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Peridot

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Olivine, and the gem variety thereof known as Peridot. Olivine receives its name from its usual olive-green color.  The evolution of the term "olivine group" is complex, with universal acceptance of the name occurring in the twentieth century.


ï  Mineral: Olivine

ï  Chemistry: M2SiO4 where M = Ca, Fe, Mn, Ni, Mg

ï  Class: Orthosilicate

ï  Crystal system: Orthorhombic

ï  Color: Olive green, Yellowish green to bright green, Greenish black, Brownish green to brown

ï  Refractive index:  1.630 to 1.690

ï  Luster: Vitreous

ï  Specific gravity: 3.2-4.4

ï  Mohs Hardness: 6.5 - 7

ï  Cleavage: Poor

ï  Fracture: Brittle - Conchoidal


Olivine is a group name for those silicate minerals that form a solid solution between the pure end member forsterite (Mg2SiO4) and pure fayalite (Fe2Si4).  Magnesium and iron can freely substitute for each other, in any ratio, in the mineral's atomic structure.  Chemical analysis or other laboratory testing is required to determine which end member is dominant, if either.  Hence "olivine" is a quick and convenient way to put a name to the specimen.  A list of the more common olivine minerals and their composition include - Forsterite  Mg2SiO4; Fayalite  Fe2SiO4; Monticellite  CaMgSiO4; Kirschsteinite  CaFeSiO4; and Tephrolite  Mn2SiO4.


Olivines are the primary component of the Earth's upper mantle, common rock-forming minerals in Earth's crust and a component of many meteorites.  Olivines are found in mafic and ultramafic rocks (basalt, gabbro, dunite, diabase & peridotite) and in metamorphic rocks with high magnesium and low silica content (serpentine or dolomitic marble).  

Experimental data indicates that Mg-rich olivine at high pressure (12GPa - the pressure at depths of 360km/220miles) can contain 8900ppm of water.  Due to the abundance of olivine in the mantle, more water may be dissolved in olivine/the mantle than is contained in Earth's oceans.

Olivine is used in steel works - to tap blast furnaces and to remove impurities.  It is used as a sand for casting objects in aluminum foundries.  In Finland, it is marketed as the ideal rock for sauna stoves.  Its widest use these days is as the gemstone peridot Fe2SiO4. Many people are familiar with olivine because it is the mineral of a very popular green gemstone known as peridot.  

It is easily identified as the only common green-colored igneous mineral, transparent to translucent with a glassy luster, hardness between 6.5 to 7, and conchoidal fracture.


Olivine is one of the first minerals to crystallize from a magma and one of the first minerals to be altered by weathering.

Norway is the main source of olivine in Europe, about 50% of the world's olivine for industrial use is produced there. Locations for Olivine is common worldwide. Most of the mass-produced jewelry grade Peridot is mined at the San Carlos Reservation in Arziona.  The stones are typically a few carats or less.  Larger Peridot crystals of higher quality are mined in Pakistan and Myanmar.  Egyptian deposits produced much of the Peridot during ancient times. 

Some of the locales from which specimens of Olivine/Peridot are available at Taos Rockers include:

Arizona

Colorado

New Mexico (see Kilbourne Hole articles)

Pakistan


Peridot is a birthstone for the month of August ..


Melody, in her book Love Is In The Earth,says of Olivine: "....an excellent healing stone, acting as a "tonic" to both strengthen and regenerate the body...It provides for a shield of protections around the body and should be removed from ones person prior to balancing and the aligning the physical body with the outer bodies and prior to cleansing chakras...It also helps to regulate cycles in ones life."

 

References:  mindat.org; wikipedia.org; Love is in the Earth - Melody; and geology.com

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