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Olivine is a series between forsterite
Mg2SiO4 and
fayalite Fe2SiO4. These are both nesosilicates
(insular SiO4 groups).
Crystal System: Orthorhombic
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:
Plutonic igneous environments
Volcanic igneous environments
Metamorphic environments
Olivine is a common primary, rock-forming mineral, varying in
amount from an accessory to a major
constituent. It is the first major mineral to crystallise in the discontinuous branch of the
Bowen reaction series.
Olivine is an essentail constituent of
peridotite and
kimberlite.
It is a common constituent of
gabbro,
dunite,
peridotite and
basalt.
It also may be found in
andesite,
diorite,
gabbro and
granite.
In peridotite olivine is typically associated with chromium-rich
spinel and pyroxene. Some
peridotite intrusions also contain
garnet, and in these the olivine is very manganese-rich
(Lauf p21).
Olivine nodules are commonly found in alkaline basalt; those from
shallower levels in the mantle are spinel-bearing, and those from deeper levels are
garnet-bearing
(Lauf p21).
Olivine can form from the progressive metamorphism of serpentinite,
such as in the cascades, Washington, USA, where in the contact aureole of a
granodiorite intrusion
lizardite - chrysotile
serpentinite was transformed first to
antigorite serpentinite,
then olivine serpentinite, then an assemblage of
forsterite, talc and
tremolite
(Lauf p22).
Olivine is associated with plagioclase feldspar,
pyroxene,
magnetite,
corundum,
chromite and
serpentine.
quartz never occurs with olivine.
Olivine is a mineral of the granulite facies.
Localities
At Maimecha-Kotuy, Krasnoyarsk Krai, Russia, gem olivine is localised in veins of
clinohumite - serpentine -
phlogopite - olivine
(GAHK 2019 p14).
Alteration
cummingtonite-grunerite and
olivine to enstatite-ferrosilite
and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌
9(Mg,Fe2+)SiO3 + H2O
(DHZ 2A p138)
enstatite-ferrosilite,
Fe-rich diopside and Fe, Cr-rich spinel
to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4
⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 +
(Mg,Fe)2SiO4
(DHZ 2A p258)
Mg-rich greenalite to olivine,
Mg-rich grunerite and H2O
18(Fe2+, Mg))3Si2O5(OH)4 →
20(Fe,Mg)2SiO4 + 2(Fe2+,Mg)7Si8O22(OH)2
+ 34H2O
(DHZ 1A p266)
Mg-rich greenalite to olivine,
SiO2 and H2O
2(Fe2+, Mg))3Si2O5(OH)4 →
3(Fe,Mg)2SiO4 + SiO2 + 4H2O
(DHZ 1A p265)
enstatite-ferrosilite,
augite and Fe and Cr-rich spinel to
garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌
Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4
(DHZ 2A p258)
olivine and CO2 to enstatite-
ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 +
(Mg,Fe)CO3
(DHZ 2A p139)
olivine and H2O to serpentine,
magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O →
3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4
+ H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite
(R&M 90.6.521).
olivine and quartz to enstatite -
ferrosilite
(Mg,Fe)2SiO4 + SiO2 → 2(Mg,Fe2+)SiO3
(DHZ 2A p139)
orthopyroxene, Fe-rich diopside and
Fe and Cr-rich spinel to
Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 +
(Mg,Fe)(Al,Cr)2O4 ⇌
(Mg,Fe)2Ca(Al,Cr)2Si3O12 +
(Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing
peridotites
are considered to have originated in a high-pressure environment according to the above reaction
(DHZ 2A p123).
Mg-rich siderite and quartz to
olivine,
orthopyroxene and CO2
3(Fe,Mg)(CO3)→ (Fe,Mg)2SiO4 + 2SiO2 →
(Fe,Mg)2SiO4 + 3CO2
(DHZ 1A p266)
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