Forsterite

Mineral Name: Forsterite

Chemical Composition: Mg2SiO4 (contains two magnesium, one silicone, and four oxygen molecules). (3)

Color: The color can be colorless, green, pale-yellow, yellow, yellow-green, or white. (1)

Streak: The streak can be colorless or white. (4)

Hardness: According to Mohs scale of hardness, forsterite has hardness that can range from 6 to 7. It will scratch glass and, maybe, porcelain, but not topaz, or corundum. (8)

Cleavage/Fracture: Forsterite can be distinctly noticed by its poor cleavage. The cleavage runs in two directions at 90 degrees. The cleavage is indistinct and rarely noticed. The fracture is conchoidal, which is characterized by smooth, curving surface. (1)(12)

Crystal Form: Its crystal form is orthorhombic, which means it has three planes that intersect at angles other than 90 degrees. The mineral can also be dipyramidal, having a structure similar to two pyramids stacked base-to-base. (1)

Luster: The luster is vitreous (glassy). (1)

Special Features: Forsterite has a high melting point of 1,760 degrees celsius, and is the first mineral to crystallize in mafic magma. Significantly sized crystals can be found in many iron-nickel meteorites. Sometimes the crystals can occupy over 50% of the meteorites volume. (1)(7)(12)

Varieties: Chrysolite, Olivine, and Peridot. (1)

Mineral Group: Forsterite is a member of the nesosilicate and silicate groups. The mineral forsterite is an end member of the Olivine Series, with the other end member being the iron-rich, fayalite. All olivine group members are essentially a mix of the two end members, with some percentage of each. (3)(12)

Environment: Forsterite can be found in a wide variety of igneous rocks such as ultramafic igneous rocks with a high iron and magnesium content and occasionally occur from the metamorphism of dolomitic limestones. Olivine is commonly found in xenoliths within igneous rocks that form when magma or lava cools. As magma migrates to the surface, or near surface, and cools to form igneous rock, it may pick up inclusions of foreign rock. (1)(5)(12)

Associated Rock types: Forsterite minerals are found in basalt, gabbro, dunite, diabase, and peridotite. (9)

Occurrence in North America: Forsterite can be found in locations such as Chihuahua, Mexico; Notre-Dame-du-Laus, Quebec Canada; Salt Lake Crater, Oahu, Hawaii; North Carolina; Bolton, Massachusetts; Crestmore, California; New Mexico; and Peridot Mesa, San Carlos Apache Reservation, Gila County, Arizona. (12)

Economic Uses: Due to its-hardness and color range, forsterite is a popular gemstone under the trade name Peridot. Characteristics that affect the grade and economic value are, size, color, clarity (i.e translucency to transparency), the quality of any cutting or faceting, and an absence of fractures and inclusions.

Peridot is the birthstone for August. This, along with its popular green color, contributes to its appeal and demand as a focal piece for birthday jewelry. Formerly an eminent source is Burmese Peridot, which was noted of having significant size and quality, up to 40 carats. Though this source has been limited due to political change. The Peridot market from the Southeast Asian country of Myanmar (formerly Burma), has gone dark, and is no longer a viable source of the gemstone. The loss of this source has been a gain to America, specifically Arizona’s San Carlos Apache Reservation, who now have the largest source of jewelry-grade Peridot in the world. The stone size and quality at the high end of the scale is 22 carats; although, most of the reserve gems are at one carat or less of tumbled, or small faceted stones. These stones are harvested and usually sold in lots by the Apache Nation, directly to jewelry makers and wholesalers. (10)

Industrial Uses: Estimations between 200,000 and 250,000 metric tons of forsterite are used annually primarily in insulating bricks for maintaining iron in a molten state due to its structural integrity at temperatures up to 1400-1450 degrees Celsius. When combined with the minerals spinel and periclase, as well as taking into account its availability, forsterite was preferred to chromite-periclase bricks. Other uses of forsterite include usage as an abrasive, for soil conditioning, in foundry sand, and for heat storage. Forsterite is an ore of magnesium and can be used for mineral specimens. (6)(7)(11)

First Notable Identification: The first notable identification comes from Johan Gottschalk Wallerius in 1747. The name chrysolit (chrysolite) was given to the olivine group mineral species. (3) Forsterite has been known by other names in the past, the name forsterite comes from Serve-Dieu Abailard “Armand” Levy in honor of Adolarious Jacob Forster in 1824. (3)

How We Identified It: We identified it by the unmistakable green to yellow color. It also appears to not have cleavage.

Don’t Confuse It With: Do not confuse it with Epidote (Ca2(Al,Fe)2(SiO4)3(OH)), which has cleavage and in a hand sample can be the color of pistachio-green. (4) Due to its green color, may also be mistaken for green beryl (Be3Al2Si6O18 ), more commonly known as emerald.