These minerals are quartz (Q), Alkali feldspars (A), plagioclase feldspars (P), and feldspathoids (F). The QAPF diagram uses the proportions of four minerals, or mineral groups. QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).Īn exact name can be given only if the mineralogical composition is known, which cannot be determined in the field. TAS is also used if volcanic rock contains volcanic glass (such as obsidian). 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. QAPF diagrams are mostly used to classify plutonic rocks ( phaneritic rocks), but are also used to classify volcanic rocks ( aphanitic) if modal mineralogical compositions have been determined. Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks. QAPF diagrams were created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks įostered by Albert Streckeisen (whence their alternative name: Streckeisen diagrams). Q, A, P and F percentages are normalized (recalculated so that their sum is 100%). These are the mineral groups used for classification in QAPF diagram.
The acronym QAPF stands for " Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". Classification system for igneous rocks QAPF diagram for classification of plutonic rocks Streckeisen diagramĪ QAPF diagram is a double quasi- ternary diagram which is used to classify igneous rocks based on mineralogic composition.
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