The equilibrium curve for the reaction 3 dolomite + 1 K-feldspar A-1 tt~O = phlogopite-~ 3 calcite A-3 CO s was determined experimentally at a total gas pressure 2 000 bars using two different methods.
a) In the first case water alone was added to the reactants. The CO2 component of the g phase was produeted solely by the reaction under favourable P-T conditions. This manr of carrying out the reaction is called the ~ water method ". With this method sufficient tiI must be allowed for the gas phase to attain a constant composition (see Fig. 1). Revel reactions were carried out using reaction products of the forward reaction.
b) In the second case silver oxalate -}-water were added to the reactants. Breakdown of t silver oxalate leads to formation of a C02--H~O gasphase of definite composition. At consta temperature and gas pressure the Xco 2 determines whether the reaction products will phlogopite + calcite or dolomite ~-K-feldspar. In this case it is not necessary to wait t equilibrium to be attained. This method is abbreviated the "oxalate method". Reactants t reverse reactions are not identical with the products of the forward reaction.
At high temperatures the results of the two different methods agree well (see Tables 1 and: Equilibrium was attained in one case at 490~ and Xco . of approximately 0.77, and in t other case at 520~ and Xco 2 of 0.90.
At lower temperatures there are considerable differences in the results. With the wat method an Xco ~ of about 0.25 was reached at 450~ With the oxalate method doloml K-feldspar and water still react with each other at even higher Xco ~ values. Phlogopi calcite and CO s are formed together with metastable talc. There are no criteria to indics which of the methods is the correct one at lower temperatures and in Fig. 2, therefore, be equilibrium curves are plotted.