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Fayalite
Current inventory:  1 gem
 

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

Discovered in 1840;   IMA status: Valid (pre-IMA; Grandfathered).
Fayalite is named after Fayal Island in the Azore Islands, Portugal.

Classification

Mineral Classification:

Silicates

Strunz 8th Ed. ID:

8/A.04-20

Nickel-Strunz 10th Ed. ID:

9.AC.05

9 : SILICATES (Germanates)
A : Nesosilicates
C : Nesosilicates without additional anions; cations in octahedral [6] coordination
05 : Olivine Group; Fayalite-Forsterite series

Crystal Data

Crystallography:

Orthorhombic - Dipyramidal

Crystal Habit:

Crystals are thick to thin, tabular, typically with wedge-shaped terminations, producing eight-sided cross sections. Commonly granular, compact, or massive.

Twinning:

On [100]; also on [031], as trillings.

Physical Properties

Cleavage:

[010] Imperfect, [100] Imperfect

Fracture:

Conchoidal

Tenacity:

Brittle

Hardness (Mohs):

6.5 - 7.0

Density:

4.392 (g/cm³)

Luminescence:

None

Radioactivity:

Not Radioactive

Optical Properties

Color:

Greenish Yellow, Yellow-Brown, Brown, rarely Colorless

Transparency:

Transparent, Translucent

Luster:

Vitreous, Greasy

Refractive Index:

1.731 - 1.875  Biaxial ( - )

Birefringence:

0.042 - 0.051

Dispersion:

Weak; r > v

Pleochroism:

Faint; X = Z = pale yellow; Y = yellow-orange, reddish brown.

Occurances

Geological Setting:

In ultramafic volcanic and plutonic rocks, less commonly in felsic plutonic rocks; rarely in granite pegmatite; in lithophysae in obsidian. In metamorphosed iron-rich sediments and impure carbonate rocks.

Type Locality:

Faial Island (Fayal Island), Azores District (Azores Islands), Portugal

Year Discovered:

1840

View mineral photos:

Fayalite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Fayalite is a very rare member of the Olivine group of minerals that includes Dunilite, Fayalite, Forsterite and Peridot (Olivine)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.

Fayalite is found at several locations around the world and also in many iron-nickel meteorites. Fayalite was even found at the Sea of Tranquility on the Moon in lunar basalt by the Apollo 11 and 14 mission astronauts.
 

  
Fayalite gems for sale: