Olivine–metal mixtures: Spectral reflectance properties and application to asteroid reflectance spectra

Abstract

Olivine-rich asteroids appear to be common in the main asteroid belt as well as present in the near-Earth asteroid population. There are a number of meteorite classes that are dominated by olivine±metal. To determine whether relationships exist between these asteroids and meteorites, we spectrally characterized a number of olivine+meteoritic metal powder intimate and areal mixtures, pallasite slabs, and olivine powders on a metal slab. Our goal is to understand the spectral characteristics of olivine+metal assemblages and develop spectral metrics that can be used to analyze reflectance spectra of olivine-dominated asteroids. We found that the major olivine absorption band in the 1μm region is resolvable in intimate mixtures for metal abundances as high as ∼90wt.%. The wavelength position of the 1μm region olivine absorption band center is sensitive to Fa content but insensitive to other variables. However, the band minimum position moves to shorter wavelengths with increasing metal abundance due to changes in spectral slope. The full width at half maximum (FWHM) of this band and reflectance at 1.8μm are both most sensitive to olivine Fa content, metal abundance, and grain size, and much less to the presence of nanophase iron that reddens spectra. Reflectance at 0.56μm and the 1.8/0.56μm reflectance ratio are sensitive to these same parameters as well as to nanophase iron-associated spectral reddening. The wavelength position of the local reflectance maximum in the 0.7μm region moves to longer wavelengths with increasing metal abundance and is most useful for constraining metal abundance in high metal-content mixtures. Pallasite slab spectra differ in a number of respects from powdered assemblages and multiple spectral parameters can be used to discriminate them. The spectra of increasingly fine-grained olivine+metal assemblages and those involving low-Fa olivine show increasing spectral dominance by metal. Systematic application of multiple spectral metrics allows olivine+metal assemblage properties such as Fa content, olivine/metal ratio, and grain size to be quantified or constrained. Analysis of reflectance spectra of 22 presumed olivine±metal-rich asteroids indicates that most of them possess low- to medium-Fa content olivine (Fa<∼67), with variable abundances of macroscopic metal. A number exhibit visible region absorption bands that are indicative of some fraction of coarser-grained olivine (>45μm). Most asteroid spectra can be plausibly linked to specific olivine±metal-bearing meteorite classes. Most of the asteroid spectra examined exhibit some degree of spectral reddening below ∼1.8μm which is most consistent with the presence of fine-grained nanophase iron, likely produced by space weathering.