Carbon dioxide in the air dissolves in water to form carbonic acid H2CO3, consisting of Hydrogen H 1+ and bicarbonate (HCO3) 1- ions. If conditions of concentration, acidity and temperature are favourable the molecule might dissociate into Hydrogen gas H2 and the carbonate ion CO3 2-. The latter is highly reactive and will combine with any locally present metal ions to form a group of minerals called Carbonates. These have the general formula M2CO3, the metal is monovalent, or MCO3 if the metal is biavelent. In other words if the respective metal carries one or two positive charges.

The structure of these metallic salts typically consists of the triangular carbonate ions held together by the metal ions in a crystalline lattice. The (CO3) 2- groups are isolated and do not form chains, rings or layers. Of the nearly 70 carbonate minerals, only Calcite and Dolomite are common. These are important rock-forming minerals in sedimentary rocks such as chalk and limestone, and metamorphic rocks like marble. They are also common in hydrothermal veins. Other notable Carbonates are those containing copper, Malachite and Azurite, and those containg lead, such as Cerrusite. Although typically rare, these form locally important ores because the copper or lead are much easier to obtain from these minerals than from the corresponding sulphides or oxides.

Nitrates are structurally almost identical to the Carbonates. The similarity arises from the NO3 1- ions being almost the same spatially as CO3 2- except that they carry one less negative charge. The triangular nitrate ion typically combines with Group 1 metals which keep the crystal structure together. Two common structures prevail, those nitrates resembling calcite, such as Soda Niter (NaNO3) and aragonite, such as Niter (KNO3).

Due to their high solubility in water, few Nitrates commonly occur in nature except in highly anhydrous conditions. There are approximately ten naturally occuring nitrates, of which only Niter and Soda Niter are found in significant amounts. The others include Ammonia Niter (NH4NO3), Nitrobarite (Ba(NO3)2), Nitrocalcite (Ca(NO3)2.4H2O) Nitromagnesite (Mg(NO3)2.6H2O), Gerhardtite (Cu2(NO3)(OH)3) and Buttgenbachite (Cu19Cl4(NO3)2(OH)32·2(H2O) and Darapskite (Na3(SO4)(NO3)·(H2O)).

The Carbonates are divided into the following groups:

Calcite Group - contains minerals with a cubic structure resembling Halite. In this case the spatial position of Cl- is replaced by (CO3)2- and Na+ by Ca2+. Unlike halite, however, the cubic arrangement is compressed along one axis. Each Ca ion is surrounded by six oxygen atoms of the CO3 groups. Like NaCl, Calcite has perfect cleavage in three directions but not at right angles. There is extensive solid solution substitution amongst the members of the group. For example Calcium can be gradually replaced by Cobalt or Manganese, giving rise to Cobaltocalcite and Rhodochrosite as end members of two series respectively.

Aragonite Group - minerals in this group differ from those of the calcite group by the fact that the metal ions are shielded by nine oxygen atoms rather than six. The additional shielding is required because these minerals contain metal ions that are much larger, including Barium, Strontium and Lead. Importantly, Calcium can also adopt this configuartion and therefore its carbonate can crystalise either as Calcite or as Aragonite. Arrangement of the Calcium and Carbonate ions in the latter structure is nearly hexagonal, which explains the frequent pseudohexagonal habit.