Cross-Over Infrared Spectroscopy: A New Tool for the Remote Determination of Olivine Composition

Olivine ([Mg,Fe](2)SiO4) has been remotely detected on numerous solar system bodies, but its composition has been only partially constrained using visible-near infrared (VNIR, 0.5-3 mu m) and midinfrared (MIR, 8-15 mu m) spectra. Meanwhile, the spectral character of olivine in the "cross-over" region of the infrared (4-8 mu m) remains generally unexplored. We examine the relationship between olivine Mg# (molar Mg/[Mg + Fe] x 100) and the wavelength positions of two strong, discrete overtone-combination bands at similar to 5.6 and similar to 6.0 mu m. In reflectance and thermal emission spectra measured for 47 synthetic and natural olivine samples that span the Mg-Fe solid solution, we find that the 5.6 and 6.0 mu m bands shift systematically to longer wavelengths by 0.11 and 0.10 mu m, respectively, with increasing Fe content. Our results indicate that olivine Mg# may be determined within +/- 10 mol% forsterite using the positions of these two bands, making "cross-over" region spectroscopy a potentially powerful remote sensing tool.