The table seen here lists the most common cations and anions in Earth’s crust. For the most part, these are the same elements that we discussed at the beginning of this chapter when we considered the composition of the crust and mantle.
|Common Cations, Anions, and Anionic Groups|
While thinking of individual atoms bonding together to form minerals seems straightforward, in reality atoms are seldom unbonded to others. Single atoms are very reactive. They tend, when possible, to bond to other atoms to form molecules and often compounds. Sometimes, they bond to other atoms of the same element. For example, N2, composed of molecules containing two nitrogen atoms, dominates the Earth’s atmosphere. Small atoms with several valence electrons, such as silicon or carbon, are especially reactive. They seldom exist by themselves, readily combining with oxygen, and sometimes other elements, to form strongly bonded anionic units called anionic groups, also called molecular ions or polyatomic ions. Molecular ions are so strongly bonded that they behave like individual anionic units in many minerals. So when minerals dissolve, molecular ions such as (CO3)2- will not dissociate in water.
The right-hand column of the table lists the most common and important anionic groups. They include, from top to bottom, the nitrate, hydroxyl, silicate, sulfate, carbonate, tungstate, borate and phosphate groups. As pointed out in the previous chapter, we generally classify minerals based on their anion species because the properties of minerals with the same anions or anionic groups are generally very similar. So, we often write mineral formulas with parentheses to emphasize any anions or anionic groups that are present.
Any of the cation species listed in the table can combine with the anions or molecular anions to produce a long list of different ionic compounds. For example, all these minerals contain Ca2+:
Lime, fluorite, larnite, anhydrite, calcite, and scheelite belong, respectively to the oxide, halide, silicate, sulfate, carbonate, and tungstate mineral groups. Additional Ca2+ minerals with more complicated formulas include sinjarite (CaCl2\(\cdot\)2H2O), colemanite (Ca2B6O11\(\cdot\)5H2O), nitrocalcite (Ca(NO3)2\(\cdot\)4H2O), and apatite (Ca5(PO4)3(OH)). Thus, Ca2+ can combine to make minerals with all except one of the anion and anionic species listed in the table above. The lone exception is nitrate, (NO3)–. But, synthetic calcium nitrate (CaNO3), called Norwegian saltpeter is sometimes manufactured for industrial use.