Felsic rocks are characterised by a high silica content of > 69 m/m% and high concentrations of alkali metals such as sodium and potassium. Their principal mineralogy is comprised of quartz, sodium- and or potassium-rich feldspar varieties, but may also contain micas i.e. muscovite. On average the continental crust can be described as being felsic. Mafic minerals are virtually absent.