Chemical, optical and infrared absorption, Mössbauer and X-ray data are reported for eight cordierite samples. Fe2+ in the octahedral and channel sites is identified in optical and Mössbauer spectra, which indicate that generally less than 5 percent of the total iron is Fe2+ in the channels. The single-crystal Mossbauer data of Duncan and Johnston (1974) are reinterpreted. Two types of H20 are identified and are found to be oriented in the (100) plane with their H-H directions parallel to [001] (Type I) and [010] (Type II). Type II H20 is found to correlate to the amount of cations in the channels. Optical spectra provide a measure of the hexagonality of cordierite, but these measurements are not correlated with the distortion index, Δ. The color and pleochroism are suggested to arise from intervalence charge-transfer between octahedral Fe2+ and channel Fe3+. The strong enhancement of intervalence intensity after dehydration is interpreted in terms of migration of Fe3+ from the six-membered tetrahedral rings to the walls of the channel cavities. It is proposed that migration of cations, mainly Na+, from the rings into the cavities is primarily responsible for changes in the distortion index which accompany dehydration. Values of ε (molar absorptivity) for the H20 absorption bands are determined.

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