With the site distributions for Fe--Mg cordierites as follows,

the ideal mixing activities are given as

and coded up for THERMOCALC,

% ===============  Ideal mixing in hydrous cordierites =============
cd 3 
   crd      1    2      1  1 -1  1  2    1  1 -1  2  1
   
   fcrd     1    2      0  1  1  1  2    1  1 -1  2  1
      bulk  1    1      0  1  1  1
      
   hcrd     1    2      1  1 -1  1  2    0  1  1  2  1    
      bulk  1    1      0  1  1  2
      
   x(cd) 0.3
   h(cd) 0.1
 

The thermodynamic properties of the hydrous cordierite end-member have been updated slightly from HP90 to take account of the measured heat capacities of Carey (1993) and the enthalpy of hydration measured by Carey & Navrotsky (1992), as well as the new water content data of Skippen & Gunter (1996). Taking the heat capacity of hydrous cordierite as 8.3 J larger than anhydrous cordierite (Carey, 1993), together with the assumption of 1 mole of H2O in the hydrous end-member, leads to

for the equilibrium hcrd = crd + H2O. A least squares fit to the hydration data of Mirwald et al. (1979) and Skippen & Gunter (1996) yields an enthalpy of reaction which is within error of the value of 41.8 ±1.6 kJ measured calorimetrically by Carey & Navrotsky (1992). It was therefore decided to accept the calorimetric enthalpy as a constraint and to retrieve the entropy from the hydration data. The resulting values used in the data set are

for the reaction crd + H2O = hcrd. The level of agreement between the calorimetric data and the measured water contents makes a satisfactory justification of the assumption of ideal mixing of cordierite and hydrous cordierite with one mole of H2O, and lends weight to the model as a tool for calculating the thermodynamic properties of water-bearing cordierites.