Images
Formula: Mg2SiO4
Nesosilicate (insular SiO4 groups),
olivine group, forms a series with
fayalite
and with tephroite.
Crystal System: Orthorhombic
Specific gravity: 3.275 measured, 3.271 calculated
Hardness: 6½ to 7
Streak: White
Colour: Yellow, green, brown, black
Melting point: About 1,900oC at atmospheric pressure
(JVW p275)
Solubility: Insoluble in water and sulphuric acid; soluble in hydrochloric and nitric acid forming an insoluble
silica gel
Common impurities: Fe
Environments:
Volcanic igneous environments
Carbonatites
Metamorphic environments
Forsterite is a primary mineral occurring in
mafic and
ultramafic igneous rocks, and also in metamorphosed impure
dolostone and skarn, where
it can be produced by both regional and
contact metamorphism; it is also found as glassy grains in stony
and stony-iron meteorites. It is an important constituent of dunite and
peridotite, and is commonly found in
kimberlite, alkaline
basalt,
gabbro, and some volcanic rocks
(Lauf p28).
Forsterite occurs in the mantle above a depth of about 400 km. If the melt is low in silica, but above
average in magnesium
and iron, olivine will crystallise out.
Forsterite is often associated with
pyroxene,
plagioclase feldspar,
magnetite,
corundum,
chromite and
serpentine.
Forsterite is a mineral of the
blueschist
greenschist,
amphibolite and
granulite facies.
At Jacupiranga, Brazil, forsterite occurs in the carbonatite deposit
(Lauf p28).
At the Parker mine, Notre Dame du Laus, Quebec, Canada, forsterite occurs in
limestone associated with black
spinel
(Lauf p28).
At St John's island, Egypt, forsterite occurs in serpentinised
peridotite (Lauf p28).
At Dun mountain, New Zealand, forsterite occurs in dunite
(Lauf p28).
At Tafjord, south Norway, forsterite occurs in garnet
peridotite (Lauf p28).
Sapat Gali, Naran, Kaghan Valley, Mansehra District, Khyber Pakhtunkhwa Province, Pakistan, is a remote,
high-altitude string of pits where large gem-quality crystals of forsterite (peridot) have been found. They
can reach 15 cm long and 2 kg in weight, and they occur in veins, shears and open pockets in
dunite; they were probably deposited after the main period of
serpentinisation, from a hydrothermal fluid rich in carbon dioxide
and boron. The forsterite crystals are nearly always accompanied by
serpentine with minor magnetite,
magnesite, talc,
calcite and chromium-rich
clinochlore. Rarely, horsetail inclusions of black
ludwigite may be seen in the gemmy interiors.
Forsterite from Sapat is transparent to translucent and pale to dark yellowish green or greenish yellow in
colour; pure forsterite would be colourless, but iron replacing some of the magnesium produces the green
shades. The colour may also be related in part to a higher titanium content.
The mineral does not fluorescence under long wave or short wave UV
(MinRec 51.6.785-801).
At Kolonne, Uva province, Sri Lanka, forsterite occurs with pargasite
(Lauf p28).
At Prairie creek, Arkansas, USA, and Alno Island, Sweden, forsterite occurs in
kimberlite
(Lauf p26).
In Hawaii, USA, forsterite occurs in scoria
(Lauf p28).
At Bolton, Massachussets, USA, forsterite is found in limestone associated
with diopside and actinolite, and
more rarely with chondrodite, apatite
and titanite. Scapolite is rarely
found with forsterite (Lauf p31).
Alteration
The main alteration products of forsterite in ultrabasic rocks are the three
serpentine minerals, lizardite,
antigorite and chrysotile.
Pseudomorphs of serpentine after forsterite are known (Lauf p26).
During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral
formation is often found, beginning with the lowest temperature product:
talc,
tremolite,
diopside,
forsterite,
wollastonite,
periclase and
monticellite.
anthophyllite and forsterite to enstatite
and H2O
2☐Mg2Mg5Si8O22(OH)2 + 2Mg2SiO4 ⇌
9Mg2Si2O6 + 2H2O
At 2 kbar pressure the equilibrium temperature is about 690oC
(pyroxene-hornfels facies). The equilibrium moves
to the right at
higher temperatures and to the left at lower temperatures
(SERC).
antigorite to
forsterite, talc and H2O
5Mg3Si2O5(OH)4 = 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
This reaction may occur in olivine-diopside-
antigorite schist within
the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which
produces forsterite and tremolite
(DHZ 1A p258).
