POSSIBLE EXPLANATIONS FOR THE 506 nm FEATURE IN TELESCOPIC SPECTRA OF VESTA , VESTOIDS AND HED METEORITES

Introduction: Telescopic spectra of Vesta, ves-toids and HED meteorites show a 506 nm absorption feature whose wavelength position varies for different Vestoids, howardites vs eucrites vs diogenites, and across the surface of Vesta [1, 2, 3]. [1] believe that the shift is due to the change in composition of pyrox-enes, specifically that increases in calcium content are correlated with increases in the wavelength position of this band., as postulated in [2]. In an extensive examination of pyroxene reflectance spectra, we only observe the 505 nm feature in orthopyroxene spectra (Fig. 1), but not in clinopyroxene spectra (Fig. 2). Therefore we believe that the band shift seen in HEDs, Vesta, and vestoids may be due to another mechanism. Experimental Procedure: Reflectance spectra of 50 pyroxene samples (45-90 µm grain size) were measured with an Ocean Optics S-2000 spectrometer, covering the wavelength range 200-600 nm with ~0.3 nm spectral resolution. A total of 300 spectra (30 msec/spectrum) were averaged to improve signal to noise ratio for each sample. The spectra were measured at i=0°, e=45° relative to halon. Halon's reflec-tance begins to decline shortwards of ~250 nm, thus spectral slopes in this range are not reliable. In addition , spectral acquisition was optimized for the 400-600 nm range, and data shortward of 400 nm are increasingly noisy. Continuum removal applied to the spectra involved division of a straight line tangent to the reflectance spectrum on either side of the feature of interest. Results: Our high resolution orthopyroxene spectra display two bands at 504.5 nm and 508.0 nm (Figs. 1 and 2). These bands do not shift with changes in Fe 2+ content, but the depth of the 508 nm band appears to increase with increasing Fe 2+ content. Given these observations it appears that shifts in 505 nm region band minima seen across the surface of Vesta, between different vestoids, and between HED meteorites [1, 2] are not attributable to changes in either Fe 2+ or Ca content in pyroxenes. In transmission spectra of lunar samples [2] see a doublet in low calcium pyroxene and pigeonite spectra , and a singlet in high calcium pyroxene spectra in the 505-510 nm region. We believe this is because the lunar samples may be exsolved into calcium-rich and calcium-poor regions. The 505 nm feature in these spectra is likely attributable to the calcium-poor regions. We see a doublet in all of our low calcium py-roxene …