ATHERMAL SOLID SOLUTION MODELS FOR SI-AL SUBSTITUTION IN ANALCIME AND CHABAZITE

Accurate depiction of phase relations involving zeolites requires explicit consideration of the consequences of solid solution and disordering in these minerals. Quantitative determination of the relative proportions of Si(nAl) tetrahedral structural units (where n=0, 1, 2, 3, or 4) through analysis of ²⁹Si MAS NMR spectra was used to assess the state of Si-Al disorder in the zeolites analcime [(NaAl)_xSi_3-xO₆·H₂O where x=0.8 to 1.1] and chabazite [(Ca,Na)_xAl)_xSi_6-xO₁₂·6H₂O where x=1.1 to 2.5]. The results of this study and previously reported ²⁹Si MAS NMR spectra suggest that the aluminosilicate frameworks of natural analcimes and chabazites are more ordered than their synthetic counterparts. Natural analcime is more ordered than natural chabazite. Synthetic varieties of both zeolites exhibit ²⁹Si MAS NMR spectra consistent with Al avoidance as the only ordering mechanism. Cluster variation calculations indicate that the configurational entropy (S_con) due to Si-Al disorder in synthetic and natural analcimes and chabazites is a regular function of Si-Al content. The calculations predict that Si-Al solid solutions in natural and synthetic analcime and chabazite can be described by two stoichiometric ordered endmembers, one corresponding to a mol fraction of Al (X_Al ) of 0 (fully siliceous) in all of the solutions. The other endmembers occur at X_Al=~0.42 (synthetic analcime and chabazite), X_Al=~0.36 (natural analcime) and XAl=~0.40 (natural chabazite). The difference between the calculated entropies and ideal entropies of mixing between these pairs of endmembers permits direct assessment of the excess entropy of mixing (S^ex). Combined with the assumption that the excess enthalpy of mixing in these solid solutions is negligible (Petrovic and Navrotsky, 1994; Shim et al., 1999), these calculations permit direct assessment of endmember activites as a function of composition. The solid solution model thus derived for natural analcimes was tested by assessing published experimental observations of fluid compositions coexisting with analcime and albite at elevated temperatures and pressures. The calculations indicate that the athermal solution model developed in this study suitably describes the effect of Si-Al substitution on the stability of analcime in experimental and geologic systems.