A QAPF diagram is a double triangle 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%). Sample flowchart diagram A diagram is a simplified and structured visual representation of concepts, ideas, constructions, relations, statistical data, anatomy etc used in all aspects of human activities to visualize and clarify the topic. ... Volcanic rock on North America Plutonic rock on North America Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Mineralogy is an earth science that involves the chemistry, crystal structure, and physical (including optical) properties of minerals. ... Acronyms and initialisms are abbreviations formed from the initial letter or letters of words, such as NATO and XHTML, and are pronounced in a way that is distinct from the full pronunciation of what the letters stand for. ... Quartz is the most abundant mineral in the Earths crust. ... Plagioclase feldspar Plagioclase feldspars are a very important group of igneous rock forming tectosilicate minerals. ... The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. ... This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band). ...

QAPF diagrams are mostly used to classify plutonic rocks (phaneritic rocks) (Figure 1), but are also used to classify volcanic rocks (Figure 2) 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 detrermined, instead a TAS diagram (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). Pluton redirects here. ... Phaneritic is a term usually used to refer to igneous rock grain size. ... Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. ... Pyroclastic rocks are formed from lavas which are ejected into the air, as occur in pyroclastic flows or Plinian eruptions. ... Obsidian from Lake County, Oregon Top stone is obsidian, below that is pumice and in lower right hand is rhyolite (light color) Obsidian is a type of naturally occurring glass, produced from volcanoes when a fluid felsic lava cools rapidly and freezes without sufficient time for crystal growth, for example... In geology, mafic minerals are silicate minerals, magmas, and volcanic and intrusive igneous rocks that have relatively high concentrations of the heavier elements. ... This article is about minerals in the geologic sense; for nutrient minerals see dietary mineral; for the band see Mineral (band). ... Peridotite Peridotite is a dense, coarse grained ultrabasic rock, consisting mainly of the minerals olivine and pyroxene. ... Pyroxenite is a rock consisting essentially of minerals of the pyroxene group, such as augite and diallage, hypersthene, bronzite or enstatite. ...

An exact name can be given only if the mineralogical composition is known, which cannot be determined in the field. Therefore, provisional or field classifications (Figures 3 and 4) are used, which are simplified versions of Figures 1 and 2.

QAPF diagrams were created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks. The diagrams are accepted by geologists worldwide as a classification of igneous, especially plutonic rocks. The International Union of Geological Sciences (IUGS) was founded in 1961 and is a Scientific Union member of the International Council for Science (ICSU), which it recognizes as the coordinating body for the international organization of science. ...

Figure 1. This classification is used if rock sample has phaneritic texture (plutonic rocks).

Figure 2. This classification is used if rock sample has fine-grained texture (volcanic rocks) but modal mineralogical composition can be determined.

Figure 3. This is provisional classification for phaneritic igneous rocks (field classification).

Figure 4. This is provisional classification for fine-grained igneous rocks (field classification).

References

• 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.