Åkermanite

sorosilicate

metamorphic

Formula:Ca2MgSi2O7 Sorosilicate (Si2O7 groups), melilite group
Near end-member åkermanite is very rare, possibly unknown in nature.
Specific gravity: 2.90 to 2.97
Hardness: 5 to 6
Streak: White
Colour: Colourless, grey, green, brown
Melting point: 1,454oC at a pressure of 1 kbar
Environments:

Plutonic igneous environments
Metamorphic environments

Commonly found in slags. Åkermanite is a product of contact metamorphism of siliceous limestone and dolostone, and it also forms from alkalic magmas rich in calcium.
It is a common component of skarn, and a mineral of the sanidinite and granulite facies.

Alteration

As åkermanite cools from its melting point it is stable down to 1345oC, when the stable mixture is åkermanite and wollastonite. From 1240oC down to 1050oC a mixture of åkermanite, wollastonite and diopside is stable, and at lower temperatures åkermanite dissociates to form wollastonite and monticellite. The stability of åkermanite is limited to pressures of less than 14 kbar in an anhydrous environment, and less than 10.2 kbar with excess H2O (DHZ 1B p307).

åkermanite to wollastonite and monticellite
Ca2MgSi2O7 ⇌ CaSiO3 + CaMgSiO4
The forward reaction occurs at temperature less than 1,050oC (DHZ 1B p307)

åkermanite and CO2 to diopside and calcite
Ca2MgSi2O7 + CO2 ⇌ CaMgSi2O6 + CaCO3
The maximum stability limit of åkermanite in the presence of excess CO2 is about 6 kbar. Below that pressure, at relatively lower temperatures, åkermanite reacts with CO2 to form diopside and calcite according to the above reaction (JVW p144).

diopside and monticellite to åkermanite and forsterite
CaMgSi2O6 + 3CaMgSiO4 ⇌ 2Ca2MgSi2O7 + Mg2SiO4 (DHZ 1B p313)

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 ⇌ 2Ca2MgSi27 + 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).

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