Olivine Crystal Structure: Earth’s Simple Starter Mineral

Olivine crystal structure is foundational for its own significant forms and processes in the Earth. The Earth is mostly made of elements silicon and oxygen, when mixed with smaller amounts of other elements silicate minerals form. The backbone of silicate minerals is the silicon-oxygen tetrahedra. Olivine has tetrahedra that are set loose from one another, with iron or magnesium, another two common elements, spacing the tetrahedra apart in the solid rock.

By volume olivine is one of the most abundant minerals on Earth. Olivine makes up the majority of the upper mantle, over 50%. It’s been found in meteorites and at volcanoes, and one of olivine’s forms is gemstone peridot.

Olivine’s Stats

Olivine group minerals have a unit cell containing SiO₄^-4 (the silicon-oxygen tetrahedron) ionically bonded with neutralizing cations, most commonly iron and/or magnesium. Usually “olivine” refers to minerals in the Fayalite-Forsterite series (Fe₂SiO₄, Mg₂SiO₄). (To understand better about the chemical formulas in silicate minerals, read about the silicate chemical formula “recipe”.) Those chemical formulas only refer to the idealized olivine crystal structure for revealing bulk properties. Real olivine has some impurities like manganese, nickel, aluminum, chromium, or boron.

Other Olivine Group Minerals

The above statistics and crystal structure information below pertain to the simple fayalite-forsterite forms of olivine. The Olivine group overall also includes minerals:

• Fayalite (Fe₂SiO₄)
• Forsterite (Mg₂SiO₄)
• Tephroite (Mn₂SiO₄)
• Monticellite (CaMgSiO₄)
• Larnite (Ca₂SiO₄)
• Kirschsteinite (CaFeSiO₄)

Forsterite has a very high melting temperature of 3,450°F (1,900 °C), while fayalite’s is lower at 2,190°F (1,200 °C). Within forsterite and fayalite olivine inclusions of larnite, tephroite, and monticellite are common.

Olivine Crystal Structure

Recall the silicon-oxygen tetrahedra, the building block and defining feature of all silicate minerals.

The best way to visualize these linkages is scrolling to the ball model for Olivine on mindat, and set the load to “2x2x2” and show to “all polyhedra.” The “2x2x2” viewing option helps because when only viewing one unit cell the sides of the cell cut off the tetrahedra. Rotate the model to notice where two of each of the four corners of every tetrahedra is connected to a cation. Olivine’s crystal system is orthorhombic, the atoms can be placed into repeating rectangular prisms, which you can see as the black box around one unit cell. There is glide symmetry and mirror symmetry about all three axes.

When we look at the oxygen atoms (red in the picture), we can describe olivine crystal structure as hexagonal close packed (HCP), a highly efficient packing ratio. In the oxygen array, the octahedral vacancies are occupied by Mg, while the tetrahedral vacancies are occupied by Fe in half of them and Si in an eighth of them, the other three-eighths of vacancies remaining empty. [Ernst].

Olivine is very resistant to erosion. It has a high density, well-packed, and no cleavage from a lack of directional symmetry (anisotropy).

Polymorphs of Olivine

Polymorphs have the same chemical formula but different structures. If a mineral forms under higher pressure conditions compared with another, the energy present in forming the bonds is higher so the crystal structure can find stability in another configuration. Up to 410 km into the Earth, forsterite exists stably. Beyond that, the crystal structure changes giving a different minerals with different properties. But these high-pressure polymorphs aren’t quite well-studied yet. Rather than actually go that deep to harvest samples, researchers have had better luck just creating synthetic versions in lab to look at.

Olivine’s polymorph forms are:

• Wadsleyite (a sorosilicate with twinned tetrahedra) – 410 km / 250 mi
• Ringwoodite (spinel) 520 mk /320 mi
• Silicate perovskite ((Mg,Fe)SiO3) makes up the lower mantle and possibly core 660 km / 410 mi
• Ferropericlase, a further transformed silicate perovskite ((Mg,Fe)O) 660 km / 410 mi

The depth of formation helps researchers parse out the polymorphs. Just like water, vapor, and ice, olivine transforms at given temperature and pressure conditions. Studying olivine polymorphs at different depths below the Earth’s surface has helped create a phase transition map for olivine. Thus, researchers can make ringwoodite in lab and study it. Ringwoodite and other high pressure olivine polymorphs have been found in meteorites and are likely present on other planets too. If you’re interested more about minerals in the deeper mantle, Dr. Kawazoe made a really good round-up page.

Geology of Olivine

Since silicon and oxygen are the most abundant elements in Earth’s crust and mantle, it is no wonder that one of the “simplest” silicates, olivine, is too very abundant. Olivine is “mixed in” to other rock, and has structurally similar silicates like pyroxene often found alongside it.

