The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.
In the Tholen classification, the X-group consists of the following types:[1] Since in this scheme, the albedo is crucial in discriminating between the above types, some objects for which albedo information was not available were assigned an X-type.
[citation needed] The SMASS classification does not use albedo, but several spectral types are distinguished on the basis of spectral features which were too subtle to be visible in the broad-band ECAS survey used for the Tholen 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 Tholen E, M, and P-types.
All the types in the X-group contain a mixture of asteroids classified as either type E, M, or P.