QAPFdiagrammia

QAPF diagrammia is a graphical method used in petrology to classify igneous rocks based on their mineral composition. The "QAPF" refers to the mineral groups that form the axes of the diagram: Quartz (Q), Alkali Feldspar (A), Plagioclase (P), and Feldspathoid (F). These minerals are typically the most abundant light-colored (felsic) components of igneous rocks.

The diagram is a double tetrahedron, or more commonly, a ternary diagram projected from a tetrahedron. The primary representation is often a pair of ternary diagrams. One ternary diagram plots the relative proportions of quartz, alkali feldspar, and plagioclase. The other ternary diagram plots the relative proportions of alkali feldspar, plagioclase, and feldspathoid. Rocks containing feldspathoids are undersaturated in silica and therefore lack quartz.

Rocks are plotted onto the diagram based on their modal or normative mineral percentages. The resulting position within the diagram indicates the rock's classification, such as granite, granodiorite, tonalite, syenite, monzonite, or various types of volcanic equivalents like rhyolite or trachyte. It is particularly useful for classifying felsic and intermediate plutonic and volcanic rocks. The QAPF diagram is a fundamental tool for geologists to communicate and understand the composition and relationships between different igneous rock types.