Thin Film Absorbance Spectra of Forsterite and Fayalite Dust Grains

Introduction: Forsteritic (Mg2SiO4, Fo) and fayalitic (Fe2SiO4, Fa) olivines exist as micron-sized dust grains in a variety of astrophysical and planetary environments. While an estimated ~ 95% of the silicate material that dominates the interstellar medium by mass is assumed to be amorphous [1], crystalline silicate dust particles are known to be present in dust disks around main-sequence stars, some solar-system comets, and outflows from evolved stars (c.f., [1-3]). Forsterite and fayalite are also important constituents in differentiated meteorites and occur in a range of (Mg, Fe) compositions, e.g., Fo10-20 for eucrites and pallasites ranging to Fa9-21 for some angrites [4]. Further evidence from comets, meteorites, interplanetary dust particles, and asteroids suggests that the occurrence of forsterite or fayalite may correspond to primordial conditions or to the presence of (aqueous) alteration on planetesimal surfaces, respectively. The spectral detection of Fe-rich olivine dust particles around a remote astronomical object (e.g., a Vega-like star [5]) could imply the existence of a surrounding asteroid belt or a series of planetesimals. Given the presence of micron-sized crystalline silicate dust grains in so many environments and the fact that dust particles of a given size, temperature, crystal structure, and composition may be differentiated by their distinct spectral signatures over infrared (IR) wavelengths (~ 200-2.5 μm, or ~ 50-4000 cm), obtaining detailed spectroscopic information on intermediate and end member forsterite and fayalite compositions over a broad range of infrared wavelengths is critical for proper analysis of astrophysical and planetary dust grains. In this work, we present midand far-IR laboratory diamond anvil cell (DAC) thin film absorbance spectra and peak position wavenumbers for approximately 20 different olivine compositions along the Fo/Fa binary. Experimental Samples and Methods: Natural forsterite and fayalite samples (Fo93, Fo91, Fo82, Fo68, Fo63, Fo54, Fo46, Fo41, Fo40, Fo38Te6, Fo31Te1, Fo14Te2, Fo9Te2, Fo6Te2, Fo0Te5) supplied by R. M. Hazen, S. A. Morse, H. S. Yoder, and the Harvard collection were selected for spectral measurement. Of the ~ 20 olivine samples measured, one sample was meteoritic: Fo82, Alice Springs Meteorite. Additional synthetic olivines were provided by C. Koike (Fo100), H. K. Mao (Fo100), R. M. Hazen (Fo80, Fo75, Fo67), C. B. Finch (Fo0), B. Fegley and R. G. Burns (Fo50), and Alfa Aesar supply company (Fo100). Chemical compositions for all samples were either previously published in the literature [6-9] or measured directly via electron microprobe at Washington University by D. Kremser (Table 1). Only a few samples studied were found to have high Mn content (3-4.5 wt. %). Because Mn2SiO4 spectra closely resemble Fe2SiO4 spectra (c.f., [10]), these low Mn amounts should not affect our results.