  M-type asteroids, as defined in the Tholen taxonomy (Tholen, 1984), are
medium albedo bodies supposed to have a metallic composition and to be the
progenitors both of differentiated iron-nickel meteorites and enstatite
chondrites. We carried out a spectroscopic survey in the visible and near
infrared wavelength range (0.4-2.5 micron) of 30 asteroids chosen from the
population of asteroids initially classified as Tholen M -types, aiming to
investigate their surface composition. The data were obtained during several
observing runs during the years 2004-2007 at the TNG, NTT, and IRTF telescopes.
We computed the spectral slopes in several wavelength ranges for each observed
asteroid, and we searched for diagnostic spectral features. We confirm a large
variety of spectral behaviors for these objects as their spectra are extended
into the near-infrared, including the identification of weak absorption bands,
mainly of the 0.9 micron band tentatively attributed to orthopyroxene, and of
the 0.43 micron band that may be associated to chlorites and Mg-rich
serpentines or pyroxene minerals such us pigeonite or augite. A comparison with
previously published data indicates that the surfaces of several asteroids
belonging to the M-class may vary significantly. We attempt to constrain the
asteroid surface compositions of our sample by looking for meteorite spectral
analogues in the RELAB database and by modelling with geographical mixtures of
selected meteorites/minerals. We confirm that iron meteorites, pallasites, and
enstatite chondrites are the best matches to most objects in our sample, as
suggested for M-type asteroids. The presence of subtle absorption features on
several asteroids confirms that not all objects defined by the Tholen M-class
have a pure metallic composition.
