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  
      x(cd) 0.3
      h(cd) 0.5
      
  p(crd)    1 1      1  2 -1  x -1  h
  p(fcrd)   1 1      0  1  1  x
  p(hcrd)   1 1      0  1  1  h
  
  ideal

  4  x(Mg)    1 1      1  1 -1  x
     x(Fe)    1 1      0  1  1  x  
     h        1 1      0  1  1  h  % H2O  
     noth     1 1      1  1 -1  h  % not H2O

  crd     1    2      x(Mg)  2    noth  1      
  fcrd    1    2      x(Fe)  2    noth  1              
  hcrd    1    2      x(Mg)  2       h  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.