Grain size is one of the classification methods used when identifying igneous rocks. They are also classified by the silica content or silica saturation. Other methods of classification include the total alkali versus silica scheme and the International Union of Geological Sciences scheme.
The grain size of igneous rock reflects the depth at which the molten rock formed within the Earth. The silica content controls the minerals that crystallise, and the silica saturation allows distinction between feldspathoid-bearing and feldspathoid-free rocks. The total alkali versus silica classification scheme uses chemistry to classify volcanic rocks. The International Union of Geological Sciences classification scheme uses the minerals present to classify them.