QAPF classification

The name and its history

IUGS Subcommission on the Systematics of Igneous Rocks, associated with Albert Streckeisen · IUGS recommendations in 1970s (not an individual first coinage)

What does the word literally mean?

The gloss ‘quartz · alkali feldspar · plagioclase · feldspathoid’ describes the linguistic allusion. The original name and its modern scientific definition must be kept distinct.

What did the original author mean?

The initials of four mineral or mineral-group components measured in an igneous rock.

Why was the name needed?

Petrologists needed consistent mineral-proportion criteria because historical rock names arose from multiple regional mapping and petrographic traditions.

What evidence supported the original concept?

Quantified modal abundances, the proportions of Q, A, P and F recalculated to a 100% total, and boundaries agreed by the international nomenclature subcommission.

How has geology changed the concept?

The QAP and FAP triangles codify inherited names such as granite and syenite. Quartz and feldspathoids are normally mutually exclusive in the classified assemblage; ultramafic rocks and many volcanic rocks require other methods.

Further historical context

Streckeisen’s 1976 paper demonstrates how an international system attempted to standardise inherited petrological terms.

The name QAPF is best learnt by decoding its constituent minerals, not by trying to translate QAPF as a single word.

Related names

Quartz · Alkali feldspar · Plagioclase · Feldspathoid · Feldspar · Granite · Granodiorite · Tonalite · Syenite · Monzonite · Diorite · Gabbro · Peridotite

Sources

1. Streckeisen (1976), To each plutonic rock its proper name — Original IUGS classification account
2. IUGS QAPF classification overview — Diagram reading and conventions; secondary overview