Forsterite - Encyclopedia

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : Mg2SiO4
    Rarity : Very common

Forsterite is the magnesium pole of the olivine group, it is by far the most common mineral in this group and one of the most abundant minerals on the globe. It forms two series, one with fayalite (ferriferous) and the other with tephroite (manganiferous). Forsterite is the fundamental constituent of peridotites, often present in gabbros and basalts and also in certain high-grade metamorphic rocks (dolomitic marble). It was named in honor of Adolarius Jacob Forster, an English mineral merchant. Forsterite crystals are frequently flattened, longitudinally striated, with sharp faces and octahedral terminations, sometimes centimeter-sized. Forsterite is also often in granular masses composed of grains with rounded surfaces, sometimes large. With a glassy luster, it has a characteristic greenish-yellow to green color. It is easily altered to minerals from the serpentine group (chrysotile and antigorite), a phenomenon which can continue with the formation of talc or magnesia. Peridot is the gemmy forsterite, it is used in jewelry, but it is also a highly sought-after industrial mineral for the manufacture of refractories (foundries, ceramics, etc.).

Main photo : Forsterite from Soppat, Manshera, Naran-Kagan Valley, Kohistan, Northwest Frontier, Pakistan

Forsterite crystal from Soppat, Manshera, Pakistan
Forsterite gemstone (peridot) from Sapat Gali, Pakistan
Forsterite from Mont Croustet, Haute-Loire, France
Peridotite from Mesa, San Carlos Volcanic Field, Arizona, USA

Forsterite in the World

The best forsterite crystals in the world come from Zebirget Island, better known as St. John Island in the Egyptian Red Sea, where they reach 10 cm ! But magnificent peridots of similar quality are known in Pakistan, near Basham, in Tanzania and in the gemmiferous alluvium of Sri Lanka. We must also mention the magnificent green and green-yellow gem crystals, weighing more than 2 kg, from Jacupiranga (Sao Paulo, Brazil) and those extracted from the lavas of the Nyiragongo volcano in the Democratic Republic of Congo. Apart from gem crystals, forsterite has produced crystals exceeding 30 cm in Oka and Notre-Dame-du-Laus (Quebec, Canada), and crystals of 10 cm in the Kovdor massif, in Russian Karelia. Less impressive crystals are known in the Urals, in Norway (Snarum), and in Kyavkpon (Myanmar) where they exceed 5 cm in altered serpentinites.

Forsterite in France

In France, forsterite is common in centimeter-sized crystals in many basalts of the Massif Central (Suc d'Eyme, Grand Champagnac, etc...). It is also worth noting the very beautiful automorphic forsterite crystals from Mont Croustet, they constitute pretty olive green to brown tabular floaters with perfect shapes and which can reach dimensions of more than 1.5 cm. Decimetric nodules of forsterite (peridotite), of a very beautiful green, are known in abundance in the basalts of Sauterre (Puy-de-Dôme), and in the projections of Mont Briançon near Langeac (Haute-Loire).


Twins are known on {100}, {011} and {012}.

Fakes and treatments

Synthetic forsterites have been produced in Russian laboratories since 1999. However, the synthetic crystals which are cut for jewelry are blue-violet, a color which does not exist for this species in nature... this synthesis actually aims to imitate the tanzanite...

Hardness : 7
Density : 3.27
Fracture : Conchoidal
Streak : White

TP : Translucent to transparent
RI : 1.636 to 1.772
Birefringence : 0.033 to 0.042
Optical character : Biaxial +
Pleochroism : Visible
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : Paramagnetic
Radioactivity : None


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