CORDIERITE III: THE SITE OCCUPATION AND CONCENTRATION OF FE3+

Abstract

Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18.x(H2O,CO2), but it must contain some Fe^3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe^3+ in two Mg-rich natural cordierites have been investigated by EPR and 57^Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000 °C for 2 days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000 °C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe^2+ or Fe^3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000 cm^−1 originating from charge-transfer between Fe^2+ in the octahedron and Fe^3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe^3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.