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 foreight cordierite samples. Fe2+
in the octahedraland channel sites is identified in optical andMössbauer spectra, which indicate that
generally less than 5 percent of the total iron is Fe3+inthe 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 withtheir H-Hdirections parallel to [001] (Type I)
and [010] (Type II). Type II H2O is found tocorrelate to the amount of cations in
the channels. Optical spectra provide a measure of thehexagonality of cordierite, but these
measurements are not correlated with the distortionindex, A. The color and pleochroism are
suggested to arise from intervalence charge-transferbetween octahedral Fe2+ and channel Fe3+. The strong enhancement of intervalence intensityafter 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, mainlyNa+, from the rings into the cavities is
primarily responsible for changes in the distortionindex which accompany dehydration.
Values of eplison (molar absorptivity) for the H2O absorptionbands are determined.
Room-temperature infrared spectrum of cordierite from Manitouwadge in the water stretching (left) and bending (right) regions.