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Peridot  (variety of Forsterite; Olivine Group)
Current inventory:  0 gems
 

Chemistry:  (Mg,Fe)₂SiO₄  [Magnesium Iron Silicate]

Forsterite: Discovered in 1824;   IMA status: Valid (pre-IMA; Grandfathered).
The name Peridot is from the thirteenth-century English peridota. Olivine is from the Latin oliva for olive because of the similarity in color.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/A.04-10

Nickel-Strunz 10th Ed. ID:

9.AC.05

9 : SILICATES (Germanates)
A : Nesosilicates
C : Nesosilicates without additional anions; cations in octahedral [6] coordination

Related to:

Olivine Group. Fayalite - Forsterite Series. Forsterite - Tephroite Series. The magnesium analogue of Fayalite, Tephroite and Calcio-Olivine. Polymorph of Ringwoodite and Wadsleyite.

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

In euhedral or subhedral crystals, typically thick, with striations parllel to elongation, with wedge-shaped terminations, to 17 cm. Commonly granular or compact massive.

Twinning:

On [100], [011], [012]

Physical Properties

Cleavage:

[010] Perfect, [100] Imperfect

Fracture:

Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

7.0

Density:

3.275 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Yellowish Green, Olive Green, Greenish Black, Reddish Brown

Transparency:

Transparent to Translucent

Luster:

Vitreous

Refractive Index:

1.635 - 1.772  Biaxial ( + )

Birefringence:

0.0330 - 0.0420

Dispersion:

Relatively Weak; r > v or r < v

Pleochroism:

None

Occurances

Geological Setting:

In mafic and ultramafic igneous rocks and thermally metamorphosed impure dolomitic limestones.

Common Associations:

Amphiboles, Antigorite, Augite, Brucite, Calcite, Chromite, Corundum, Diopside, Dolomite, Enstatite, Magnetite, Phlogopite, Plagioclase, Spinel.

Common Impurities:

Cr, Fe

Type Locality:

Peridot: Prehistoric; Forsterite: Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy

Year Discovered:

Peridot: Prehistoric; Forsterite: 1824

View mineral photos:

Olivine Mineral Photos and Locations

More Information

Mindat.org  (Peridot)
Mindat.org  (Forsterite)
Mindat.org  (Olivine)
Webmineral.com  (Forsterite)
Webmineral.com  (Olivine) 

Peridot is the green variety of the Olivine Group of minerals. Olivine is not an individual mineral but a group of minerals that includes Fayalite, Forsterite, Liebenbergite, Tephroite and Laihunite. Fayalite (Mg2SiO4) is the iron rich member of the Fayalite - Forsterite mineral series, Forsterite (Fe2SiO4) is the magnesium rich member. Peridot (Mg2Fe2SiO4) is the green variety of Forsterite containing both Iron (Fe) and magnesium (Mg). Peridot's color can vary from pale to rich green (for which Peridot is famous) to brown. The considerable variation in shades of green to brown is dependant on locality of origin and amount of ferrous iron. The ideal green Peridot has an iron content of about 12-15%. More iron results in a muddy color. Very bright green Peridot may also contain traces of chromium.

Peridot is not a recognized mineral name. It's hard to believe that Peridot is the equivalent of a trade name, not a recognized mineral name since it is one of the most popular and well known gems in the world. Peridot has been a popular gem throughout history.

Peridot is available from many localities worldwide including these well known sources: Mogok, Sagaing District, Mandalay Division, Burma (Myanmar); St John's Island (Zagbargad; Zebirget; Topazios), Red Sea, Egypt; Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan; and Peridot Mesa, San Carlos, San Carlos Indian Reservation, Gila Co., Arizona, USA
 

  
Peridot gems for sale:

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