QAPF diagram
Encyclopedia
A QAPF diagram is a double triangle diagram
which is used to classify igneous rock
s 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%).
(IUGS): Subcommission on the Systematics of Igneous Rocks. The diagrams are accepted by geologists worldwide as a classification of igneous, especially plutonic rocks.
(phaneritic
rocks) (Figure 1), but are also used to classify volcanic rock
s (Figure 2) if modal mineralogical compositions have been determined. QAPF diagrams are not used to classify pyroclastic rock
s or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification
(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
mineral
s make up more than 90% of the rock composition (for example: peridotite
s and pyroxenite
s).
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.
Diagram
A diagram is a two-dimensional geometric symbolic representation of information according to some visualization technique. Sometimes, the technique uses a three-dimensional visualization which is then projected onto the two-dimensional surface...
which is used to classify igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s based on mineralogic
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
composition. The acronym, QAPF, stands for "Quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, Alkali feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, Plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, Feldspathoid
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
(Foid)". These are the mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
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 SciencesInternational Union of Geological Sciences
The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...
(IUGS): Subcommission on the Systematics of Igneous Rocks. The diagrams are accepted by geologists worldwide as a classification of igneous, especially plutonic rocks.
Usage
QAPF diagrams are mostly used to classify plutonic rocksIntrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
(phaneritic
Phaneritic
Phaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock are large enough to be distinguished with the unaided eye as opposed to aphanitic . This texture forms by slow cooling of magma deep underground in the plutonic environment...
rocks) (Figure 1), but are also used to classify volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
s (Figure 2) if modal mineralogical compositions have been determined. QAPF diagrams are not used to classify pyroclastic rock
Pyroclastic rock
Pyroclastic rocks or pyroclastics are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed volcaniclastic...
s or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification
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 proportions of alkalis and silica play an important role in...
(Total-Alkali-Silica) is used. TAS is also used if volcanic rock contains volcanic glass (such as obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
). QAPF diagrams are also not used if mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s make up more than 90% of the rock composition (for example: peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
s and pyroxenite
Pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
s).
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.