At 10 kbar pressure the equilibrium temperature is about 600oC
(amphibolite facies), with equilibrium to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(SERC).
antigorite and
calcite to
forsterite,
diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 →
4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar
pressure and 400 to 500oC (greenschist facies)
(DHZ 1A p263).
antigorite and
magnesite to
forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4
+ CO2 + 2H2O
(DHZ 1A p263)
brucite and antigorite to forsterite
and H2O
Mg(OH)2 + Mg3Si2O5(OH)4 ⇌ 2Mg2SiO4
+ 3H2O
The equilibrium temperature for this reaction at 8 kbar pressure is about 450oC
(greenschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures. The reaction also may occur in the
albite-epidote-hornfels and
blueschist facies
(SERC).
corundum and forsterite to spinel and
enstatite
2Al2O3 + 2Mg2SiO4 ⇌ 2MgAl2O4 +
Mg2Si2O6
At 10 kbar pressure the equilibrium temperature is about 570oC
(amphibolite facies). The equilibrium moves to the right at
higher temperatures and to the left at lower temperatures
(SERC).
diopside and antigorite to forsterite,
Mg-rich tremolite and H2O
2CaMgSi2O6 + 5Mg3Si2O5(OH)4 ⇌
6Mg2SIO4 + Ca2Mg5Si8O22(OH)2 + 9H2O
At 10 kbar pressure the equilibrium temperature is about 580oC
(amphibolite facies)
(SERC).
diopside and dolomite to
forsterite,
calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 →
2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC
(MOM, DHZ 5B p213).
diopside, forsterite and
calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 +
2CO2
This reaction requires a high temperature
(DHZ 2A p271).
dolomite and
quartz to
forsterite,
calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3
+ 2CO2
In siliceous dolostone dolomite and
quartz may react to form either diopside
or forsterite, with diopside forming at a lower temperature
than forsterite
(DHZ 2A p270, 1A p264).
dolomite and tremolite to
forsterite, calcite, CO2
and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2
→ 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
(DHZ 5B p213)
dolomite, tremolite and
forsterite to
diopside, enstatite
and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌
2CaMgSi2O6 + H2O
At a pressure of 4 kbar the equilibrium temperature is about 840oC
(granulite facies)
(JVW p97).
enstatite and H2O to
forsterite and
cummingtonite
9MgSiO3 + H2O = Mg2SiO4
+ Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism
according to the above reaction
(DHZ 1A p259).
enstatite and calcite to forsterite,
diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 +
2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above
(DHZ 2A p135).
Al-rich enstatite and Al-rich diopside
to forsterite, enstatite,
diopside and anorthite
Mg9Al2Si9O30 +
Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4
+ 3Mg2Si2O6 + 3CaMgSi2O6 +
2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure
(DHZ 1A p233).
enstatite and spinel to forsterite and
cordierite
5Mg2Si2O6 + 2MgAl2O4 ⇌ 5Mg2SiO4 +
Mg2Al4Si5O18
At 4 kbar pressure the equilibrium temperature is about 715oC
(amphibolite facies). The equilibrium moves to the right at
higher temperatures and to the left at lower temperatures
(SERC).
forsterite and CO2
to enstatite and
magnesite
Mg2SiO4 + CO2 ⇌ MgSiO3 + MgCO3
(DHZ 2A p105)
forsterite and H2O to
serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4
+ Mg(OH)2
The forward reaction is highly exothermic.
At 5 kbar pressure the equilibrium temperature is about 420°C
(greenschist facies)
(WJ).
forsterite, SiO3 and H2O to
serpentine
3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5
(OH)4
This reaction is highly exothermic
(Wiki Serpentinite).
forsterite and
åkermanite to
diopside and
monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 +
3CaMg(SiO4)
forsterite and
anorthite to clinoenstatite,
diopside and
spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 +
CaMgSi2O6 + MgAl2O4
forsterite and anorthite to
enstatite, diopside and
spinel
2Mg2SiO4 + Ca(Al2Si2O8) =
Mg2Si2O6 + CaMgSi2O6 +
MgAl2O4
(DHZ 1A p242)
forsterite, calcite and quartz to
diopside and CO2
Mg2SiO4 + 2CaCO3 + 3SiO2 → 2CaMgSi2O6 +
2CO2
In high temperature environments with excess SiO2 diopside may form according to the above reaction
(DHZ 2A p271).
forsterite,
calcite and quartz to
monticellite and CO2
Mg2SiO4 + 2CaCO3 + SiO2 → 2CaMg(SiO4) + 2CO2
forsterite and cordierite to pyrope and
quartz
2Mg2SiO4 + Mg2Al4Si5O18 ⇌
2Mg3Al2(SiO4)3 + SiO2
Increasing pressure favours the forward reaction
(SERC).
forsterite,
diopside and calcite to
monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) +
2 CO2
This reaction occurs during contact metamorphism of magnesian
limestone
(DHZ 1A p353).
forsterite, dolomite and H2O to
calcite, hydroxylclinohumite
and CO2
4Mg2SiO4 + CaMg(CO3)2 + H2O →
Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich
dolomite in the aureole of the Alta
granodiorite in Utah, USA, is
probably due to the above reaction
(DHZ 1A p264).
forsterite, enstatite and H2O to
serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O →
2Mg3Si2O5(OH)4
serpentine is not stable in the presence of carbon dioxide, and may further
react with it to form talc and
magnesite
(R&M 90.6.521).
