G-type asteroids are a relatively uncommon type of carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

aceous asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

. The most notable asteroid in this class is 1 Ceres

1 Ceres

Ceres, formally 1 Ceres, is the smallest identified dwarf planet in the Solar System and the only one in the asteroid belt. With a diameter of about 950 km, Ceres is by far the largest and most-massive asteroid, comprising about a third of the mass of the asteroid belt. Discovered on 1 January 1801...

.

Characteristics

Generally similar to the C-type objects, but containing a strong ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 absorption feature below 0.5 μm. An absorption feature around 0.7 μm may also be present, which is indicative of phyllosilicate minerals such as clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

s or mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

.

In the SMASS classification
the G-type corresponds to the Cgh and Cg types, depending on the presence or absence (respectively) of the absorption feature at 0.7 μm. The G-type, C-type

C-type asteroid

C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...

 and some rare types are sometimes collected together into a wider C-group of carbonaceous

Carbonaceous

Carbonaceous is the defining attribute of a substance rich in carbon. Particularly, carbonaceous hydrocarbons are very unsaturated, high-molecular-weight hydrocarbons, having an elevated carbon:hydrogen ratio....

asteroids.

Reference

D. J. Tholen Asteroid taxonomic classifications in Asteroids III

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