Cordierite III: the site occupation and concentration of Fe3+

Cordierite has the ideal formula (Mg,Fe)(2)Al4Si5O18.x(H2O,CO2), but it mus t contain some Fe3+ to account for its blue color and strong pleochroism. T he site occupation and concentration of Fe3+ in two Mg-rich natural cordier ites have been investigated by EPR and Fe-57 Mossbauer spectroscopy. In add ition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra ind icate that Fe3+ is located on T(1)1 in natural cordierites and not in the c hannels. The amount in Mg-rich cordierites is very small with an upper limi t set by Mossbauer 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 degreesC for 2 days causes an oxidation and/or loss of Fe2+ on T(1)1, toget her with an expulsion of Na+ from the channels, whereas heating at the Fe-F eO buffer produces little Fe3+ in cordierite. Heating at 1,000 degreesC rem oves all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treat ed samples was found. The blue color in cordierite arises from a broad abso rption band (E//b and weaker with Ella) around 18,000 cm(-1) originating fr om charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shar ed T(1)1 tetrahedron. It therefore appears that all natural cordierites con tain some tetrahedral Fe3+. 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 t hrough TEM study.