A QAPF diagram is a double ternary diagram which is used to classify

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

s based on mineralogic composition. The

acronym An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...

QAPF stands for "

Quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, Alkali feldspar,

Plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

, Feldspathoid (Foid)". These are the

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized (recalculated so that their sum is 100%).

Origin

QAPF diagrams were created by the International Union of Geological Sciences (IUGS): ''Subcommission on the Systematics of Igneous Rocks'' fostered by

Albert Streckeisen Albert Streckeisen (8 November 1901 – 29 September 1998) was a Swiss petrographer and petrologist, the son of Basel forensic scientist Adolf Streckeisen. Biography He studied geology, mineralogy and petrology in Basel, Zürich and Berne. He su ...

(whence their alternative name: Streckeisen diagrams). Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks.

Usage

QAPF diagrams are mostly used to classify plutonic rocks ( phaneritic rocks), but are also used to classify

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

s 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 The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportio ...

(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

s 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 Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

(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 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). This diagram makes no distinction between rock types of the same chemical composition in QAPF, but different chemical compositions with respect to other minerals (such as gabbro, diorite, and

anorthosite Anorthosite () is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most ...

). The QAPF diagram is not used for all plutonic rocks; ultramafic plutonic rocks are the most important group that have separate classification diagrams.

References

External links

* {{DEFAULTSORT:Qapf Diagram Igneous petrology Igneous rocks Petrology concepts

Origin

Usage

Reading QAPF diagram

References

Further reading

External links