Olivine
Introduction
Olivine is a group of rock-forming minerals that are typically found in mafic and ultramafic igneous rocks such as basalt, gabbro, dunite, diabase, and peridotite. They are usually green in color and have compositions that range from magnesium-endmember forsterite (Mg2SiO4) to iron-endmember fayalite (Fe2SiO4).
Composition and Structure
Olivine is a nesosilicate, or orthosilicate, mineral with a formula of (Mg, Fe)2SiO4. This indicates that it is part of the larger silicate group, which is the most abundant group of minerals in the Earth's crust. Olivine's structure is composed of isolated silica (SiO4) tetrahedra that are bonded by divalent cations (Mg, Fe) in octahedral coordination with oxygen. The ratio of magnesium to iron varies between the two endmembers of the solid solution series: forsterite (Mg-endmember) and fayalite (Fe-endmember). Intermediate compositions are common and are denoted as the percentage of forsterite (Fo) or fayalite (Fa) present (e.g., Fo70Fa30).
Physical Properties
Olivine is usually green in color, although its shades can range from a light olive green to a dark green, depending on the iron content. The mineral has a vitreous luster and a hardness of 6.5–7 on the Mohs scale. It exhibits conchoidal fracture and its cleavage is poor. Olivine is birefringent, meaning it splits a beam of light in two. Its refractive index and specific gravity also increase with iron content.
Formation and Occurrence
Olivine is a common mineral in the Earth's upper mantle, making up as much as 50% of it. It is also found in lunar rocks and meteorites. On the Earth's surface, olivine is found in mafic and ultramafic igneous rocks, and it can also be found in metamorphic rocks under high-temperature and pressure conditions. Olivine weathers rapidly at the Earth's surface, especially when it's finely disseminated in volcanic ash and in hot climates.
Economic Significance
Olivine has several economic uses. It is mined for use as a refractory material, thanks to its high melting point and resistance to chemical erosion. It is also used in blast furnaces to remove impurities from steel and to form a slag. Some olivine is used in ceramics and glass. Gem-quality olivine is a beautiful green stone known as peridot, which is the birthstone for the month of August.
Environmental Impact
Olivine has been proposed as a potential tool for combating climate change through a process known as enhanced weathering. When olivine reacts with water and carbon dioxide, it forms a stable, non-toxic material and releases magnesium ions. This process effectively sequesters carbon dioxide, a greenhouse gas, and could help to mitigate global warming.