MINERAL EQUILIBRIA INVOLVING MIXED-VOLATILES IN A C-O-H FLUID PHASE - STABILITIES OF GRAPHITE AND SIDERITE

The stability of graphite in the presence of a C-O-H fluid-phase was calculated for pressures below 10 kb and temperatures above 400 degree C. The modified Redlich-Kwong technique was used to model the behavior of the fluid phase. A compositional variable, X//c, has been defined (X//C equals X (C O) //2 plus X//C//O plus 1/3 X//(C H) //4) which allows one to depict the graphite-fluid equilibria as an isobaric, isothermal univariant curve on a log f//(O) //2-X//C diagram. The log f O //2-X//C diagrams produced by the calculations presented here are applied to show how graphite veins can form by oxidation of a methane-rich fluid phase and how it is possible for carbon to be transported in the fluid phase from a graphite-bearing gneiss to a graphite vein. The diagrams are also useful to understanding the stability of siderite in the system Fe-C-O-H.