Hydrous species reaction in rhyolitic melt

Another geochemical reaction that has been investigated extensively as a geospeedometer by high-temperature geochemists is the hydrous species reaction in rhyolitic melt. As the H2O component dissolves in silicate melt, it partially reacts with oxygen in the melt to form OH groups (Reaction 1-10)  

there are at least two hydrous species in silicate melt, H2O molecules (referred to as H20m in this section) and OH groups (referred to as OH). Total H2O will be referred to as H20t. The equilibrium constant for the interconvert reaction is 

The above takes the following form when expressed in terms of mole fractions  

Mass balance for intracrystalline reactions must consider the number of moles of the site per formula unit, as shown in Box 2-4. To clarify this point further, here is an example. Consider exchange reaction between 

OH groups and units in alunite, KAl3(S04)2(0H)6. The equilibrium constant is 

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