Cordierites
With the site distributions for Fe--Mg cordierites as follows,
| oct | channel | |
| crd | Mg | - |
| fcrd | Fe | - |
| hcrd | Mg | H2O |
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.
Last updated on 20-Jan-09 13:45