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 with less than 10% forsterite is classified simply as fayalite. Confusingly, the term chrysolite is also sometimes used as a synonym for olivine in general.
Magnesium-rich olivine is the majority ingredient of the rock peridotite, the main component of Earth's upper mantle. The interface between the underside of the crust and the olivine-rich peridotite of the mantle, is called the Mohorovicic discontinuity or Moho for short, and is of great importance in seismology . The Moho is generally located at a depth of 3.7 mi (6 km) beneath the oceans and 19 mi (30 km) beneath the continents.
Compression of olivine's atomic structure to its spinel phases under extreme pressure causes a second seismic discontinuity at approximately 250 mi (400 km) and a third at approximately 420 mi (670 km). These olivine–spinel phase transitions affect the mechanical properties of the whole mantle, which in turn determine the convective flow processes that drive plate tectonics and thus much of the geological history Earth.
Implosive collapse of olivine to spinel in slabs of oceanic crust being subducted rapidly into the mantle may be violent enough to generate deep-focus earthquakes.
Olivine is readily altered to the mineral serpentine by the hydration of its crystal structure by hot (400–800°C) water . This process occurs along mid-ocean ridges and other places where mafic rock is exposed to superheated water.
Peridot (pronounced PER-ih-do) is a transparent variety of olivine valued as a gemstone. Olivine is used industrially as a lining in furnaces due to its heat resistance.
See also Bowen's reaction series; Earth, interior structure; Mantle plumes