Olivine is one of the most common minerals within the earth, and is a prime rock forming mineral. Despite this, desirable specimens and huge crystals are unusual and fashionable. Only few localities yield large examples of this mineral, even though small and microscopic grains are determined worldwide. It is likewise determined in meteorites, and massive grains were suggested in many of them.
Name: Olivine derives its name from the usual olive-green color of the mineral, and is the term usually given to the species when speaking of it as a rock-forming mineral. Peridot is an old name for the species.
Alteration: Very readily altered to serpentine and less commonly to iddingsite. Magnesite and iron oxides may form at the same time as a result of the alteration.
Diagnostic Features: Distinguished usually by its glassy luster, conchoidal fracture, green color, and granular nature.
Composition: Silicate of magnesium and ferrous iron, (Mg,Fe)2Si0 4 . A complete isomorphous series exists, grading from forsterite, Mg2Si04, to fayalite, Fe2Si04. The more common olivines are richer in magnesium than in iron
Crystallography: Orthorhombic; dipyramidal. Crystals usually a combination of prism, macro- and brachypinacoids and domes, pyramid and base. Often flattened parallel to either the macro- or brachypinacoid. Usually in imbedded grains or in granular masses.
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Occurrence and Formation of Olivine
Most olivine found at Earth’s floor is in dark-colored igneous rocks. It usually crystallizes inside the presence of plagioclase and pyroxene to form gabbro or basalt. These varieties of rocks are maximum not unusual at divergent plate limitations and at hot spots within the centers of tectonic plates.
Olivine has a totally high crystallization temperature as compared to other minerals. That makes it one of the first minerals to crystallize from a magma. During the slow cooling of a magma, crystals of olivine may additionally shape and then settle to the lowest of the magma chamber because of their particularly high density. This focused accumulation of olivine can result in the formation of olivine-wealthy rocks which includes dunite inside the lower components of a magma chamber.
The transparent green variety is known as peridot. It was used as a gem in ancient times in the East, but the exact locality for the stones is not known. At present peridot is found on St. John’s Island in the Red Sea, and in rounded grains associated with pyrope garnet in the surface gravels of Arizona and New Mexico. Crystals of olivine are found in the lavas of Vesuvius. Larger crystals, altered to serpentine, come from Sharum, Norway. Olivine occurs in granular masses in volcanic bombs in the Eifel District, Germany, and in Arizona. Dunite rocks are found at Dun Mountain, New Zealand, and with the corundum deposits of North Carolina
Olivine Composition
Olivine is the name given to a set of silicate minerals which have a generalized chemical composition of A2SiO4. In that generalized composition, “A” is generally Mg or Fe, however in unusual situations can be Ca, Mn, or Ni.
The chemical composition of most olivine falls somewhere between pure forsterite (Mg2SiO4) and pure fayalite (Fe2SiO4). In that series, Mg and Fe can alternative freely for each other in the mineral’s atomic structure – in any ratio. This form of non-stop compositional variation is called a “strong solution” and is represented in a chemical components as (Mg,Fe)2SiO4.
| Mineral | Chemical Composition |
| Forsterite | Mg2SiO4 |
| Fayalite | Fe2SiO4 |
| Monticellite | CaMgSiO4 |
| Kirschsteinite | CaFeSiO4 |
| Tephroite | Mn2SiO4 |
Olivine Physical Properties
Olivine is typically inexperienced in color however also can be yellow-inexperienced, greenish yellow, or brown. It is obvious to translucent with a glassy luster and a hardness between 6.5 and 7.0. It is the simplest not unusual igneous mineral with these residences. The properties of olivine are summarized within the table.
| Chemical Classification | Silicate |
| Color | Usually olive green, but can be yellow-green to bright green; iron-rich specimens are brownish green to brown |
| Streak | Colorless |
| Luster | Vitreous |
| Diaphaneity | Transparent to translucent |
| Cleavage | Poor cleavage, brittle with conchoidal fracture |
| Mohs Hardness | 6.5 to 7 |
| Specific Gravity | 3.2 to 4.4 |
| Diagnostic Properties | Green color, vitreous luster, conchoidal fracture, granular texture |
| Chemical Composition | Typically (Mg, Fe)2SiO4. Ca, Mn, and Ni rarely occupy the Mg and Fe positions. |
| Crystal System | Orthorhombic |
| Uses | Gemstones, a declining use in bricks and refractory sand |
Olivine Optical Properties
| Formula | (MgFe)2SiO4 |
| Crystal System | Orthorhombic |
| Crystal Habit | Granular masses or rounded grains |
| Cleavage | Poor cleavage on (010) and (110) |
| Color/Pleochroism | Olive or yellowish-green in hand samples. Colorless to pale green in thin section. Weak, pale green pleochroism in thin section. |
| Optic Sign | Biaxial (-); or Biaxial (+) |
| 2V | 82-90; forsterite 46-90; fayalite |
| Optic Orientation | X=b Y=c Z=a O.A.P. = (001) |
| Refractive Indices alpha = beta = gamma = delta = | forsterite-fayalite 1.635-1.827 1.651-1.869 1.670-1.879 0.035-0.052 |
| Extinction | parallel |
| Dispersion | Relatively weak |
| Distinguishing Features | Olivine is commonly recognized by it high retardation, distinctive fracturing, lack of cleavage, and alteration to serpentine. Colorless to olive green in thin section. Second-order interference colors. High relief. Lack of cleavage. H= 7. G = 3.22 to 4.39. Specific gravity increases and hardness decreases with increasing Fe. Streak is colorless or white. |
| Sources | Nesse (1986) Introduction to Optical Mineralogy. Mindat.org. |
Olivine Uses
Olivine is a mineral that isn’t regularly utilized in enterprise. Most olivine is used in metallurgical strategies as a slag conditioner. High-magnesium olivine (forsterite) is introduced to blast furnaces to take away impurities from metallic and to shape a slag.
Olivine has also been used as a refractory material. It is used to make refractory brick and used as a casting sand. Both of those uses are in decline as opportunity substances are less highly-priced and simpler to obtain.
- Olivine is likewise the mineral of the gemstone referred to as “peridot.” It is a yellow-green to green gemstone that is very popular in earrings. Peridot serves as a birthstone for the month of August. The most valued hues are dark olive green and a shiny lime inexperienced. These specimens are of the mineral forsterite due to the fact the iron-wealthy fayalite is mostly a brownish, much less perfect color.
- Much of the arena’s peridot utilized in mass-manufacturing earrings is mined on the San Carlos Reservation in Arizona. There, a few basalt flows containing nodules of granular olivine are the supply of the peridot. Most of the stones produced there are some carats or less in size and regularly incorporate visible crystals of chromite or other minerals. They are cut in Asia and lower back to the US in business earrings.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Dana, J. D. (1864). Manual of Mineralogy… Wiley.
- Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
- Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
