DARKENING IN BLACK AND GAS-RICH ORDINARY CHONDRITES - THE SPECTRAL EFFECTS OF OPAQUE MORPHOLOGY AND DISTRIBUTION

Darkening of ordinary chondrite surface material by regolith processes such as shock-induced heating may hinder identification of main asteroid belt parent bodies for ordinary chondrites. There are two types of darkened ordinary chondrites, the black chondrites and the gas-rich chondrites. Relative to normal ordinary chondrites, they show reduced reflectance, a modest red continuum slope, and subdued absorption features that can make mineral identification challenging. Both meteorite types are darkened by the dispersion of micron-scale FeNi metal and troilite. The two major morphologies of metal dispersion are (1) a random dispersion of 0.5-5 mu m diameter blebs of FeNi metal and troilite throughout the silicates and (2) fine networks of veins up to 5-mu m thick. Gas-rich ordinary chondrites acquired their darkening while directly exposed to the space environment on the surface of ordinary chondrite parent bodies. Black chondrites probably represent material that was originally more deeply buried and was shocked, heated, and darkened by major impacts. The implications of these darkening processes are that shock can significantly alter the spectral signature of ordinary chondrite material and that a ''mature'' ordinary chondrite regolith soil on a large parent body may be spectrally altered.