The study of crystal chemistry shows how the chemical composition (chemical formula), internal structure, and physical properties of minerals are linked together.

Minerals are classified according to their chemical composition. They are grouped into classes according to the dominant anion (negatively charged element) present. The reasons for classifying minerals this way are listed below:

Minerals with the same anion show unmistakable family resemblances (look similar, have similar physical properties, etc.) more so than minerals that have the same dominant cation (positively charged atom).

Minerals with the same anion tend to occur together or in the same or similar geologic environment. Examples include the sulfides, which occur in vein or replacement type of deposits and the silicates, that make up the great bulk of the rocks of the Earth’s crust.

Classifying minerals this way is similar to the current chemical practice of naming and classfiying inorganic compounds.