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OlivineThe mineral olivine is an orthosilicate of magnesium with the formula
(Mg,Fe)2SiO4, in Olivine is actually a name for a series between two end members, Fayalite
and forsterite. Fayalite is the iron rich member with a pure formula of
Fe2SiO4. Forsterite is the magnesium rich member with a pure formula of
Mg2SiO4. The two minerals form a series where the iron and magnesium are
substituted for each other without much effect on the crystal structure.
Fayalite due to its iron content has a higher index of refraction, is
heavier and has a darker color than forsterite. Otherwise they are difficult
to distinguish and virtually all specimens of the two minerals contain
both iron and magnesium. For simplicity sake and general public recognition,
they are often treated as one mineral, olivine. Olivine however is not
officially recognized as a mineral (see other non-minerals such as apophyllite,
tourmaline, mica, serpentine, chlorite and apatite). Olivine's gemstone variety, known as peridot, is one of the most mispronounced of gemstone names. The correct pronunciation has peridot rhyming with doe or depot. But peridot is often pronounced incorrectly so that it rhymes with dot. Peridot is the birthstone of August and is usually a very affordable colored gemstone. Unfortunately it is often compared to the rich dark green of emerald and in this comparison it is often found lacking. But peridot has its own unique green-yellow color that is different from emerald and this comparison is rather unfair. Most peridot is actually the magnesium rich forsterite and its color is caused by the presence of iron ions. Fatality’s higher iron content make for darker, less attractive specimens that are not generally used as gemstones. The best colored peridot has an iron percentage less than 15% and includes nickel and chromium as trace elements that may also contribute to the best peridot color. Peridot is a colorful, affordable and attractive gemstone. Olivine is found in ultramafic igneous rocks and marbles that formed from metamorphosed impure limestones. Mafic is a word that is used to define igneous rocks with a high iron and magnesium content. The "MA" is for magnesium while the "F" is for ferrum, the latin word for iron. The olivine minerals have a high melting point and are the first minerals to crystallize from a mafic magma. Forsterite crystallizes first with fayalite crystallizing last when other minerals such as the pyroxenes are just beginning to form. The early crystallization of olivine is the reason that molten lavas can contain already crystallized grains of olivine. Some ultramafic rocks can be composed of almost all olivine and these are called dunites or peridotites. Peridotites contain the same chemical makeup as the molten magma in the Earth's mantle. Thus peridotite could be called the most common rock by volume in the Earth, although on the Earth's surface and in the crust it is not nearly as well represented.
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