Thermodynamics of water in cordierite and some petrologic consequences of cordierite as a hydrous phase

Abstract

The H₂O content of cordierite is often regarded as incidental to its stability, probably because cordierite has substantial fields of stability at low pressures both in wet and dry experimental systems. In this paper we show that, in contrast, the molecular water content of cordierite has profound effects on many equilibria involving this phase.

Mg-cordierite has been modelled as an ideal solid solution of the hydrous and anhydrous end-members Mg₂Al₄Si₅O₁₈·1.2H₂O and Mg₂Al₄Si₅O₁₈ respectively. The H₂O-solubility data of Mirwald and Schreyer (1977) fit this model within experimental uncertainty and yield 1 bar enthalpy and entropy changes for the reaction:

$$\begin{gathered} Mg_2 Al_4 Si_5 O_{18} + 1.2H_2 O = Mg_2 Al_4 Si_5 O_{18} \cdot 1.2H_2 O \hfill \\ cordierite fluid codierite \hfill \\ \end{gathered} $$

of −12,300 cal and −32.87 cal/K. This implies that the partial molal entropy of H₂O in cordierite at 298 K/l bar is almost exactly the same as the molar entropy of liquid water (16.9 cal/K as opposed to 16.7 cal/K) and that the interaction energy of liquid water with cordierite is only of the order of a few hundred calories per mole.

Application of the model to the hydrous experiments of Fawcett and Yoder (1966) and Chernosky (1974) yields a value for ΔG ⁰_f,298 of anhydrous Mgcordierite of between −2,062.71 and −2,074.21 Kcal per mole. This in in good agreement with the calorimetric data of Charlu, Newton and Kleppa (1975) which yield ΔG ⁰_f,298 of −2,067.03±1.18 Kcal.

Water pressure has a considerable influence on the (Mg, Fe) isopleths of coexisting cordierite and garnet, and hence, their use as geobarometric curves. Pressures estimated from the Mg/Fe ratios in the high-Mg range can vary by two kilobars or more, depending on the assumed$P_{H_2 O}$, with highest estimates for$P_{H_2 O} = P_{total}$.

The stability field of the talc-kyanite “white-schist” assemblage (Schreyer, 1973) is found to expand appreciably as$P_{H_2 O}$ is lowered. Thus the minimum pressure required to form this assemblage can be considerably less than the 10 kb required under conditions of$P_{H_2 O} = P_{total}$=P _total, as anticipated by Schreyer (1977).

The high partial molal entropy of H₂O in cordierite results in small entropy changes coupled with large volume changes in dehydration reactions forming cordierite. This greatly influences the slopes and positions of univariant reactions involving cordierite. The stability of cordierite is promoted to higher pressures in H₂O-bearing systems where none of the cordierite breakdown products is a hydrate. Cordierite-forming reactions from hydrates can have the H₂O released on the relatively low-temperature sides of the reaction curves, an anomalous situation known only in zeolite stability curves. These considerations can have profound effects on model “petrogenetic grids” involving cordierite.