For this example the rock can be classified as a Monzogranite.įor example: a plutonic rock that contains no alkali feldspar and no feldspathoids, but contains many pyroxenes (unlabeled in QAPF diagram), plagioclase-feldspar, and few quartz grains is probably gabbro (located at the right edge of the diagram, near P). The rock can now be plotted on the diagram by finding a horizontal line representing 37.5% quartz and then plotting a point on it 60% of the way across from the A side to the P side. Of these, the (again) normalised relative proportions of A and P are 37.5/62.5 = 60% and 25/62.5 = 40%. To use this classification method, the concentration (the mode) of the 4 minerals must be determined (or estimated) and normalized to 100%.įor a rock identified as having, say, 20% mica, 30% quartz (Q), 30% alkali feldspar (A) and 20% plagioclase (P), the mica is disregarded and the normalised ratios of Q, A and P are therefore 37.5%, 37.5% and 25% = 100%. The QAPF diagram is composed of two quasi-ternary plots (QAP and FAP) joined along one side.
Other minerals may occur in samples, but they are disregarded in this classification method. F and Q cannot form in plutonic rocks simultaneously due to the difference in their respective silica contents. 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)". A QAPF diagram is a double quasi- ternary diagram which is used to classify igneous rocks based on mineralogic composition.
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