Olivine

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Chemical Name:	Magnesium Iron Silicate
Chemical Formula:	(Mg,Fe)2SiO4

Olivine is not officially a mineral, but is actually the name for a series ranging from fayalite to forsterite. Fayalite is the iron based member with the formula Fe₂SiO₄. Forsterite is the magnesium based member with the formula Mg₂SiO₄. Ranging between the two, Iron and Magnesium are substituted for each other with very little effect on the crystalline structure. Almost all naturally occuring samples contain some of both iron and magnesium. Olivine can be found in virtually all mafic (Ma for magnesium, F for the Latin ferrum) magmas, has a high melting point and is usually the first mineral to solidify.

Olivine has a gemstone variety called Peridot (rhymes with depot or doe). Peridot is actually the magnesium rich forsterite and it's color is caused by the presense of iron ions. The gem has a green yellow color and is sometimes called "Evening Emerald" by jewelers in an attempt to increase its value.