Spectral reflectance characteristics of the meteorite classes

Spectral reflectance curves (0.35–2.5 μm) have been measured for about 150 individual meteorite specimens which represent nearly all the types and subtypes of meteorites present in terrestrial meteorite collections. The spectra of members of each meteorite class are discussed in terms of the composition, abundance, and distribution of the component mineral phases. The range of meteoritic minerals is divided into four classes based on the optical properties of a mineral as defined by the optical density and conductivity of that mineral. These classes are (1) metals (e.g., the Ni-Fe minerals taenite, kamacite, etc.), (2) dielectric transparent phases which contain transition metal ions (Fe²⁺, Ti³⁺, etc.) (e.g., the common rock-forming silicates pyroxene, olivine, feldspar), (3) dielectric transparent phases which do not contain transition metal ions (e.g., the pure magnesium enstatite pyroxenes of enstatite chondrites and achondrites), and (4) opaque semiconductor phases (e.g., troilite (FeS), carbon, and carbon compounds typical of the carbonaceous chondrites). The mixing of phases follows the rule that the spectral contribution of a phase at a given wavelength is proportional to the relative optical density of that phase times the effective abundance of the phase. The important results of this study are as follows: (1) The spectral reflectance curves of a meteorite class, representing a particular mineral assemblage and metamorphic grade, do not differ significantly. (2) The variations between the spectra of different meteorite types are significant and are understandable in terms of differences in the composition, abundance, and distribution of the component mineral phases. These variations are of sufficient magnitude to permit discrimination of meteorite types from spectral data. (3) There exist a series of diagnostic features in the spectrum of a meteoritic material (absorption band presence, position, symmetry, and intensity as well as the continuum slope, curvature, and inflection points) which can be utilised in the general interpretation of telescopic spectral reflectance measurements of asteroids. (4) The positions of the 1.0- and 2.0-μm absorption features common in meteoritic materials are sensitive indicators of pyroxene compositions and of the olivine/pyroxene ratio. (5) The physical properties of a surface material (particle size, packing, illumination angle) for a reasonable range of these parameters do not significantly affect the normalized spectral reflectance curve measured for that material.