X-type asteroid
The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.
Tholen classification
In the Tholen classification the X-group contains the types:
E-type – with high albedos, typically above 0.30
M-type – intermediate albedo, "metallic", the largest grouping,
P-type – low albedos, typically below 0.10
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.
SMASS classification
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:
core X-type containing the asteroids with the most "typical" spectra
Xe-type of asteroids whose spectra contain a moderately broad absorption band around 0.49 μm. It has been suggested that this indicates the presence of troilite (FeS). There is some correlation between this group and the Tholen E-type.
