Monticellite

nesosilicate

metamorphic

Formula: CaMg(SiO4) nesosilicate (insular SiO4 groups) olivine group
Specific gravity: 3.2
Hardness: 5½
Streak: White
Colour: Colourless, grey or greenish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with gelatinous residue in hydrochloric acid
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.

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.

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.

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).

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).

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

Common impurities: Ti,Al,Fe,Mn,Zn,H2O

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