Physical properties of M class asteroids

The available data on M class asteroids have been collected and analysed. The possible number of M-asteroids in the main asteroid belt larger than 30 km was estimated to be about 90 objects. A list of the 60 asteroids currently classified as M is presented. Such parameters as UBV and JHK colour indices, colours from the ECAS data, albedo, rotation rate, lightcurve amplitude, opposition effect value of magnitude-phase dependences, and the depth of the negative polarization branch of M-asteroids are considered in the comparison with the data for other asteroid classes and the laboratory data for meteorites. The reliability of metal content determination from spectral observations is discussed. On average the physical properties of the M class are close to those of the S class. However a diversity in composition inside the M class is evident.