Channel constituents in cordierite

Don S. Goldman and George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125

Wayne A. Dollase
Department of Geology
University of California
Los Angeles, CA 90024

Abstract

Chemical, optical and infiared absorption, Mössbauer and X-ray data are reported for eight cordierite samples. Fe²⁺ in the octahedraland channel sites is identified in optical and Mössbauer spectra, which indicate that generally less than 5 percent of the total iron is Fe³⁺ in the channels.The single-crystal Mössbauer data of Duncan and Johnston (1974) are reinterpreted. Two types of HrO 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 H₂O 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, A. 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 eplison (molar absorptivity) for the H₂O absorption bands are determined.

Room-temperature infrared spectrum of cordierite from Manitouwadge in the water stretching (left) and bending (right) regions.

last updated: 9-Apr-2023