COMBINED INFRARED AND ANALYTICAL ELECTRON-MICROSCOPE STUDIES OF INTERPLANETARY DUST PARTICLES

Silicate grains in thin sections of eight chondritic interplanetary dust particles (IDPs) were studied using analytical electron microscopy (AEM) and infrared (IR) microspectroscopy. Relative abundances of olivine, pyroxene, layer silicates, and glass were determined by collecting 200-700 quantitative point-count analyses, each with (X-ray) spatial resolution less than 0.05-mu-m, from each thin section. Three of the IDPs are pyroxene-rich, two are olivine-rich, one is layer silicate-rich, and two are glass-rich. IR transmission spectra were obtained within the spectral range 2.5-16.0-mu-m (625-4000 cm-1) from the thin sections. The 10-mu-m "silicate feature" of the pyroxene-rich IDPs (U222B42, W7027H14, U230A3) consists of two principal bands at 9.1-9.4-mu-m (1064-1099 cm-1) and 10.5-10.75-mu-m (930-953 cm-1). The bands are consistent with monoclinic clinopyroxene whose presence in the thin sections was confirmed using electron diffraction. The olivine-rich IDPs, U220A14 and U230A4, each produce an intense silicate band at 11.2-11.3-mu-m (885-893 cm-1), with less intense bands at 10.1 and 10.75-mu-m (930, 991 cm-1), while the layer silicate-rich IDP (U230A43) produces a principal silicate band at almost-equal-to 9.8-mu-m (1031 cm-1). (An additional band at 6.8-mu-m (1471 cm-1) indicates the presence of carbonates in U230A43). The glass-rich IDPs, W7027A11 and U219C11, produce broad silicate band structure between 8 and 12-mu-m, with a prominent peak at 11.3-mu-m. The 10-mu-m silicate features of the olivine, pyroxene, and layer silicate-rich IDPs are unlike those of comets, but those of W7027A11 and U219C11 are similar to comets like Halley and Bradfield in terms of overall bandwidth and shape, as well as the 11.3-mu-m "olivine" feature. The mineralogy and petrography of U219C11 and W7027A11 resemble those of comet grains analyzed by the PIA and PUMA instruments during the Halley encounters. Chondritic IDPs dominated by glass and submicrometer silicate crystals are strong candidates for the dust grains responsible for silicate IR emission from comets.