Mineralogists divide carbonate minerals into three main groups based on atomic arrangement: the calcite group, dolomite group, and aragonite group. Several other species that have more complex structures and chemistries are classified separately.
Calcite group minerals have hexagonal structures related to the structures of halite and galena: six Ca, Mg, Fe, Mn, or Zn surround anionic (CO3)-2 groups and each metal atom is surrounded by six carbonate groups. Dolomite group minerals have structures similar to calcite’s, but Ca and Mg, Fe, or Mn occupy alternate layers. Aragonite group minerals are orthorhombic. Solid solutions are common within a structural group, although some carbonates, such as aragonite, are almost always close to end-member composition.
niter (saltpeter) KNO3
Due to very high solubility in water, nitrate minerals are rare. They have structures similar to carbonates, but contain monovalent rather than divalent cations because the (NO3)– anionic group is monovalent. Over half a dozen nitrates are known, but nitratite and niter are the only common ones in more than just a few localities.
For more general information about carbonates and nitrates, see Section 7.4.3 of Chapter 7.