Igneous rocks can vary widely in composition and grain size, which are two important characteristics helping to identify an igneous rock and determine its geological origin.
Classified by composition, igneous rocks can be considered mafic, enriched in magnesium and iron silicate minerals like olivine and pyroxene. Felsic igneous rocks are magnesium-poor and contain much greater amounts of silica (SiO2), along with light minerals such as quartz, mica and feldspar.
Mafic igneous rocks are usually partial melts of ultramafic peridotite rock in the upper mantle. Ultramafic rocks contain even more magnesium and iron silicate minerals than mafic rocks. A partial melt of peridotite produces a basaltic magma, which forms basalt rock on Earth's surface or gabbro rock if a magma chamber solidifies inside the Earth.
Partially melting basaltic/gabbroic rock produces intermediate compositions such as andesite (extrusive) or diorite (intrusive), which contain less olivine, less pyroxene, and more amphibole (hornblende) and biotite mica. If diorite or andesite partially melts a more felsic granite or rhyolite is produced.
Bowen's reaction series is a characteristic sequence of minerals that are produced by cooling of mafic magma or lava. The first minerals to crystalize from a freezing mafic magma are olivine, then pyroxene, then amphibole & biotite, then K-feldspar, muscovite mica and finally quartz. The former minerals comprise a discontinuous reaction series; each mineral forms during a specific phase of cooling. Plagioclase feldspars crystalize in a continuous reaction series, beginning with calcium-rich and ending with sodium-rich plagioclase grains.