M-type asteroids are asteroids of partially known composition; they are moderately bright (albedo 0.1–0.2). Some, but not all, are made of nickel–iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts, and are thought to be the source of iron meteorites. M-type asteroids are the third most common asteroid type.
There are also M-types whose composition is uncertain. For example, 22 Kalliope has an accurately known density that is far too low for a solid metallic object or even a metal rubble pile: a rubble pile of iron-nickel metal would need about 70% porosity which is inconsistent with packing considerations. 22 Kalliope and 21 Lutetia have features in their spectra which appear to indicate the presence of hydration minerals and silicates, anomalously low radar albedos inconsistent with a metallic surface, as well as characteristics more in common with C-type asteroids. A variety of other M-type asteroids do not fit well into a metallic body picture.
M-type spectra are flat to reddish and usually devoid of large features, although subtle absorption features longward of 0.75 µm and shortward of 0.55 µm are sometimes present.