Equilibria decarbonization

Siderite + hematite + magnetite equilibrium depends to a considerable extent on pressure, inasmuch as the slope of the P-T curves of decarbonation reactions are much steeper than those of dehydration reactions. It is presumed that the fluid consists mainly of carbon dioxide. Therefore the reaction in question does not strictly determine the lower temperature limit of the greenschist facies, and siderite plus hematite are often retained alongside grunerite all the way to temperatures of the order of 380-420°C at Pqq = 5-7 kbar. 

The above discussion suggests that there may be a number of reaction paths by which dolomite and calcite can decarbonate. Campbell has presented the evidence for reactions (1) and (2) being spontaneous at 600°C and at 1 atm pressure (1, 20). Metamorphic studies give arguments for all three of the above reactions having equilibrium constants greater than one at 600°C and 1 atm (15, 19). The conclusion is that, from a thermodynamic viewpoint, decarbonation of oil shale should be complete at 600°C. 

This suggested mechanism is consistent with a number of observations made here and with previous work reported in the literature. For example it was found that recarbonation rates were relatively insensitive to temperature. This would indicate non-activated chemisorption and, as Fischbeck and Snaidt report (10), mineral recarbonation is often independent of temperature when the temperature dependencies of the decarbonation rate conr-stant and the equilibrium constant are similar. This is exactly what is observed in western oil shales (Ref (9), Equations (6) and (7)). Previous work has also pointed to the role of chemisorption phenomena in mineral decomposition reactions. Spencer and Topley (11) have suggested that finely grained oxides can chemisorb HoO as well as H2 and Soni and Thomson (12) observed higher recarbonation rates when CO2 was produced on the surface during oil shale char combustion. 

Another common use of the equilibrium constant is to arrange for it to equal the fugacity of a gaseous species in equilibrium with a mineral assemblage. This usually has reference to dehydration, decarbonation, or desulfidation reactions, where all reacting constituents are minerals except one, which is the gas species. For example, consider one of the simplest and most studied reactions... 

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