Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2025 / Paper 3 / 10 / c

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- Solution c

Solution

For pure albite, pure nepheline and unit water activity,

Q_{III} = a_{H_{4} Si O_{4}}^{2}

. Consequently

x = \frac{1}{2} lo g_{10} K_{III} = - 5.515,

a vertical line through the intersection of (I) and (II). It does not depend on

y

because the net reaction has no hydrogen or sodium ions.

Figure 1.
Stable and metastable boundaries for albite, nepheline and corundum
. Original diagram derived from the two supplied equilibrium constants. Solid black segments delimit the stable mineral fields; dashed grey continuations are metastable pairwise equilibria. Logarithms are base ten and the plotted silica activities are below one. Details.

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