HDNB data

Equation (8.9) would be the procedure used with the HDNB data set, or any other set that gives standard thermodynamic parameters at 298 K, 1 bar, plus Maier-Kelley heat capacity coefficients. The RHF tables hst AjG° at various temperatures above 298 K at one bar, so that these can be used directly to give the first two terms on the right-hand side of equation (8.6). In other words... 

Fig. 13.7. Brucite-periclase-water equilibrium temperatures predicted from RHF and HDNB data at various pressures compared to the experimental brackets of Barnes and Ernst (1963).

Figure 6.12 shows A,.G° for the calcite aragonite reaction from HDNB data, for 1 bar and 4140 bars." Also shown are the four experimental points at that pressure. As you can see, shifting the A,G° curve down by only 15 cal mor would shift the A G° = 0 intersection by about 20 °C, which would be within the experimental bracket at 4140 bars. Clearly, the amount of temperature change for a given change in A G° depends on the slope of the line as it passes through A,.G° = 0, and the slope of the line is of course -A S°. The relationship is just a variation of Equation (4.42), or... 

This phase boundary may also be calculated directly by supcrt92, using the univariant curve option. Surprisingly, the results are worse than using the HDNB data. For many minerals, SUPCRT92 uses HDNB data but for calcite and aragonite the values of AG° are slightly different. 

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