The carbonates and related nitrates and borates are common constituents of the earth's near-surface crust. This is a structurally-related as well as chemically-related group The basic anionic (negatively charged) unit of this class consists of a triangle where at the center resides either a carbon, nitrogen or boron atom. At every corner of the triangle sits an oxygen atom. The threefold symmetry of the triangle explains the trigonal symmetry that many members of this class possess. As long as the triangles of the anionic group fall in a plane parallel with the plane of the triangle and all other bonds in the structure, when viewed perpendicular to this plane, are multiples of three, and are evenly separated from each other, the mineral will have a trigonal symmetry.

Although somewhat varied, this class' properties can be generalized more so than the other classes. Typical carbonates are transparent, lightly colored with a white streak, average to above average in density, soft with good to perfect cleavage, soluble to at least some degree in acidic solutions, and tend to originate in sedimentary and oxidizing environments with the exception of carbonatite igneous intrusions. Most of these common characteristics are due to the common chemistry the group shares and members that diverge from the norm do so because of the effects of metal cations such as lead, copper, manganese and iron.

Carbon dioxide in the air dissolves in water to form carbonic acid H₂CO₃, consisting of hydrogen ions H 1+ and bicarbonate (HCO₃) ions. If conditions of concentration and temperature are favourable, the carbonate ion will combine with metal ions to form carbonates. The stucture of these inorganic salts consists of triangular (CO₃)^2- ions held together by the metal ions in a crystalline lattice. The (CO₃)^2- groups are otherwise isolated in the structure and do not form chains, rings or layers. Of the nearly 70 carbonate minerals, only calcite and dolomite are commonly found species. These are important minerals in sedimentary rocks such as chalk and limestone, and metamorphic rocks like marble. They are also common in hydrothermal veins.