QAPF diagram

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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%).

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).

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.

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.

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