olivine

olivine is a common rock-forming nesosilicate mineral found predominantly in mafic and ultramafic igneous rocks. Its name derives from its typical olive-green color, though it can range from yellow to brownish-green. The mineral is a solid solution series between the magnesium-rich end-member forsterite and the iron-rich end-member fayalite, with its composition and properties varying accordingly. Olivine is a primary constituent of the Earth's upper mantle and is abundant in basaltic lavas and meteorites.

Properties

The physical and chemical properties of olivine vary continuously between its two main end-members. Forsterite (Mg₂SiO₄) is magnesium-rich, has a higher melting point, and is less dense, while fayalite (Fe₂SiO₄) is iron-rich and denser. The mineral typically exhibits a vitreous luster and conchoidal fracture, with a hardness between 6.5 and 7, making it moderately resistant to abrasion. Its color intensifies with increasing iron content, and it is notably susceptible to chemical weathering processes, especially in the presence of water and carbon dioxide. Olivine has a high crystallization temperature, making it one of the first minerals to crystallize from a cooling magma, as described by Bowen's reaction series.

Occurrence

Olivine is a major component of the Earth's mantle, particularly within the asthenosphere, and is brought to the surface via tectonic processes and volcanic activity. It is a dominant mineral in peridotite and dunite, which are key rocks of the upper mantle, and is common in basalts such as those found in Hawaii and Iceland. Significant terrestrial deposits are mined in locations like the Åheim region of Norway and the San Carlos area in Arizona. Olivine is also a principal constituent of stony-iron meteorites, specifically pallasites, where crystals are embedded in a nickel-iron matrix, and it has been identified on the Moon and Mars by missions like NASA's Mars Exploration Rover.

Uses

Due to its high-temperature stability and magnesium content, olivine is primarily used as a refractory sand in foundries for metal casting, notably in the production of steel. It is also employed as a slag conditioner in blast furnace operations. In recent decades, olivine has gained attention for its potential in carbon sequestration projects, as it naturally reacts with carbon dioxide to form stable carbonate minerals like magnesite in a process called mineral carbonation. The gem-quality variety, known as peridot, has been prized since antiquity, with historic sources including Zabargad Island in the Red Sea, and is the birthstone for August.

Mineralogy and crystal structure

Olivine crystallizes in the orthorhombic crystal system, with the space group Pbnm. Its structure consists of isolated silicate tetrahedra (SiO₄) linked by divalent cations of magnesium and iron in octahedral coordination. This independent tetrahedral structure classifies it as a nesosilicate. The complete solid solution between forsterite and fayalite is possible due to the similar ionic radii of Mg²⁺ and Fe²⁺. The mineral's refractive index and specific gravity increase linearly with iron content, a relationship fundamental to its optical and physical characterization in geological studies.

Identification and alteration

Olivine is often identified in hand specimen by its distinctive green color, vitreous luster, and granular habit. In thin section under a petrographic microscope, it displays high birefringence and parallel extinction. A primary alteration product of olivine is serpentine, through a hydration process known as serpentinization, which is significant in the geology of ophiolite complexes. In the presence of carbonated water, olivine can alter to form magnesite or silica, a reaction central to natural and engineered carbon capture. The mineral is also susceptible to iddingsite, a mixture of clay minerals and iron oxides, commonly observed in weathered basalts and some martian meteorites.