The IUGS systematics of igneous rocks

TLDR

The IUGS established a Subcommission on the Systematics of Igneous Rocks to develop a universally applicable, sustainable classification of igneous rocks. The classification is built on ten principles—using descriptive, property‑based, and simple criteria—and distinguishes pyroclastic, carbonatitic, melititic, lamprophyric, and charnockitic rocks before applying the QAPF scheme or, when mineral mode is indeterminate, a TAS chemical classification. The system requires only 297 distinct rock names, a dramatic reduction from the roughly 1,500 names previously in use.

Abstract

In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the IUGS Subcommission on the Systematics of Igneous Rocks. In the course of creating the classification, the Subcommission has established ten principles for its construction and for defining an appropriate nomenclature. The principles are: (1) use descriptive attributes; (2) use actual properties; (3) ensure suitability for all geologists; (4) use current terminology; (5) define boundaries of rock species; (6) keep it simple to apply; (7) follow natural relations; (8) use modal mineralogy; (9) if mode not feasible, use chemistry; (10) follow terminology of other IUGS advisory bodies. These principles and their rationale have not previously been enunciated. The classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering the main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F). Rocks with mafic content >90% have their own classification. If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used. The nomenclature for these classifications necessitates only 297 rock names out of the c . 1500 that exist.