Formation of Olivine Crystal Structure

Most olivine comes from solidified magma from the upper layers of the mantle. Materials from the mantle get naturally “piped” up like capillary action in a straw through structures called kimberlitic pipes. The kimberlitic rock making up the pipes are an igneous rock, a variant of peridotite, named after town of Kimberly in South Africa. These vertical pipes coming through the crust provide information about the mantle because they reach very deep and pipe up diamonds, garnets, peridot, and more.

To understand how crystals grow in magma, you can visualize the permeating high temperatures like wind blowing across fluid. Nesosilicates (single islands of tetrahedra) form at the highest temperatures versus tectosilicates. For nesosilicates the heat has “blown” through and dispersed the silicon-oxygen tetrahedra into their single islands.

Olivine’s Associated Composite Rocks

Olivine is purified from composite rocks that contain many other minerals. Some composite rocks containing high amounts of olivine, like dunite, a type of peridotite, gabbros and basalts. (You can read about some of these rocks and their broader mineral classifications here.)

Forsterite (Mg₂SiO₄) is more common than fayalite (Fe₂SiO₄), found either in cooled magma (igneous rocks) that is low in silica and high in magnesium (gabbro, basalt making up peridotite), sedimentary rock like limestone, or metamorphosed sedimentary rock like dolomite. Fayalite occurs in igneous granites and rhyolites, much less commonly. The iron-rich fayalite can co-exist with quartz (SiO₄) and tridymite (SiO₄). Meanwhile, forsterite would react with the excess silica and make orthopyroxene ((Mg,Fe)₂Si₂O₆), so forsterite is not commonly found with other silica rich minerals like quartz.

Olivine is very common in mafic and ultramafic rocks, which are igneous rocks with 55% silica or less. When the mafic or ultramafic rock has over 50% olivine, it is periodotite, and for less than 50%, pyroxenite.

Peridotite

Periodotites such as gabbro-peridotite are an ultramafic rock with over 40% olivine, with dunite having over 90% olivine. Peridotite rock dominates the Earth’s upper mantle, and occurs along the kimberlitic pipes too. They capture CO₂ in their crystal structure and are highly enriched in magnesium. Peridotites have different percentages of pyroxenes, the second most complex silicate mineral. There are even more complex silicates mixed in in smaller amounts, like hornblende, an amphibole present in peridotite. Biotite, magnetite, garnet and spinels also co-exist in peridotite.

Lherzolite	40-90% olivine
Harzburgite	partially melted Lherzolite, often erupting on basalt
Dunite	90+% olivine

Remember from the types of silicates that olivine is a nesosilicate of the least complex structure, and pyroxene is just one level up in complexity of silicate bonding. Whereas nesosilicates like olivine have only islands of silicon-oxygen tetrahedra, inosilicates like pyroxene have those tetrahedra bonded into simple chains. So peridotites contain more silicates than just olivine, and having as complex the inosilicate amphibole and the tectosilicate plagioclase.

The gemstone peridot is the more transparent form of olivine. Peridot is a mixture of proportions of forsterite and fayalite which determines the shade of green. More often peridot is mainly forsterite (Mg). It can further oxidize beginning at 600º C to magnetite and pyroxene. In its natural environment peridot weathers rapidly, and so the larger peridot crystals are quite rare.

Serpentization of Olivine Crystal Structure into Serpentine

Olivine also makes a cool transformation with natural water. When seawater and groundwater flows through olivine-coated veins in the ground, the dissolved ions react with the rocks in contact. Inosilicates like pyroxene and amphiboles undergo this too.

Here’s how it achieves serpentization chemically:

Here you can also see some beautiful pictures of serpentine itself, an industrial source of magnesium and asbestos.

Olivine Crystal Structure in Context

Olivine-rich areas also make some nice landforms like the Olivine pine forest in Norway and the Olivine sand beach in Hawaii. Applications of olivine other than general ores could include carbon dioxide sequestering.

But also importantly, olivines are the “starter kit” from which you can understand how more complex silicates like zeolites can precipitate.

Sources

Demouchy, Sylvie. “Defects in olivine.” European Journal of Mineralogy 33.3 (2021): 249-282.

Ernst, W. G. Earth Materials. Englewood Cliffs, NJ: Prentice-Hall, 1969. p. 65

The rate of olivine weathering, an expensive myth, R.D.Schuiling, Faculty of Geoscience, Utrecht, https://smartstones.nl/the-rate-of-olivine-weathering-an-expensive-myth/

Oliveira, S.M.B., Melfi, A.J., Carvalho, A., Friedrich, G., Marker, A. and Kanig, M. (1988) Lateritic evolution of the Jacupiranga Complex, S.P. Geochimica Brasiliensis, 2 (2), 119-126.

Alex Strekeisen’s bibliography:
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Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.