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:
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. An example of this is 50 Virginia.
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.