CORDIERITE IV: STRUCTURAL HETEROGENEITY AND ENERGETICS OF MG-FE SOLID SOLUTIONS

Abstract

The local structural heterogeneity and energetic properties of 22 natural Mg–Fe cordierites, ideal formula (Mg,Fe)2Al4Si5O18·x(H2O,CO2), were investigated at length scales given by powder infrared spectroscopy (IR) and also by published electronic absorption spectra. The studied samples have iron mole fractions from XFe = 0.06 to 0.82 and cover most of the Mg–Fe cordierite binary. Variations in wavenumbers and line widths of the IR bands were determined as a function of composition. Most modes shift linearly to lower wavenumbers with increasing XFe, except those at high wavenumbers located between 900 and 1,200 cm-1. They are vibrations that have a large internal (Si,Al)O4 character and are not greatly affected by Mg–Fe exchange on the octahedral site. The lower wavenumber modes can be best characterized as lattice vibrations having mixed character. The systematics of the wavenumber shifts suggest small continuous variations in the "average" cordierite structure with Mg–Fe exchange and are consistent with an ideal volume of mixing, ΔVmix= 0, behavior (Boberski and Schreyer 1990). IR line broadening was measured using the autocorrelation function for three wavenumber regions in order to determine the range of structural heterogeneity between roughly 2 and 100 Å (0.2–10.0 nm) in the solid solution. In order to do this, an empirical correction was first made to account for the effect that small amounts of channel Na have on the phonon systematics. The results show that between 1,200 and 540 cm-1 the line widths of the IR bands broaden slightly and linearly with increasing XFe. Between 350 and 125 cm-1 nonlinear behavior was observed and it may be related to dynamic effects. These results suggest minimal excess elastic enthalpies of mixing for Mg–Fe cordierite solid solutions. Channel Na should affect measurably the thermodynamic properties of natural cordierites as evidenced by variations in the IR spectra of Na-containing samples. Occluded H2O (Class I) and CO2 should have little interaction with the framework and can be considered nearly "free" molecules. They should not give rise to measurable structural heterogeneity in the framework. The contribution of the crystal field stabilization energy (CFSE) of octahedral Fe2+ to the energetics of Mg–Fe cordierites was also investigated using published electronic absorption spectra (Khomenko et al. 2001). Two bands are observed between 8,000 and 10,500 cm-1 and they represent electronic dd-excitations of octahedral Fe2+ derived from the 5T2g → 5Eg transition. They shift to higher wavenumbers with increasing XMg in cordierite. An analysis gives slightly asymmetric excess -ΔCFSE across the Mg–Fe cordierite join with a maximum of about −550 J/mole towards iron-rich compositions.