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Formula: Fe2+2(SiO4)
Nesosilicate (insular SiO4 groups),
olivine group, forms series with
forsterite and with tephroite.
Varieties
Hortonolite is manganese and magnesium bearing variety of fayalite
Properties of fayalite
Crystal System: Orthorhombic
Specific gravity: 4.39 measured
Hardness: 6½
Streak: White
Colour: Green, yellow, brown
Solubility: Insoluble in water and nitric acid; soluble in hydrochloric acid forming an insoluble silica gel
Common impurities: Mn
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-enstatite-ferrosilite
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 greenalite
3Fe2SiO4 + SiO2 aqueous + 4H2O →
2Fe3Si2O5(OH)4
CM 36.157).
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
greenalite to grunerite,
fayalite and H2O
9Fe3Si2O5(OH)4 →
Fe2+2Fe2+5Si8O22(OH)2 +
10Fe2SiO4 + 17H2O
CM 36.157).
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)
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