A spectral analysis of ordinary chondrites, S-type asteroids, and their component minerals: genetic implications

Three salient features of visible and infrared reflectance spectra of ordinary chondrites (OCs) and S-type asteroids are (1) albedo at 0.56 μm, (2) continuum slope, and (3) depth of the electronic absorption band due to octahedrally coordinated Fe²⁺ in olivine and pyroxene. These quantities were numerically extracted from the spectra of 23 OCs representing all metamorphic grades and 39 S-type asteroids to be plotted in a three-dimensional coordinate system. When the region containing the 39 S-asteroid spectra is compared with that of the altered and unaltered OCs, it is found that not one of the OCs falls within the S-asteroid region. The range of S-asteroid parameters is then compared with potential pure "end-member' components most likely to result from magmatic differentiation of a chondritic protoasteroid. The S-asteroid array is consistent with random mixtures of the differentiated components except for a notable dominance of the spectral characteristics of the opaque (metallic) component. These results suggest that the M-asteroids may form a composition continuum with the S-asteroids. Petrological domains of the S-asteroids (beneath a mixed regolith) may be large and supply achondrites, irons, and stony irons to Earth rather than well-mixed breccias of these components. The (smaller) OC protoasteroids may still be abundant in the asteroid belt but, if small enough to escape differentiation, may also be small enough to escape Earth-based identification. -from Authors