Channel constituents in cordierite

Abstract

Chemical, optical and infrared absorption, Mössbauer and X-ray data are reported for eight cordierite samples. Fe^(2+) 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 Fe^(2+) in the channels. The single-crystal Mössbauer data of Duncan and Johnston (1974) are reinterpreted. Two types of H_2O 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 H_2O 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 Fe^(2+) and channel Fe^(3+). The strong enhancement of intervalence intensity after dehydration is interpreted in terms of migration of Fe^(2+) 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 є (molar absorptivity) for the H_2O absorption bands are determined.