilite Rocks. Recommendations and Suggestions. Neues Jahrbuch für Mineralogie, Abhandlungen, Vol. 141, 1-14.
- Le Maitre,R.W. 2002. Igneous Rocks: A Classification and Glossary of Terms : Recommendations of International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press, 236pp.
- Classification of Igneous Rocks - IUGS Classification, Geological Sciences Department - Cal Poly Pomona, archived from the original on 30 Sep 2011
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