A mineral is a naturally occurring, inorganic solid, which can be written with one chemical formula and which has a repeating atomic lattice structure. This later statement just means that a mineral is able to form crystals. Crystals have very specific shapes and ratio and these shapes and ratios are directed by their internal structure. Early mineralogists made very good guesses about these internal structures by using an instrument called a goniometer to measure the ratios of angles and faces in crystals of many minerals. Most of these guesses were proved to be either correct, or very close approximations when the minerals were investigated with more advanced technologies, such as x-ray diffraction.
The reason that the definition of a mineral concentrates on the lattice structure of the material, is because it is possible to have multiple minerals with the same chemical formula, but with different atomic arrangements. Materials which share the same chemical formula but different periodic arrangements are called polymorphs. Polymorphs have different crystal structures and different physical properties.
This system was initially designed by Dana, it is a methodical way to list minerals based on their chemistry. The system we are using here is very condensed and is missing all the subcategories. There is a fuller and far more complete updated system available.