The olivines are a class of common silicate minerals named for their greenish or olive color. They are glassy, fracture conchoidally (i.e., along curving cleavage surfaces), and are often found in meteorites and in mafic igneous rocks such as basalt, dunite, gabbro, and peridotite.

Like the feldspars, the olivines consist of a silicon (Si) and oxygen (O) framework interspersed with atoms of a metallic additive, usually magnesium (Mg) or iron (Fe) but sometimes calcium (Ca). Forsterite (Mg₂SiO₄) is olivine containing no additive but magnesium, while fayalite (Fe₂SiO₄) is olivine containing no additive but iron. Between these two minerals there is a continuum of olivines containing varying percentages of forsterite and fayalite in solid solution. Olivine with 10–30% fayalite is defined as chrysolite; 30–50%, hyalosiderite; 50–70%, hortonolite; and 70–90%, ferrohortonite. The remainder in all cases is forsterite. An olivine with less than 10% fayalite is classified simply as forsterite, while one...