Fayalite - Encyclopedia

Class : Silicates
Subclass : Nesosilicates
Crystal system : Orthorhombic
Chemistry : Fe₂SiO₄
Rarity : Common

Fayalite is the iron-bearing pole of the olivine group. It forms two series, one with forsterite (magnesian) and the other with tephroite (manganiferous). Fayalite is rarer than forsterite. It is mainly found in metamorphic sediments rich in iron, but can be present in ultrabasic magmatic rocks, occasionally in granites and pegmatitic veins, it is also present in neoformation in foundry slag. It takes its name from its discovery location : Fayal Island in the Azores (Portugal). The crystals are thin to thick, frequently ending in bevels, sometimes centimetric in size. Fayalite is brown in color, sometimes brownish gray. It has a very marginal role in the refractory industry.

Main photo : Fayalite from Caspar quarry, Ettringen, Vordereifel, Germany © Michael Förch

Fayalite from Coso Hot Springs, Inyo County, California, USA © Matteo Chinellato

Fayalite de Coso Hot Springs, Inyo County, California, USA © Tony Peterson

Fayalite from Mount Somma, Vesuvius, Italy © Gianfranco Ciccolini

Fayalite from Caspar quarry, Ettringen, Vordereifel, Germany © Volker Betz

Fayalite in the World

Fayalite is found in beautiful crystals in the basalts of Fayal (Azores) and the Skaergaard intrusion (Greenland). It is also reported in beautiful crystals at Crestmore (California), in the marbles of Bolton (Massachussetts), and in the obsidian lithophyses of Obsidian Cliff in Yellowstone Park (Wyoming). Many basalts from the Scottish and Italian islands also contain crystals.

Fayalite in France

In France, fayalite is known at Roc du Capucin (Puy-de-Dôme), in the mica-schists of Collobrières (Var), in Evisa and Tolla (Southern Corsica), on the Kerguelen Islands and in the slag of Lapanouse (Aveyron) and Vialas (Lozère).

Twinning

No twin known for this mineral species.

Fakes and treatments

Large centimeter-sized crystals can be produced in foundries and identified as coming from Romania (Baia Sprie), but they are rare on the mineral market.

Right photo : Fayalite analyzed by the University of Vienna given as coming from Baia Sprie, Maramures, Romania - 12 cm group of crystals © Vilhelm Farago

Hardness : 7
Density : 4.39
Fracture : Conchoidal
Streak : White

TP : Translucent to transparent
RI : 1.731 to 1.875
Birefringence : 0.042 to 0.051
Optical character : Biaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Hydrochloric acid

Magnetism : Paramagnetic
Radioactivity : None

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