Chemical Composition

The crystal chemical formula is an empirical approximation of the actual composition of a mineral. For example _, a mineral with the formula _Fe₃Al₂ [ _SiO₄ ] _₃, upon precise measurement, actually has the formula (Ca _0.7 ^Fe²⁺ _2.0 Mg _0.3 Mn _0.1 ) _₃ (Al _2.0 )(Si _2.9 Al _0.1 ) _{₃O₁₂ .} Minerals often contain both covalent and ionic bonds between individual atoms/ions or molecules. In the crystal chemical formula, substituting ionic cations (positively charged, e.g., Fe, Ca, Mg, Mn, etc.) are usually written in parentheses, and anionic complexes (-) with internal covalent bonds are written in square brackets (e.g., [ _SiO₄ ] ^²⁻ ). An ionic bond usually exists between cations (+) and the anion complex(es). Many minerals also contain volatile anion complexes, such as ( ^OH⁻ , ^F⁻ , ^Cl⁻ ), which can also substitute for each other. Certain minerals, such as phyllosilicates, also exhibit intermolecular van der Waals forces between their mineral layers. Since pure metals like gold (Au), platinum (Pt), and osmium (Os) are also minerals, metallic bonds are also found in the mineral kingdom. Metallic bonds between certain elements are also found in other minerals (e.g., sulfides). The chemical bonding types of a mineral have a direct impact on many other properties, including solubility, hardness, and melting point.

Minerals are classified and cataloged in most systems based on their chemical composition. The naming is based on the anion complex with the largest proportion; thus, _Fe₃Al₂ [ _SiO₄ ] _₃ is a silicate. The most famous systems are the one used in America by James Dwight Dana (1837) and the _one most widely used in Europe by Hugo Strunz (1941).