Gabbro is a coarse-grained igneous rock that is relatively low in silica and rich in iron, magnesium, and calcium.Such rock is described as mafic.Gabbro is composed of pyroxene and calcium-rich plagioclase, with minor amounts of hornblende, olivine, and accessory minerals.
Pure forsterite is composed of magnesium, oxygen and silicon. The chemical formula is Mg 2 SiO 4.Forsterite, fayalite (Fe 2 SiO 4) and tephroite (Mn 2 SiO 4) are the end-members of the olivine solid solution series; other elements such as Ni and Ca substitute for Fe and Mg in olivine, but only in minor proportions in natural occurrences.