Classification of igneous rocks

Igneous rocks

Igneous rocks are formed when magma cools and solidifies. They are classified by using grain size, silica content, and/or silica saturation. View some examples of igneous rocks from the Australian Museum's Mineralogy Collection.

Classification schemes

Grain size

Grain size reflects the depth at which molten rocks form within the Earth. Classification of igneous rocks can be based on their predominant grain size. Three types of rock can be identified:

·         Volcanic rocks: solidify close to the Earth's surface. Because they cool quickly they have a finer-grained matrix (called groundmass). They may contain some larger crystals that formed earlier further down (called phenocrysts).

·         Hypabyssal or subvolcanic rocks: form at intermediate depths (generally as dykes and sills) and so tend to be medium-grained.

·         Plutonic rocks: form deeper within the Earth and the slower cooling allows them to crystallise as coarse-grained rocks.

Silica content

Silica (SiO2) content also controls the minerals that crystallise and is used to further classify igneous rocks as follows:

·         Acid: rocks with above 63% silica (mostly feldspar minerals and quartz), e.g. granite.

·         Basic: rocks that are about 45 to 55% silica (mostly mafic minerals plus plagioclase feldspar and/or feldspathoid minerals), e.g. basalt.

·         Ultrabasic: rocks that are usually less than 45% silica (mostly mafic minerals such as olivine and pyroxene), e.g. peridotite.

Silica saturation

This is similar to silica content, but allows distinction between feldspathoid-bearing and feldspathoid-free rocks. Feldspathoids belong to a group of rock-forming minerals chemically similar to the feldspars but containing less silica. This scheme classifies igneous rocks into three categories:

·         Rocks that are under-saturated in silica will crystallise silica-poor minerals like feldspathoids or olivine.

·         Those that are just saturated in silica will lack feldspathoids or quartz.

·         Those that are over-saturated in silica will crystallise quartz among the minerals.

International classification

The International Union of Geological Sciences (IUGS) classification scheme is the standard scheme for igneous rocks and uses the minerals present to classify them. The scheme takes the form of triangular diagrams where each point of the triangle represents 100% of a mineral. It splitsrocks into feldspathoid-bearing (i.e. nepheline, sodalite) and feldspathoid-free rocks. Then it takes the relative percentages of quartz (Q), plagioclase (P) and alkali feldspars (A). The values of Q, P, and A determines a given field for each rock type. Similar schemes also classify ultramafic rocks (but using olivine, orthopyroxene and clinopyroxene) and gabbroic rocks.

TAS classification

The TAS (Total Alkalis vs Silica) classification scheme uses chemistry to classify volcanic rocks. The analysis is recalculated to 100% after removing water (H2O) and carbon dioxide (CO2). It is a simple X-Y graph with the X-axis showing silica (SiO2) Wt% and the Y-axis showing alkalis (Na2O+K2O) Wt%.