Muséum national d'histoire naturelle

GENERAL DESCRIPTION

Forsterite is a mineral from the olivine series of minerals, like fayalite. Forsterite and fayalite forms the 2 end-members of a "solid solution". A "solid solution" is a series of minerals that possess the same atomic structure, but their chemical composition varies from one end-member to another. Fayalite (Fe2SiO4) is the ferrous end-member of the olivine, whereas forsterite is the magnesium-rich end-member (Mg2SiO4).

In nature, the pure end-members of these « solid solutions » are never encountered because they always contain impurities from the other end-member. Then, natural forsterite always contains some iron (5-15 wt.%) resulting in the green colour of most forsterites. Some very rare forsterites do contain a few wt.% of iron: they are then gray in colour. Olivine (or peridot, or chrysolithe) is a forsterite containing usually 10 wt. % of fayalite.

Forsterite is found in mafic volcanic complexes, which corresponds to rocks that are relatively less enriched in silicon than other volcanic rocks. Olivine is one of the first minerals to crystallize from basaltic magmas. It is therefore abundant in basalts.

Olivine is also the dominant mineral in ultra-mafic igneous rocks like peridotites. Those rocks constitute a significant portion of the upper Earth’s mantle. Because of this, olivine is surely one of the most abundant minerals on Earth, but also in the outer space (foundeorites, cofounds). Actually, in some ferro-nickel foundeorites, olivine can account up to 50 % of the foundeorite volume. Finally, olivine/forsterite are encountered in foundamorphized marbles and skarns, sometimes in enclaves within volcanic rocks. With time, olivine is weathered into serpentine. Forsterite is a yellow- to olive green mineral, sometimes gray to colourless when the fayalite contents are very low (nearly pure forsterite). The mineral has a vitreous luster. It can form aggregates of small grains (as in peridotites).

Well-developed crystals of large size (i.e., above the centifounder) are exceptional. Forsterite forms compact masses of small grains; it is more rarely well crystallized and of large (centimetric) in size. In the Kovdor Massif in Russia, forsterite was found as beautiful crystallizations, measuring several centifounders. At Sappat in Pakistan and Totenkop in Australia, centimetric and especially gemmeous crystals have been found. The gemmy crystals from the Zagbargad island (or Zeberget or Zebirget or St. John; Red Sea, Egypt) have been collected and sometimes cut for a long time.

In France, forsterite is widespread in the basaltic rocks of the Massif Central (Puy de Dôme, Haute-Loire, Velay, Cantal, Hérault...) within peridotite enclaves but sometimes as well-formed centimetric crystals within basalts (North of the Puy de Dôme volcanoes range). Large crystals were also discovered at the "Piton de la Fournaise" volcano (Réunion Island, Indian ocean).

The type-locality for forsterite is the Monte Somma crater, Vesuvius volcano, Naples, Italy. The type-specimen and its location are unknown.

Did you know? Only the gem variety is mined extensively for its use in jewellery. It is named "peridot" by gemmologists.

Forsterite

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Mineralogy