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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 .

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

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