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Forsterite
Current inventory:  0 gems
 

Chemistry:  Mg₂(SiO₄)  [Magnesium Silicate]

Discovered in 1824;   IMA status: Valid (pre-IMA; Grandfathered).
Forsterite is named after Adolarius Jacob Forster (1739 - 1806), English mineral collector and dealer.

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 and the Forsterite-Tephroite Series. The magnesium analogue of Fayalite, Tephroite and Calcio-Olivine. A polymorph of Ringwoodite and Wadsleyite

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

As euhedral or subhedral crystals, typically thick, with striations, 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 Radioacitve

Optical Properties

Color:

Colorless, Green, pale Yellow, White

Transparency:

Transparent, Translucent

Luster:

Vitreous

Refractive Index:

1.635 - 1.670  Biaxial ( + )

Birefringence:

0.0330 - 0.0420

Dispersion:

r > v or r < v

Pleochroism:

Weak; X = colorless, Y = colorless, Z = colorless

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

Type Locality:

Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy

Year Discovered:

1824

View mineral photos:

Forsterite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Forsterite is a rare member of the Olivine Group that includes Dunilite, Fayalite, Forsterite and Peridot although Peridot is not recognized as a separate mineral. Fayalite and Forsterite are simply known as Olivine. Fayalite is the iron rich member of the series and Forsterite is the magnesium rich member. Because of its iron content, Fayalite has a higher index of refraction, is heavier, and usually has a darker color. Otherwise, they are difficult to distinguish and virtually all specimens of the two minerals contain both iron and magnesium. The best color of Peridot has an iron content less than 15% and includes traces of nickel and chromium.

The gem variety of Forsterite is usually known as the bright green Peridot. But a recent find of colorless Forsterite in Mogok, Myanmar (Burma) has seemingly brought a new variety of gemstone to the market. Colorless Forsterite gems were virtually unheard of prior to this find.

Forsterite is also found in many iron-nickel meteorites as large crystals. Thinly cut and polished slices of these meteorites are extremely attractive with the bright steel gray of the iron contrasted by the gemmy green Forsterite crystals.

There are many locations to find green Forsterite but the main location for colorless Forsterite is in the Mogok district and at Pyaung Gaung, Myanmar (Burma).
 

  
Forsterite gems for sale:

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