The Ferromagnesian Minerals of the Discontinuous Branch
The ferromagnesian minerals contain magnesium and iron. In order from mafic to felsic, these are olivine, pyroxenes such as augite, amphiboles such as hornblende, and biotite. Olivine looks like rounded olive-green grains. The others are greenish-black.
Olivine crystallizes first. It forms with its silicon arranged in isolated tetrahedrons of SiO4. With a general chemical formula of (Mg,Fe)2SiO4 (ranging from Mg2SiO4 (forsterite) to MgFeSiO4 to Fe2SiO4 (fayalite)), olivine has proportions of one part silicon to two parts iron or magnesium (a 1:2 ratio), which means it depletes the magma of iron and magnesium and leaves the remaining liquid with proportionately more silicon.
When the magma cools to the temperature for pyroxene to crystallize, some of the olivine reforms into pyroxene, which has the form of tetrahedrons in single chains (for each silicon atom, two oxygen atoms shared with another silicon). The general chemical formula is MgSiO3 (enstatite) or FeSiO3 (ferrosilite), giving it a 1:1 ratio of silicon to magnesium and iron. (Or CaSiO3 (wollastonite), or any combination of Fe, Mg, and Ca with Si2O6.) If there's still magma left over, it has even more silicon in it than before.