IRTF spectra for 17 asteroids from the C and X complexes: A discussion of continuum slopes and their relationships to C chondrites and phyllosilicates

In order to gain further insight into their surface compositions and relationships with meteorites, we have obtained spectra for 17 C and X complex asteroids using NASA’s Infrared Telescope Facility and SpeX infrared spectrometer. We augment these spectra with data in the visible region taken from the on-line databases. Only one of the 17 asteroids showed the three features usually associated with water, the UV slope, a 0.7 μm feature and a 3 μm feature, while five show no evidence for water and 11 had one or two of these features. According to DeMeo et al. (2009), whose asteroid classification scheme we use here, 88% of the variance in asteroid spectra is explained by continuum slope so that asteroids can also be characterized by the slopes of their continua. We thus plot the slope of the continuum between 1.8 and 2.5 μm against slope between 1.0 and 1.75 μm, the break at ∼1.8 μm chosen since phyllosilicates show numerous water-related features beyond this wavelength. On such plots, the C complex fields match those of phyllosilicates kaolinite and montmorillonite that have been heated to about 700 °C, while the X complex fields match the fields for phyllosilicates montmorillonite and serpentine that have been similarly heated. We thus suggest that the surface of the C complex asteroids consist of decomposition products of kaolinite or montmorillonite while for the X complex we suggest that surfaces consist of decomposition products of montmorillonite or serpentine. On the basis of overlapping in fields on the continuum plots we suggest that the CI chondrites are linked with the Cgh asteroids, individual CV and CR chondrites are linked with Xc asteroids, a CK chondrite is linked with the Ch or Cgh asteroids, a number of unusual CI/CM meteorites are linked with C asteroids, and the CM chondrites are linked with the Xk asteroids. The associations are in reasonable agreement with chondrite mineralogy and albedo data.