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Formula: CaMg(SiO4)
Nesosilicate (insular SiO4 groups), olivine group
Crystal System: Orthorhombic
Specific gravity: 3.03 to 3.27 measured, 3.058 calculated
Hardness: 5½
Streak: White
Colour: Colourless, grey or greenish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with gelatinous residue in hydrochloric acid
Common impurities: Ti,Al,Fe,Mn,Zn,H2O
Environments:
Volcanic igneous environments
Carbonatites
Metamorphic environments
Monticellite occurs as a relatively common mineral formed during metamorphism of siliceous
olivine
dolostones, in
contact metamorphic deposits between
limestones
and olivine gabbros and in
skarns at
granite-dolomitic
limestone contacts. It occurs less frequently in
kimberlites.
It is a mineral of the granulite facies.
Alteration
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, monticellite
åkermanite to monticellite and
wollastonite
Ca2MgSi2O7 → CaMg(SiO4) + CaSiO3
diopside, forsterite and
calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 +
2CO2
This reaction requires a high temperature
(DHZ 2A p271).
forsterite and
åkermanite to
diopside and monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 +
3CaMg(SiO4)
forsterite,
calcite and quartz to
monticellite and CO2
Mg2SiO4 + 2 CaCO3 + SiO2 → 2CaMg(SiO4) + 2 CO2
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).
grossular, diopside, monticellite,
calcite and H2O to
vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4)
+ 2CaCO3 + 8H2O ⇌
2Ca19Al10Mg3(SiO4)10
(Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability
will tend to increase with increasing water and decrease as the activity of CO2 rises.
(DHZ 1A p714).
monticellite and CO2 to
åkermanite, forsterite and
calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi27 +
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
At a pressure of 4.3 kbar the equilibrium temperature is about 890oC
(granulite facies)
(DHZ 1A p357).
monticellite and
spurrite to
merwinite and
calcite
2CaMg(SiO4) + Ca5(SiO4)2(CO3) ⇌
2Ca3Mg(SiO4)2 + CaCO3
monticellite, spurrite and quartz to
merwinite and CO2
5CaMg(SiO4) + Ca5(SiO4)2(CO3) + SiO2 ⇌
5Ca3Mg(SiO4)2 + 2CO2
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