1.1b The composition of magma ranges from ultramafic to felsic
Mafic magmas are rich in iron, magnesium and calcium. They tend to be composed of around 50% silica.
Felsic magmas are composed around 75% silica. They are lower in iron, magnesium and calcium than mafic magma, but contain more sodium and potassium. Felsic magmas also tend to contain more volatiles such as water, carbon dioxide and sulfur dioxide.
Source: Earle, S (2015) Physical Geology#Chapter 4 Volcanism