The concentration of silica (SiO₂) in an igneous rock, relative to the concentration of other chemical constituents in the rock which combine with the silica to form silicate minerals. On this basis, three classes of igneous rock are recognized: (a) silica-oversaturated rocks (e.g. granite), in which there is more than enough silica to satisfy the requirements of all the major silicate minerals, the free silica appearing as quartz in the rock; (b) silica-saturated rocks (e.g. diorite), in which there is just enough silica present to satisfy the requirements of all the major silicate minerals, there being neither an excess nor deficiency of silica, resulting in a lack of both quartz and feldspathoid minerals in the rock; and (c) silica-undersaturated rocks (e.g. nepheline-syenite), in which there is not enough silica present to satisfy the requirements of all the major silicate minerals, the silica deficiency being accommodated by the crystallization of feldspathoids (nepheline, leucite) in place of feldspar, the feldspathoids containing less silica in their structure than feldspars.