PERIDOT
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PERIDOT
Etymology and Appearance: The origin of the name "peridot" is uncertain, possibly derived from Anglo-Norman or Arabic words. It is a gemstone that occurs only in an olive-green color, with its intensity and tint depending on iron content. Peridot can vary from yellow to olive to brownish-green.
Occurrence: Olivine is the source of peridot. Gem-quality peridot is rare due to its chemical instability on Earth's surface. Large crystals of forsterite, used to cut peridot gems, are also rare. Peridot can be found in meteorites and can be differentiated by size and composition. The San Carlos Apache Indian Reservation in Arizona is the principal source of peridot olivine today, with other sources around the world.
Cultural History: Peridot has been prized for its alleged protective powers, though there is no scientific evidence for such claims. It is sometimes mistaken for emeralds and other green gems. Peridot is the birthstone for August, and the largest cut peridot olivine is a 310-carat specimen in the Smithsonian Museum.
The earliest use in England is in the register of the St Albans Abbey, in Latin, and its translation in 1705 is possibly the first use of peridot in English. It records that on his death in 1245, Bishop John bequeathed various items, including peridot, to the Abbey.[3]
Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green with no secondary yellow hue or brown mask. Lighter colored gems are due to lower iron concentrations.[4]
The molecular structure of peridot consist of isomorphic olivine, silicate, magnesium and iron in an orthorhombic crystal system. In an alternative view, the atomic structure can be described as a hexagonal, close-packed array of oxygen ions with half of the octahedral sites occupied with magnesium or iron ions and one-eighth of the tetrahedral sites occupied by silicon ions.
Oxidation of peridot does not occur at natural surface temperature and pressure, but begins to occur slowly at 600 C (870 K) with rates increasing with temperature.[5] The oxidation of the olivine occurs by initial breakdown of the fayalite component, and subsequent reaction with the forsterite component, to give magnetite and orthopyroxene.
Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, often found in lava and in peridotite xenoliths of the mantle, which lava carries to the surface; however, gem-quality peridot occurs in only a fraction of these settings. Peridots can also be found in meteorites.
Peridots can be differentiated by size and composition. A peridot formed as a result of volcanic activity tends to contain higher concentrations of lithium, nickel and zinc than those found in meteorites.[6]
Olivine is an abundant mineral, but gem-quality peridot is rather rare due to its chemical instability on Earth's surface. Olivine is usually found as small grains and tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite, the variety most often used to cut peridot gems, are rare; as a result peridot is considered to be precious.
The principal source of peridot olivine today is the San Carlos Apache Indian Reservation in Arizona.[8] It is also mined at another location in Arizona, and in Arkansas, Hawaii, Nevada, and New Mexico at Kilbourne Hole, in the US; and in Australia, Brazil, China, Egypt, Kenya, Mexico, Myanmar (Burma), Norway, Pakistan, Saudi Arabia, South Africa, Sri Lanka, and Tanzania.[citation needed]
Peridot crystals have been collected from some pallasite meteorites. The most commonly studied pallasitic peridot belongs to the Indonesian Jeppara meteorite, but others exist such as the Brenham, Esquel, Fukang, and Imi