Fayalite

forsterite

tephroite

Formula: Fe2+2(SiO4)
Nesosilicate (insular SiO4 groups), olivine group, forms series with forsterite and with tephroite.
Properties of fayalite:
Specific gravity: 4.392
Hardness: 6½
Streak: White
Colour: Green, yellow, brown
Solubility: Insoluble in water and nitric acid; soluble in hydrochloric acid forming an insoluble silica gel
Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Metamorphic environments

Fayalite is uncommon in nature, but common in man-made slags. In nature it occurs in ultramafic plutonic and volcanic rocks, in obsidian, in felsic plutonic rocks and more rarely in granite pegmatites. It can also form in thermally metamorphosed iron-rich sediments, by both regional and contact metamorphism (Lauf p27). Manganese-rich members of the tephroite - fayalite series form primarily in skarn associated with iron-manganese deposits, and in metamorphic rocks formed from manganese-rich sediments (Lauf p24). Fayalite has been found in lunar rocks, although forsterite is the more common member of the olivine group there (Lauf p25).

At the Nain complex, Labrador, Canada, metamorphic fayalite occurs in olivine - garnet - hypersthene granulite (Lauf p27).

At Baveno, Italy, fayalite occurs in granite (Lauf p27).

At the Bjerkrem-Sogndal massif, Norway, fayalite occurs in monzonite (Lauf p27).

At the Mariupol' iron deposit, Ukraine, metamorphic fayalite occurs in peridotite (Lauf p27).

At Camas Moor, Scotland, UK, fayalite occurs in olivine gabbro (Lauf p27).

In the Sawtooth mountains, Custer county, Idaho, USA, fayalite occurs in pegmatites (Lauf p27).

At Rockport, Massachussets, USA, fayalite occurs as large crystals with amphibole in granite (Lauf p27).

Alteration

fayalite and H2O to magnetite, SiO2 and H2
3Fe2+2(SiO4) + 2H2O &38594; Fe2+Fe3+2O4 + 3SiO2 + 2H2
This reaction is highly exothermic (Wiki Serpentinite).

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4
(DHZ 1A p262)

fayalite, oxygen and H2O to hematite and silicic acid
2Fe2SiO4 + O2 + 4H2O → 2Fe2O3 + 2H4SiO4
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above (KB p334).

fayalite, SiO2 and H2O to grunerite
7Fe2+2(SiO4) + 9SiO2 + 2H2O → 2Fe2+2Fe2+5Si8O22(OH)2
Fayalite may undergo partial regression to grunerite according to the above reaction (DHZ 1A p265).

ferrosilite to fayalite and quartz
Fe2Si2O6 ⇌ Fe2SiO4 + SiO2
Ferrosilite is unstable at pressure less than about 15 kbar (DHZ 1A p55).

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

siderite and quartz to fayalite and CO2
2Fe(CO3) + SiO2 = Fe2+2(SiO4) + 2CO2
This reaction occurs in cherty rocks (DHZ 1A p265, Lauf p22)

Common impurities: Mn

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