TheX-group ofasteroids collects together severaltypes with similar spectra, but probably quite different compositions.
In theTholen classification, the X-group consists of the following types:[1]
Since in this scheme, thealbedo is crucial in discriminating between the above types, some objects for whichalbedo information was not available were assigned an X-type. An example of this is50 Virginia.[citation needed]
TheSMASS classification does not usealbedo, but several spectral types are distinguished on the basis ofspectral features which were too subtle to be visible in the broad-bandECAS survey used for theTholen scheme. The X-group contains the types:
Apart from the Xe-type, there is no significant correlation between the split into these SMASS types and the TholenE,M, andP-types. All the types in the X-group contain a mixture of asteroids classified as either typeE,M, orP.
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