What is the difference between granite and rhyolite?

They have essentially the same chemistry and mineralogy, but granite is coarse-grained from slow plutonic cooling while rhyolite is fine-grained or glassy from rapid volcanic cooling.

What is a porphyritic texture?

A texture with large crystals (phenocrysts) set in a finer-grained groundmass, recording two stages of cooling: slow crystallization at depth followed by faster cooling after ascent.

Igneous Rock Classification and Textures

Igneous rocks are classified by their mineral content, chemical composition, and texture, which together record how and where they crystallized.

Definition

The systematic naming of igneous rocks based on the proportions of essential minerals or bulk chemistry, together with the description of textures that reflect their crystallization history.

Scope

This topic covers the IUGS modal classification using the QAPF double triangle, the chemical TAS classification by silica and alkali content, the contrast between phaneritic plutonic and aphanitic volcanic textures, and special textures such as porphyritic, vesicular, glassy, and pegmatitic. It connects grain size and texture to cooling rate and depth.

Core questions

How does the QAPF diagram classify igneous rocks by modal mineralogy?
How does the TAS scheme classify volcanic rocks by chemistry?
What does grain size reveal about cooling rate and emplacement depth?
How do porphyritic and glassy textures form?

Key theories

IUGS modal classification: Plutonic and volcanic rocks are named by plotting the relative proportions of quartz, alkali feldspar, plagioclase, and feldspathoids on the QAPF double triangle, giving a standardized, reproducible nomenclature.
Texture as a cooling record: Coarse phaneritic textures indicate slow cooling at depth, fine aphanitic textures indicate rapid cooling near the surface, and glassy or porphyritic textures record sudden quenching or two-stage cooling histories.

Clinical relevance

Classification and texture are the first steps in interpreting any igneous rock, providing immediate clues to its magma chemistry, depth of formation, and eruptive or intrusive origin, and underpinning correlation and mapping in the field.

History

Albert Streckeisen's work in the 1960s and 1970s on modal classification led to the IUGS-adopted QAPF system, while the total-alkali-silica diagram developed in parallel provided a chemical scheme for fine-grained volcanic rocks, both consolidated in the Le Maitre classification volume.

Key figures

Roger W. Le Maitre
Albert Streckeisen
John D. Winter

Seminal works

lemaitre2002
winter2013

Frequently asked questions

What is the difference between granite and rhyolite?: They have essentially the same chemistry and mineralogy, but granite is coarse-grained from slow plutonic cooling while rhyolite is fine-grained or glassy from rapid volcanic cooling.
What is a porphyritic texture?: A texture with large crystals (phenocrysts) set in a finer-grained groundmass, recording two stages of cooling: slow crystallization at depth followed by faster cooling after ascent.