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Definition of qapf diagram

Within the QAPFM classification scheme proposed by the IUGS, rocks with less than 90 vol-% mafic minerals are classified by their content (in volume-%) of:
Q Quartz
A Alkali-Feldspars
P Plagioclase Feldspars
F Foids (abbreviation for Feldspathoids)

The composition of the rocks are plotted into a diamond-shaped coordinate system, the QAPF diagram, also known as Streckeisen diagram (named after the author of the original article). It is made of two ternary diagrams with the corners Q, A, P and F, A, P, adjoined to each other along their A-P edge. The corners represent cases in which only one felsic component is present, effectively 100% of either quartz, alkali-feldspar, plagioclase or foid.

Because foids and quartz are mutually exclusive in an igneous rock, the QAPF classification is always based on a maximum of 3 components, either QAP or APF, and the compositions of the rocks are plotted in either the upper or lower triangle.

The QAPF diagram is divided into 15 basic fields that define ranges of mineral compositions for the different classes of rocks.

There are 2 QAPF diagrams, one for coarse grained (plutonic) or intrusive igneous rocks, one for fine grained (volcanic) or extrusive igneous rocks. They mostly have similar field divisions, so most volcanic rocks have a corresponding plutonic rock of equivalent mineralogy (for example, rhyolite and granite). Some of the fields are assigned to 2 or 3 rock types, which are further distinguished by their mafic mineral content or plagioclase type (calcic or sodic).

The official field numbers and rock definitions can be found in the description of the figures below, or see Coarse-grained ("plutonic") crystalline igneous rock or Fine-grained ("volcanic") normal crystalline igneous rock .
Plutonic Rocks
1 Quartz-Rich Granitoid
1a Quartzolite
2 Alkali-Feldspar Granite
3 Granite (3a: Syenogranite - 3b: Monzogranite)
4 Granodiorite
5 Tonalite
6 Alkali-Feldspar Syenite
6* Quartz-Bearing Alkali-Feldspar Syenite
6' Foid-Bearing Alkali-Feldspar Syenite
7 Syenite
7* Quartz-Bearing Syenite
7' Foid-Bearing Syenite
8 Monzonite
8* Quartz-Monzonite
8' Foid-Monzonite
9 Monzodiorite / Monzogabbro
9* Quartz-Monzodiorite / Quartz-Monzogabbro
9' Foid-Monzodiorite / Foid-Monzogabbro
10 Diorite / Gabbro / Anorthosite
10* Quartz-Diorite / Quartz-Gabbro
10' Foid-Bearing Diorite / Foid-Bearing Gabbro
11 Foid-Syenite (Foyaite)
12 Foid-Monzosyenite
13 Foid-Monzodiorite (Essexite)
14 Foid-Diorite
15 Foidolite

Field 3 is sometimes subdivided into fields 3a and 3b, which correspond to syenogranite and monzogranite. Usually both are treated as one field.

Volcanic Rocks

1 Not Defined
2 Alkali-Feldspar Rhyolite
3 Rhyolite
4 Dacite
5 Plagidacite
6 Alkali-Feldspar Trachyte
6* Quartz-Bearing Alkali-Feldspar Trachyte
6' Foid-Bearing Alkali-Feldspar Trachyte
7 Trachyte
7* Quartz-Trachyte
7' Foid-Bearing Trachyte
8 Latite
8* Quartz-Latite
8' Foid-Bearing Latite
9 Andesite
9* Quartz-Bearing Andesite
9' Foid-Bearing Andesite
10 Basalt
10* Quartz-Bearing Basalt
10' Foid-Bearing Basalt
11 Phonolite
12 Tephritic Phonolite
13 Phonolitic Tephrite / Phonolitic Basanite
14 Tephrite / Basanite
15a Phonolitic Foidite
15b Tephritic Foidite
15c Foidite

See also tas classification as an alternative classification scheme for fine grained igneous rocks.

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