C-type asteroids are asteroids made of carbonates. They are the most common variety taking up around 75% of known asteroids, and an even higher percentage in the farther part of the belt beyond 2.7 AU, which is dominated by this asteroid type. The amount of C-types may actually be higher than this, because C-types are much darker than most other asteroid types except D-types and others common only at the extreme farther edge of the Main Belt.
This type of asteroid has very similar spectra to carbonaceous chondrite meteorites (types CI and CM), whose chemical make-up is about the same as the Sun and the primitive solar nebula, except that they do not contain hydrogen, helium and other volatiles. Hydrated (water-containing) minerals are present.
C-type asteroids are very dark with albedos usually in the 0.03 to 0.10 range. Consequently, whereas a number of S-types can normally be viewed with binoculars at opposition, even the biggest C-types require a small telescope. The potentially brightest C-type asteroid is 324 Bamberga, but that object's very high eccentricity means it rarely reaches its maximum magnitude.
Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are very featureless but slightly reddish. The so-called "water" absorption feature around 3 μm, which can be a sign of water content in minerals is also present.
S. J. Bus and R. P. Binzel Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy, Icarus, Vol. 158, pp. 146 (2002).