LABORATORY FAR-IR SPECTROSCOPY OF PHYLLOSILICATES

This work focuses on the phyllosilicate mineral and presents far infrared absorption coefficient spectra (calculated from the measured absorption spectra) of natural phyllosilicate minerals in the wavelength range 15 - 250 µm (650 – 40 cm^-1). We report infrared absorption band tables for over 17 phyllosilicate mineral species, representing 4 structural groups: kaolinite-serpentine, smectite, chlorite, and mica, which are intended as references for quantitative mineral identification during the interpretation of spectral data return by far-infrared space missions such as Herschel Space Observatory. This is a part of the effort on building a database of laboratory far-IR spectra, and expanding the spectra presently available in other databases (typically below 25 micron, or 400 cm^-1) towards the longer wavelength of up to 250 micron (40 cm^-1). The phyllosilicates minerals were sampled from the American Museum of Natural History for diversity and astrophysical relevancy, based on their identification in STARDUST returned comet samples, in stratospheric IDP samples, and in meteorites. All samples were characterized by powder x-ray diffraction and electron microprobe to verify structure and uniformity of composition. Spectra of ~micron-sized mineral powder suspensions in polyethylene pellets reveal prominent and characteristic far-IR features. For many minerals, ours are the first far-IR spectra out to 250 µm, according to literature review. Measurements at cryogenic temperatures (sample temperatures of ~20 K) for some of the minerals reveal significant spectral changes in comparison with room temperature data. Spectra of the same mineral species from different geological localities were collected, in order to account for the variations that often occur between the spectra of the samples of the same mineral obtained from various sources, reflecting the variations in crystal structure and chemical composition due to geochemical and physical environment of the mineral formation. Far-IR peak frequencies and mass absorption coefficient values are tabulated, and comparison with the previously published spectra is made.