forsterite,
fayalite, H2O and CO2 to
serpentine,
magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 →
12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4
forsterite, kyanite and quartz to
cordierite
Mg2SiO4 + 2Al2OSiO4 + 2SiO2 ⇌
Mg2Al4Si5O18
At 6 kbar pressure the equilibrium temperature is about 400oC
(greenschist facies)
(SERC).
forsterite and kyanite to
cordierite and spinel
5Mg2SiO4 + 10Al2OSO4 ⇌ 3Mg2Al4Si5O18 +
4MgAl2O4
At 400oC the equilibrium pressure for this reaction is about 4 kBars; increasing temperature favours the forward reaction
(SERC).
forsterite and kyanite to spinel and
pyrope
5Mg2SiO4 + 4Al2OSiO4 ⇌ MgAl2O4 + 3Mg3Al2
(SiO4)3
Increasing temperature favours the forward reaction
(SERC).
forsterite and
quartz to enstatite
Mg2SiO4 + SiO2 → 2MgSiO3
forsterite is not stable in the presence of free SiO2 and will react with it to form
enstatite according
to the above reaction.
forsterite and talc to
anthophyllite and H2O
4Mg2SiO4 + 9Mg3Si4O10(OH)2 ⇌
5Mg2Mg5Si8O22(OH)2 + 4H2O
At 2 kbar pressure the equilibrium temperature is about 650oC
(pyroxene-hornfels facies), with equilibrium
to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(DHZ 1A p258, SERC).
forsterite and talc to
enstatite and H2O
Mg2SiO4 + Mg3Si4O10(OH)2 ⇌
5MgSiO3 + H2O
(JVW p103)
At 10 kbar pressure the equilibrium temperature is about 680oC
(amphibolite facies), with equilibrium to the right at
higher temperatures and to the left at lower temperatures (for the same pressure)
(SERC).
kyanite and forsterite to enstatite and
spinel
2Al2OSiO4 + 4Mg2SiO4 ⇌ 3Mg2Si2O6 +
2MgAl2O4
Increasing temperature favours the forward reaction
(SERC).
labradorite, albite,
forsterite and
diopside to omphacite,
garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) +
3Mg2SiO4 + nCaMgSi2O6 →
(2NaAlSi2O6 + nCaMgSi2O6) +
3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure
(DHZ 2A p449).
monticellite and CO2 to
åkermanite, forsterite and
calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi2O7 +
Mg2O7 + CaCO3
At 4.3 kbar pressure the equilibrium temperature is about 890oC
(granulite facies)
(DHZ 1A p357).
monticellite and diopside to
åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi2O7 +
Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar
(granulite facies)
(DHZ 1A p357).
nepheline and diopside to
åkermanite, forsterite and albite
3NaAlSiO4 + 8CaMgSi2O6 ⇌ 4Ca2MgSi2O7 +
2Mg2SiO4 + 3NaAlSi3O8
This reaction is in equilibrium at about 1180oC, with lower temperatures favouring the forward reaction
(DHZ 4 p251).
quartz, kyanite and forsterite to
pyrope
SiO2 + 2Al2OSiO4 + 3Mg2SiO4 ⇌
2Mg3Al2(SiO4)3
Increasing temperature favours the forward reaction
(SERC).
serpentine to forsterite,
talc and H2O
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
At a pressure of 10 kbar the equilibrium temperature is about 600oC. Increasing temperature favours the forward reaction
(SERC).
serpentine and
brucite to forsterite and H2O
Mg3Si2O5(OH)4 + Mg(OH)2 ⇌
2Mg2SiO4 +
3H2O
This reaction can proceed in either direction, depending on the external conditions. Early formation of forsterite
(DHZ 1A p259).
serpentine and diopside to
forsterite and talc
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
In olivine-diopside-
antigorite schist within
the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which
produces forsterite and tremolite.
serpentine and diopside
to tremolite, forsterite and
H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌
Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 +
9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite
and forsterite according to the above reaction
(DHZ 2A p271).
Fe and Cr-rich spinel, diopside and
enstatite to forsterite,
anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 +
Mg2Si2O6
⇌
2Mg2SiO4 + Ca(Al2Si2O8) +
Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure
(DHZ 1A p233).
spinel, forsterite and cordierite to
pyrope
MgAl2O4 + 5Mg2SiO4 + 2Mg2Al4Si5O18 ⇌
5Mg3Al2(SiO4)3
Increasing pressure favours the forward reaction
(SERC).
spinel and
tremolite to forsterite and
magnesio-hornblende
MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌
Mg2SiO4 +
Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite
(DHZ 1A p261).
tremolite and dolomite to
forsterite, calcite, CO2
and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 →
8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
(DHZ 1A p264)
tremolite and forsterite to
diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌
2CaMgSi2O6 + H2O
The equilibrium temperature for this reaction at 4 kbar pressure is 840oC
(granulite facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(JVW p97).
Back to Minerals