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.
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:
Silica (SiO2) content also controls the minerals that crystallise and is used to further classify igneous rocks as follows:
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:
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.
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%.
Le Maitre, R.W., 1989. A classification of igneous rocks and glossary of terms. Blackwell Scientific Publications, 193p.