Olivine is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe)2SiO4; the pyroxenes are chain silicates having the variable formula (Ca,Na,FeII,Mg)(Cr,Al,FeIII,Mg,Mn,Ti,V)Si2O6 comprising a large number of different minerals.
Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/or clinopyroxene; minerals that may be present in lesser abundance include olivine, garnet, plagioclase, amphibole, and spinel.