Cordierite I: The coordination of Fe2+

The incorporation of Fe2+ was investigated in four natural cordierite sampl es. Fe-57 Mossbauer, single-crystal UV-VIS optical absorption, and X-ray ab sorption spectroscopies, as well as X-ray single-crystal diffraction were u sed. Mossbauer, optical, and XAS spectroscopy show that Fe2+ is incorporate d on two different structural sites in two Mg-rich samples. Mossbauer measu rements give the best quantitative measure of the amounts of Fe2+, but the optical spectra are the most sensitive for determinations at low concentrat ions and at high-bulk Fe2+ concentrations in cordierite. The spectroscopic data are most consistent with small amounts of Fe2+ (i.e., 0.02 of Fe2+ per formula unit) being located on a tetrahedral site rather than in the cente r (or off center) of the six-membered tetrahedral rings or in channel cavit ies. X-ray single-crystal refinements on two Mg-rich cordierites show a ver y small excess electron density on T(1)1 and not in the channels. A third r efinement on a slightly more iron-rich sample shows, in contrast, no excess electron density on T(1)1. We interpret these data as indicating that smal l amounts of Fe2+ (0.01 to 0.02 atoms per formula unit) replace tetrahedral Al(1)1 in cordierite, where charge balance is achieved by placing Na in th e center of the six-membered rings. This substitution is consistent with th e known chemistry of natural cordierites and with simple structural energet ics. The identification and assignment of small amounts of Fe2+ on T(1)1 re quires spectroscopic determination or careful X-ray single-crystal refineme nts and cannot be achieved from composition data and structural formula cal culations.