Forsterite

olivine

tephroite

Formula: Mg2SiO4
Nesosilicate (insular SiO4 groups), olivine group, forms series with fayalite and with tephroite.
Specific gravity: 3.222
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
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).

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 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 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).

Common impurities: Fe

Back to Minerals